rural com

Application Support & TrainingIntelsat Global Service Corporation

3400 International Drive, N.W.Washington, DC 20008-3098

Phone: +1 202 944 7176Fax: +1 202 944 8214

Copyright © 2001 by Intelsat Global Service CorporationALL RIGHTS RESERVED.

Satellite Based Rural Telephony Handbook Page i

CONTENTS

1 Introduction to Rural Communications............................................................ 11.1 Overview of Rural Communications .................................................................... 11.2 Definition of Rural Area ..................................................................................... 21.3 Overview of Rural Telephony Market Projections ................................................. 3

1.3.1 Addressable Market.................................................................................... 41.4 Rural Telephony Subscribers Access Solution ....................................................... 6

1.4.1 Wired Networks (External Line Plant)............................................................ 61.4.2 Wireless Network (Wireless Local Loop)........................................................ 6

1.5 Wireless Local Loop (WLL) versus Wireline ........................................................... 71.5.1 Network Costs............................................................................................ 71.5.2 Installation time.......................................................................................... 91.5.3 Flexibility .................................................................................................. 101.5.4 Conclusion: Choice of Rural Subscriber Telephony Access Solutions............. 10

2 Introduction to VSAT and WLL technologies.................................................. 112.1 VSAT Technology ............................................................................................ 11

2.1.1 VSAT Hardware ........................................................................................ 112.1.2 VSAT Network Topology ........................................................................... 122.1.3 Overview of VSAT Applications and Services............................................... 142.1.4 Multiple Access Protocol ........................................................................... 17

2.1.4.1 Introduction.......................................................................................... 172.1.4.2 Satellite Capacity Access Protocols ......................................................... 182.1.4.3 Satellite Network Access Protocols ......................................................... 192.1.4.4 TDM/TDMA Networks ........................................................................... 202.1.4.5 SCPC/DAMA Networks.......................................................................... 212.1.4.6 FTDMA/DAMA...................................................................................... 232.1.4.7 Performance comparison between various access protocols .................... 24

2.2 WLL Technology .............................................................................................. 252.2.1 WLL Hardware.......................................................................................... 262.2.2 WLL network topology.............................................................................. 272.2.3 Review of Wireless Standards .................................................................... 28

2.2.3.1 Analog standards .................................................................................. 292.2.3.2 Digital standards ................................................................................... 30

2.2.3.2.1 TDMA ............................................................................................. 302.2.3.2.2 CDMA............................................................................................. 322.2.3.2.3 Cordless Telephones/Digital European Cordless (DECT) ...................... 34

The MC/TDMA/TDD principle........................................................................ 362.2.3.2.4 PHS................................................................................................. 372.2.3.2.5 CT-2................................................................................................ 38

2.3 Choosing the appropriate WLL Technology ....................................................... 392.3.1 Coverage comparison ............................................................................... 392.3.2 Capacity comparison................................................................................. 402.3.3 Functionality comparison........................................................................... 41

Technology .................................................................................................. 413 Rural Models and Micro- versus Macro-Cell Architecture ............................... 424 Integrated WLL-VSAT Micro Terminal............................................................ 45

4.1 Choice of WLL and VSAT solution..................................................................... 45

Satellite Based Rural Telephony Handbook Page ii

4.2 What needs to be integrated?.......................................................................... 464.3 Integrated DECT-VSAT terminal description....................................................... 47

5 The integrated WLL-VSAT network ............................................................... 495.1 DAMA – The rural telephony architecture of choice........................................... 495.2 The Distributed Call Processing Approach ......................................................... 52

5.2.1 Subscriber management............................................................................ 545.2.2 Call scenarios for integrated VSAT/WLL with local switching ....................... 55

5.2.2.1 WLL subscriber-PSTN ............................................................................. 555.2.2.2 WLL subscriber-WLL subscriber (intra-cell)............................................... 565.2.2.3 WLL subscriber-WLL subscriber (inter-cell)............................................... 56

5.3 The Call Detail Record (CDR) Collection/Public Calling Office Support ................. 575.3.1 WLL subscriber-PSTN................................................................................. 575.3.2 WLL subscriber – WLL subscriber (intra-cell)................................................ 585.3.3 WLL subscriber –WLL subscriber ( inter-cell)................................................ 58

5.4 The Centralized Network Management (and O&M) Design ................................ 606 Planning and implementation of rural communication networks, via the Intelsatsystem................................................................................................................ 61

6.1 Sizing and implementation of a VSAT DAMA SCPC network.............................. 616.1.1 Traffic estimation...................................................................................... 61

6.1.1.1 The DAMA Network Channel Pool ........................................................ 616.1.1.2 Estimation of the number of Channel Units ............................................ 62

6.1.2 Space segment ......................................................................................... 636.1.3 Earth Segment.......................................................................................... 646.1.4 Intelsat Satellite and frequency band.......................................................... 646.1.5 Topology and access alternatives ............................................................... 656.1.6 Link budget.............................................................................................. 656.1.7 Network implementation costs .................................................................. 656.1.8 Pre-implementations activities ................................................................... 676.1.9 Post-implementation issues ....................................................................... 68

6.2 Deploying WLL ................................................................................................ 686.2.1 Choosing a service offering ....................................................................... 696.2.2 The Network build-out costs...................................................................... 69

6.2.2.1 Subscriber equipment costs ................................................................... 696.2.2.2 Network costs....................................................................................... 69

6.2.3 The recurring costs ................................................................................... 706.2.4 Rolling Out the Network ........................................................................... 70

6.2.4.1 Selecting the number of cells ................................................................. 706.2.4.2 Connecting the cells to the PSTN ........................................................... 71

7 Rural Telephony via Intelsat: summary........................................................... 727.1 VSATs directly connected to Subscriber Lines..................................................... 737.2 VSAT connected to Local Loop ......................................................................... 74

7.2.1 VSAT connected to Wired Local Loop ........................................................ 747.2.2 VSAT connected to Cordless/Wireless Local Loop (DECT/PHS)...................... 75

7.2.2.1 Case Study: Intelsat Trial Project in Senegal-DAMA VSAT/DECT WLLconfiguration ...................................................................................................... 76

7.3 VSAT connected to Wireless Macro-cellular Networks........................................ 777.3.1 Case Study: Intelsat trial in Peru................................................................. 78

7.4 Generic features of rural telephony solutions..................................................... 797.5 Summary ........................................................................................................ 80

Satellite Based Rural Telephony Handbook Page iii

8 Intelsat Rural communications service portfolio ............................................. 828.1 The Intelsat value added services ...................................................................... 828.2 Type of service offered..................................................................................... 82

8.2.1 Intelsat Business Service (IBS) via VSATs...................................................... 838.2.2 Leases ...................................................................................................... 838.2.3 Intelsat DAMA.......................................................................................... 84

Disclaimer

While every precaution has been taken in the preparation andcompilation of this manual, the accuracy of the information containedherein cannot be guaranteed and Intelsat Ltd. and its affiliated entitiesmake no representations or warranties, whether expressed or implied, as tothe accuracy of the information contained herein.

This manual may be amended or modified from time to time byIntelsat Ltd. and its affiliated entities, as it deems necessary without priornotification.

Satellite Based Rural Telephony Handbook Page 1

1 Introduction to Satellite Based Rural Communications

1.1 Overview of Rural Communications

Advances in digital technology have substantially increased the availability ofcommunications, resulting in significant cost reductions of communicationsequipment and utilization charges. Nevertheless, provision of cost effective andreliable communication solutions to rural areas has always posed a great challengenot only to telecommunications operators, but also manufacturers ofcommunications equipment. Although the existence of a widely recognized demandhas been documented, almost three billion people still remain in rural areas withoutbasic telephone service due to limited cost effective communication solutions forrural applications. Many telecommunications regulators and planners have recentlyrealized how challenging it is to provide universal service to rural populations.

Satellite-based networks using Very Small Aperture Terminal (VSAT) technologyprovide simple and cost effective solutions for rapid implementation oftelecommunications infrastructure that can link these remote rural areas to the restof the world. The use of traditional terrestrial Wide Area (backbone) Network(WAN) technologies such as microwave links and optical fiber cables to connectthese remote areas to the national and international Public Switched TelephonyNetwork (PSTN) has also proved very costly when serving small pockets of isolatedsubscribers. For example, unencapsulated fiber optic cable costs over US$30 permeter to install. This could hardly be termed a cost-effective rural solution. Line-of-sight microwave systems have similar drawbacks, especially for large distances(>100 km) and difficult terrain where the installation of multiple microwave towersand repeaters is required.

Because of its ubiquitous and multi-point nature, satellite technology can ensure asingle WAN model that provides access to the national and international PSTN,irrespective of the geographical location and the nature of obstacles. Severalsatellite solutions are available (or will be soon). Communications satellite solutions,such as Global Mobile Personal Communications Systems (GMPCS), offer new abreed of L-band low Earth orbit (LEO) services aimed at mobile subscribers in areasthat do not have access to the PSTN. The mobile GMPCS subscriber typicallypurchases a $2,000 dual mode handset that use GSM or Code Division MultipleAccess (CDMA) cellular services where available, then reverts to satellite when outof terrestrial coverage. Satellite retail call costs are estimated to be from $1.50 to$5.00 per minute. Several regulatory and technical issues still remain before trueglobal "roaming" can be implemented. These constraints coupled with low incomesin rural areas might make GMPCS an expensive solution for the rural population.The use of geostationary satellites to provide services in rural areas offers cheapercost per minute rates; especially when combined with the latest Demand


Assignment Multiple Access (DAMA) technology that allows subscribers to pay forthe established connected calls on-demand (as opposed to pre-assigned circuits). Asa worldwide satellite capacity provider, with a fleet of 19 geostationary satellites,Intelsat*, is ideally positioned to offer rural areas direct access to the worldwidePSTN. Moreover, rural telephony reinforces Intelsat's role and charter to providepremier communications to all areas of the world.

This handbook is the result of an effort undertaken by various Intelsat departments(Marketing, Sales, Engineering and Corporate Development) to present someinformation about rural communications solutions that might be of interest to itscustomers. The rural communications handbook describes technologies, planning,and the applications and benefits of VSAT-based network solutions. A significantportion of this book is devoted to the integrated WLL/VSAT solution as Intelsat hasconducted a number of pilot projects and research and development activities inthis field during the last 2 years.

1.2 Definition of Rural Area

Defining rural area is quite a complex issue because it often varies from one countryto another. On the outskirts of large cities, it is sometimes difficult to draw a cleardistinction between what is urban and what is rural, as it is not always evidentwhether a built-up area should be considered urban or rural.

Based on the GAS 7 (ITU-T document), a rural area is classified as an area consistingof scattered settlements, villages, and small towns, where the area exhibits one ormore of the following characteristics:

(a) Scarcity or absence of public facilities, such as reliable electricity or watersupply, roads in poor conditions, and irregular transport;(b) Simplicity of life, where people are primarily concerned with their survivaland basic needs, locally available qualified technical personnel are scarce;(c) Severe climatic conditions, particularly tropical, semi-tropical or desert zones,that make critical demands on the life and maintenance of equipment;(d) Sparse and scattered population distribution with relatively poor and/ortemporary housing;(e) Scarcity or absence of health and education facilities;(f) Economic activities limited to basic vocations such as agriculture, fishing,livestock breeding, small-scale mining, or cottage industries.

All these characteristics can be applied in varying degrees to rural areas indeveloping and least developed countries. In addition, in 1982, in the report known*The term Intelsat stands for Intelsat Bermuda and its subsidiaries, here and later in use in the handbook


as the "Missing Link", the ITU provided yet another definition of a "rural area" bystating that it is "non-urban", which would translate into the followingdemographic parameters:

• less than 5000 people residing in the area;• population density of less than 400 per sq. km; and• more than 75 percent of the male population engaged in agriculturalactivities.

Finally, recently, the Indian government provided the following definition: ruraltelephony takes place in an area where a subscriber has to walk more than 5km oran hour to reach the nearest phone. All these definitions complement each other toprovide a more accurate picture of a rural environment.

1.3 Overview of Rural Telephony Market Projections

The following section provides some preliminary market sizing information for therural telephony application worldwide. A second portion of this chapter deals withthe overall market projections to be addressed by various technology solutionsavailable on the market today, i.e., microwave-based wireless access, wireless localloop, VSAT, and fixed Mobile Satellite Services (MSS) solutions. The marketprojection for the integrated/combined VSAT/WLL architecture is a subset of bothWLL and VSAT market projections. A conservative estimate of the WLL linesemploying a VSAT/WLL solution is 5 percent while the estimate for the VSAT marketis 15 percent.

Defining the rural telephony application and sizing the market pose manychallenges. The nature of the market differs from service to service and region toregion. In addition, many variables need to be assessed based on the followingfacts:

Cost per line and service price are the critical drivers of the market size. Cost per lineis often calculated based upon traffic (erlang per subscriber) and quality of service,regardless of the rural environment, which at times may be misleading. A moreaccurate parameter to refer to is the installed cost per line once import taxes,transportation, power supply (UPS, solar) and installation costs, all occurring in arural environment, are accounted for. Truly, the installed and operated costs per line(recurring and non-recurring expenses) are the most critical issues because manyoperating inefficiencies occur in emerging markets and, thereby, greatly impact therural telephony business case.

Availability of financing (cost of money), economical (currency fluctuation),regulatory (spectrum allocation, interconnection rules), and socio-political barriersare also notable key-drivers of the addressable market.


The problem may be approached from two different perspectives. First identifyinghow large the potential rural market is worldwide. Then, from this vast potential,filtering and narrowing the potential market down to an addressable market sizeusing both, a top-down and bottom-up approach.

Table 1.1: World Rural Telephony Market Total Potential based on Demographics1997-2004

Year World RuralPopulation

PublicPhone

ResidentialPhone

BusinessPhone

Total lines

1997 3,500,000,000 5,250,000 480,000,000 144,000,000 629,250,000

1998 3,670,000,000 5,535,000 504,000,000 150,000,000 659,535,000

1999 3,850,000,000 5,800,000 530,000,000 160,000,000 695,800,000

2000 4,030,000,000 6,000,000 550,000,000 166,000,000 722,000,000

2001 4,210,000,000 6,350,000 575,000,000 175,000,000 756,350,000

2002 4,390,000,000 6,600,000 600,000,000 180,000,000 786,600,000

2003 4,670,000,000 6,900,000 625,000,000 188,000,000 819,900,000

2004 4,750,000,000 7,150,000 650,000,000 195,000,000 852,150,000

Note: All figures are rounded. Source: Frost & Sullivan

The preceding Table 1.1 is drawn from preliminary information provided by Frost &Sullivan. These projections include rural population living in developed anddeveloping countries. The public phones would serve the poorest classes of thepopulation. As indicated, the market potential is overwhelming and greatlyoverstates the near-term addressable market.

1.3.1 Addressable Market

The addressable market may be computed using the affordability criteria to filter themarket potential information (top-down approach). As a result, Frost & Sullivanestimated the total number of addressable lines to be in the range of 50,000,000lines around the years 2000’s or approximately 8 percent of the potential market.Table 1.2 provides some detailed information per service mix.

Table 1.2 Projected World Rural Telephony Market – Top/down approachProjected Number of Lines Installed by Service Mix

1997-2004


Year Business Residential PayPhone Total Lines1997 2,614,150 5,655,000 447,450 8,716,6001998 3,488,250 7,540,650 604,100 11,633,0001999 8,040,000 17,394,000 1,374,000 26,808,0002000 12,767,500 27,619,600 2,182,900 42,570,0002001 20,057,320 43,385,360 3,431,720 66,874,4002002 25,237,564 59,738,704 8,320,948 93,297,2162003 30,323,783 80,820,115 12,522,482 123,666,3802004 35,742,068 96,762,750 15,544,477 148,049,295

Source: Frost & Sullivan

A second more conservative approach adopted here is a bottom up approach wherethe rate of technology implementation dictate the addressable market projections.For example, projected Wireless Local Loop figures and actual results from the last 2to 3 years are now well documented. Overall, the rate of roll out is much slowerthan the projected 100 million lines by the year 2003. Again, the integrated VSAT-WLL may take some of the WLL market share starting slowly in 1999, and risingrapidly by 2000 (up to 5 percent of the WLL market and 15 percent of the VSATmarket). With the introduction of the integrated VSAT-WLL, the applicability ofVSATs to medium sized villages and medium density rural areas is also enhanced.Finally, fixed MSS (Mobile Satellite Services) are shown with a very limited marketshare due to the untested nature of the technology solution. As a result, thisapproach would indicate that less than 3 percent of the potential market wouldconstitute the total addressable market for rural telephony in the near-term (2000-2001), and approximately 10 percent in the mid-term (2004 and after). Table 1.3provides some detailed information on the projected World Rural telephony linescontracted by technology rollout.

Table 1.3: Projected World Rural Telephony Market and Projected Number of LinesContracted by Technology Roll-Out

1997-2004Year Wireless Access WLL VSAT LEO/GMPCS Total Lines1997 3,000,000 3,000,000 10,000 0 6,010,0001998 3,600,000 4,000,000 15,000 0 7,615,0001999 4,500,000 5,500,000 22,000 0 10,022,0002000 6,000,000 8,000,000 40,000 12,000 14,052,0002001 9,000,000 12,000,000 90,000 50,000 21,140,0002002 13,000,000 20,000,000 200,000 100,000 33,300,0002003 18,000,000 30,000,000 400,000 300,000 48,700,0002004 30,000,000 50,000,000 1,000,000 700,000 81,700,000


1.4 Rural Telephony Subscribers Access Solution

Subscriber access in rural areas was traditionally based on copper cables connectinga local exchange to remote subscriber units. The service offered by this kind ofnetwork was often limited to subscribers located within 5-km from the exchange.More recently, advances in wireless technologies have led to the development ofnew wireless-based products that can efficiently be implemented in rural areas.Wireless Local Loop is one such technology that can replace the copper cable in thesubscriber loop of the wired network.

1.4.1 Wired Networks (External Line Plant)

The standard wireline loop in a rural environment is shown schematically inFigure1.1. There is a feeder segment (usually a large, multi-pair cable), connectedto the distribution segments (smaller gauge cables), which in turn are connected toindividual drop wire segments (single or double twisted-pairs) connecting toindividual subscribers.

The connections between the feeder segments and the drop-wire segments aretoday normally accomplished by means of cross-connection device, which definesan administrative boundary sometimes called the serving area interface.

9/02/2581Fig1_1.wmf

CrossConnect

Feeder Cable Segment

DistributionCable Segment Dropline Segment

CentralOfficeSwitch

MainDistribution

Frame(MDF)

CrossConnect

- -

Figure 1.1. Basic Wired Network

In most rural networks, only the subscribers located near the local exchange (within5-km) have access to the telephone service. The copper is normally distancesensitive, in that there is a noticeable degradation in the quality of the voice as onemoves away from the exchange.

1.4.2 Wireless Network (Wireless Local Loop)

Wireless Local Loop is the use of radiocommunications to provide communicationsto the subscribers (see figure 1.2 for the basic configuration of WLL system).


Historically, the local loop featured a copper cable buried in the ground or carriedon overhead pylons. WLL replaces the local loop section with a radio path, ratherthan a copper cable. It is concerned only with the connection from the localexchange to the susbcriber; all other parts of the network remain as for a traditionalwired arrangement.

In a WLL system, the exchange is connected to a radio transmitter via a radiocontroller. A radio receiver is mounted on the side of the house and a cable is rundown the side of the house to the socket inside the house. This is an identicalsocket to the one which users currently plug their home telephones into. Hence,apart from a small receiver on the side of the house, the home subscriber does notnotice any difference.

Base Station RadioTerminal

Switch & Network

Backhaul Distribution

Cell SiteController

Figure 1.2. Basic Configuration of WLL System

1.5 Wireless Local Loop (WLL) versus Wireline

The following sections provide some comparisons between the WLL and wiredtechnology in terms of:

• network costs;• installation time; and• flexibility especially in terms of network expansion (rollout).

1.5.1 Network Costs

When cost comparisons are computed between WLL and wireline in a ruralenvironment, it is often clear to see why the WLL option is so compelling.Implementing the last 1/4-mile of wireline infrastructure is one of the most costlyportions of the entire network due to the expenses associated with labor andmaintenance. In addition, most telephone companies are faced with significantexpenses to maintain the telephone lines, that are frequently plagued by damagedue to inclement weather, falling trees, digging, and recurring problems of copperthefts. By opting for a WLL-based solution, the network is less exposed to the harsh


environmental elements, therefore, its maintenance cost is also lower. Finally, thelabor activity required to construct a WLL versus wired solution is significantly less,resulting in a lower installation cost.

The overall cost per subscriber in a WLL network is estimated between US$1,000 toUS$2,000 as compared to US$4,000 to US$80,000 per subscriber for a wiredinfrastructure (low penetration) in all but the highest density regions, (typically over10k subscribers/sq. km). In high-density regions, a wireline solution still has a costadvantage. (See Table 1.4 for some of the publicly announced WLL pricing). Thegraphs in Figures 1.3, 1.4, and 1.5 show the relative costs of installed cable andwireless access for a range of housing density and penetration levels. The costs forWLL systems will be further reduced, as the market keeps expanding. Theadvantages of WLL as either the housing density or the penetration falls areobvious. The figures show WLL consistently providing a less expensive accesstechnology than cable, with the difference decreasing as the penetration increases.The costs of both systems rise as the density of homes falls, with WLL remaining theleast expensive alternative.

5,000

2,000

4,000

1,000

05% 10% 15% 25%20%

Penetration

Cable

WLL

3,000

9/02/2581-5_2

Figure 1.3 Relative Costs of Cable and WLL in a High-Density Case

8,000

2,000

4,000

6,000

05% 10% 15% 25%20%

Penetration

Cable

WLL

9/02/2581-5_3

Figure 1.4. Relative Costs of Cable and WLL in a Medium-Density Case


10,000

2,000

4,000

8,000

05% 10% 15% 25%20%

Penetration

Cable

WLL

6,000

9/02/2581-5_4

Figure 1.5. Relative Costs of Cable and WLL in a Low-Density Case

Overall, the advantage, in terms of network costs, of WLL compared to wireline iseven more pronounced in rural remote areas were population densities are lower.

Table 1.4. Prices for some of the WLL Products on the World Market (1997)

Company Cost perline (US$)

System Type Location

Nokia 3,192 NMT based WLL East GermanyMotorola 320 AMPS cellular St. Petersburg, RussiaEricsson 444 TACS cellular Guangdong, ChinaSiemens 1,500 GSM cellular MoroccoAT&T 500 AMPS cellular/WLL Presumed ArgentinaNorthernTelecom

900 450 MHz TDMA WLL Mexico

Stanilite 400 AMPS cellular ArgentinaHughes NetworkSystem

1,500 E-TDMA based WLL Jakarta, Indonesia

Interdigital 2,400 450 MHz proprietary Myanmar Courtesy of International Technology Consultants, Bethesda, MD, USA

1.5.2 Installation time

Another factor that tips the scales in favor of WLL is the time involved in planningfor and installing a network. In the majority of cases, building a WLL link and havinga subscriber connected to the local public telephone network is normallyaccomplished in a fraction of the time it takes to physically lay copper cables. To thelocal company, this not only translates into a much faster response time in meetingthe customers' demands for access, but also provides for a faster source of revenue,enabling them to become profitable sooner.


1.5.3 Flexibility

The concept of network flexibility or the ability to adjust/expand the networkcapacity to meet the evolving demand for telephone access is seldom considered ina wired architecture. When additional copper phones lines are added to meet thegrowing demand, the installation is usually done on a permanent basis. Also,because building new wireline infrastructure is such a time consuming and costlytask, it is added in very large increments, allowing for future population growth.The one drawback with this approach is that for a long time the percentage of thewired network that is actually used is relatively low, resulting in a much smallerreturn on net assets for the service provider.

In addition, planning for future growth in the wireline environment is also a sourcefor costly mistakes if the targets for future capacity requirements are grossly over- orunderestimated. With WLL, however, capacity can be added in small increments tomeet growing demand as needed, thereby lowering the overall expense for newinfrastructure and eliminating the cost associated with planning errors. Thus, a WLLsystem is more "forgiving" to uncertainty in subscriber demand forecasts (Fig. 1.6).Flexibility is even more important in rural areas where there has never been anyform of communications services on which to base these projections. In such areas,planning for a network and its future growth can at times be difficult.

Wireless Capacityadded Incrementally

Traffic Demand

WirelinesWireless

PlantInvestment

($)

Wireline Plant requires LargeLumpy Investment

Time

Figure 1.6. Comparison of Wireless and Wireline Systemsin Terms of Flexible Network Design

1.5.4 Conclusion: Choice of Rural Subscriber Telephony Access Solutions


Telecommunications operators would like to provide telephone services to ruralareas economically and within a short time due to the severe and extremeconditions in these areas. The advantages that WLL technology has over wirelinetechnology in terms of rapid installation time, lower network cost, and addedflexibility to expand make it a very attractive solution for providing a minimum of 30to 50 lines. VSAT remains the economical solution for a scattered population withup to 20 to 30 lines for the subscriber rural access solution.

With the majority of the rural population still lacking telephone access, wireless localloop appears destined to solve much of the challenges of providing the basic accessto telephones. A number of WLL solutions exists. Therefore, it is important todetermine the correct wireless architecture that will provide spectral efficiency, tollquality voice, and proven reliability in the macro- and micro-cell environment, whileat the same time solving the challenges of rural demand for telephone access.

However, there is no quick rule to suggest that only the WLL solution can beprovided in the rural environment, especially when considering the technicalfeatures associated with each solution, in particular the current bandwidthlimitation of a WLL solution. Therefore, Wireless and Wired media will continue tobe implemented side by side in the same network. One option is the so-calleddoughnut cell. In this scenario, the cost of connecting homes very close to theexchange might be lower by using copper instead of radio because of the shortdistances involved. However, above the radius of say, 1km, the costs are loweredusing a wireless solution.

2 Introduction to VSAT and WLL technologies

The concept of integrating WLL and VSAT access technologies is fairly new. Thesystems currently in place are based on open architecture with different vendorssupplying WLL and VSAT equipment. Starting in Chapter 2, and in subsequentchapters, the basic concepts of VSAT and WLL technologies and configurations willbe presented.

2.1 VSAT Technology

2.1.1 VSAT Hardware

A VSAT is a micro-Earth station that uses the latest innovations in the field ofsatellite communications to allow users access to reliable satellite communications.A typical VSAT consists of the communications equipment and a small antenna withdiameter of less than 3.5 meters. As depicted in Figure 2.1, a typical VSATinstallation consists of an antenna, an outdoor unit (ODU), the interfacility cable link(IFL), and the indoor unit (IDU). The antenna and the outdoor unit (ODU) providethe radio frequency conversion and amplification for the satellite uplink and


downlink. The ODU is often called the transceiver because it includes theupconverters (U/Cs); the solid state power amplifiers (SSPAs); the Low NoiseAmplifier (LNA), and the downconverters (D/Cs).

The IDU provides the baseband interfacing required to carry the user's services.VSAT terminals are generally part of a network, with a larger Earth station thatserves as a network "Hub". The Hub contains the intelligence to control thenetwork operation, configuration, and traffic. The Hub also records theperformance, status, and activity levels of each VSAT terminal. Databases generatedby the Hub contain call data records that are used for billing purposes. The Hub isusually located where the bulk of network traffic originates and/or terminates. AHub consists of an antenna, LNA, SSPA, and frequency converters.

DataTerminal

Fax

Telephone

Outdoor Equipment

Outdoor Unit

9/02/2581Figure 2_1

Inter-FacilityLink Cable

Indoor Unit

Demodulator

BasebandInterface

Modulator

User'sService

Antenna

Figure 2.1. Typical VSAT Terminal

2.1.2 VSAT Network Topology

Currently, there are two types of VSAT network topologies: star and mesh. In thestar topology, each VSAT transmits to and receives from the Hub. (See Figure 2.2(a).) This does not preclude the VSATs from communicating among themselves,because VSAT-to-VSAT communications can be routed via the Hub using a doublesatellite hop. The majority of the VSAT networks use star topology because thelarge antenna gain at the Hub optimizes the use of the space segment andminimizes the size of the VSAT. The drawback of the star topology is that the delayfor the VSAT-to-VSAT communications doubles in comparison to the single hoptransmission.


VSAT

VSAT

VSAT

HUBSAT SAT

VSAT

VSAT

VSAT

VSAT

VSAT

VSAT

VSAT

VSAT

VSAT

VSAT VSAT

VSAT

HUB

A) Star Topology B) Mesh Configuration

Figure 2.2: Commonly Used VSAT Topologies

Mesh topology (Figure 2.2 (b)) allows all terminals to communicate with each otherdirectly. A Hub must control the communications set up and tear down process, butneed not be involved in carrying traffic. Sometimes, a VSAT terminal is equippedwith the network management and control equipment, and the network is said tooperate hubless. Because each VSAT must have sufficient power and receivesensitivity (G/T) to communicate with every other VSAT, mesh topology requireslarger antennas and SSPAs than the star topology. Mesh topologies are well suitedfor applications such as telephony that cannot tolerate delay.

Hybrid topology allows a group of VSATs to communicate in mesh topology whileothers communicate only in star topology. This topology is useful for networks inwhich certain terminals have larger traffic demand between themselves than theother terminals. The terminals with larger traffic volume can be accommodated inmesh to reduce the expense of extra equipment at the Hub, and the satelliteresources required for the double hop. The rest of the network can communicatewith any of these larger terminals or each other via the star topology. An exampleof hybrid network would be used to carry data over the star network and voice overthe mesh network. Multi-star topology, which is also a hybrid of star and meshtopologies, is suitable for scattered star networks whose subscribers would like tocommunicate with other subscribers in particular star networks using single hop.Based on the called party's destination phone number, the call is routed via thenearest Gateway site of the called party. In most cases, the Gateways can belocated with the toll exchange that corresponds to particular regions serving an areacode or group area codes. This solution not only eliminates the double satellitehops, but is also economical in that it decongests the existing infrastructure.


Therefore, depending on the traffic from the remote to the Hub and intra-VSATcalling levels, the user can configure the VSAT network accordingly to suit the trafficlevels.

2.1.3 Overview of VSAT Applications and Services

VSATs are used for a wide range of telecommunication solutions for domestic,regional, or international applications. Despite the fact that our major focus in thishandbook is on VSAT applications to rural communications, there are still otherapplications that broadly fall into two categories:

• interactive or two-way applications; and• broadcasting or one-way applications.

Interactive or two-way applications: Interactive applications allow two-waycommunications via the VSAT terminal. The carrier from the hub-station to theVSAT is called "outbound", while the carrier from the VSAT to the hub is called"inbound". The interactive applications can be bundled in three categories:interactive voice services; interactive data services; and interactive video services.

A. Interactive voice: This category offers the following voice services. (SeeFigure 2.3.)

• Voice services to extend the PSTN facilities to rural areas or remote areas;and• Voice services for private networks of companies whose branches arelocated in areas where there is no access to the PSTN

A VSAT terminal is flexible enough to either handle a single telephone line for verylow traffic levels, or several lines, possibly via a local PBX.

The combination of VSAT and WLL can extend the basic phone service to placeswhere other technologies are not cost effective. For example a VSAT equipped with8 satellite channels and a WLL base station can serve a population of 500subscribers. (See Figure 2.4 for the WLL-VSAT network architecture.)


Figure 2.3 Voice Applications Examples

SATELLITEGATEWAY

VSAT & WLLTERMINAL

FARM

- -

VILLAGE

WIRELESSPUBLIC

PAYPHONE

LOCAL LOOP(WLL TECHNOLOGY)

BACKHAUL(VSAT TECHNOLOGY)

Figure 2.4 VSAT-WLL Network Architecture Diagram

B. Interactive data services: This application involves the transfer of data fromone VSAT terminal to another. This application usually involves a data transferrequest from one VSAT terminal and a subsequent response from another VSATterminal. (See Figure 2.5.) Typical applications are listed below.

• File and batch transfers for financial institutions, stock brokers, andbanks(i.e., branch offices to headquarters)


• Management of point-of-operations for supermarkets, retail shops, gasstations, fast food stores, all types of payment terminals, including Automatic TellerMachines (ATMs) and credit card transactions• Reservations requests and confirmations for airlines, hotels, car rentals, andtravel agencies• Data request retrieval from remote sensing on oil drillings, pipe lines, gas,electric, and transport industries• Remote processing and LAN extensions

C. Interactive video services: Current compression rates enablevideoconferencing at data rates as low as 64 kbps. However, the best tradeoffbetween quality and cost is achieved at 384 kbps. VSAT users generally implementoutbound video at 384 kbps and inbound video at 64 kbps. This configurationallows good quality in the outbound and rate savings in the inbound. If the userneeds symmetric quality, then the inbound needs the 384kbps, too.

HOTEL

RENT-A-CAR

Figure 2.5 Example of Applications for Interactive VSAT

Broadcasting or one-way applications: In the description above, much has beenmentioned about interactive applications (i.e., two-way communications withtransmit/receive stations). However, the importance of one-way applications shouldnot be underestimated. Typical applications are listed below:

• Price lists, inventory records• Stock, bonds, and commodity information• Weather bulletins, sports scores, news and press releases• Sound broadcasting• Digital video for conferencing or entertainment• Internet distribution(See Figure 2.6 below.)


MASTERSTATION

NARROWCASTINGGROUP

BROADCASTINGCOVERAGE AREA

Figure 2.6 Illustration of Broadcast or One-Way Applications.

2.1.4 Multiple Access Protocol

2.1.4.1 Introduction

In implementing VSAT networks, three different layers of protocols have to beconsidered: satellite access protocol, network access protocol, and user dataprotocols. (Refer to Figure 2.7.)

FEP

HOSTVBP

VBP

HBE

TERMINALS

Satellite Network Access Protocol(Satellite Efficient Access Protocol i,e, S-Aloha &TDM/TDMA)

Satellite CapacityAccess Protocol(FDMA, TDMA)

Customer's Data Protocol

HBE: Hub Baseband Equipment

FEP: Front-End Processor

VBP: VSAT Baseband ProcessorFigure 2.7 Different Layers of Protocols Used in VSAT Network


The performance of a network is directly affected by the protocol used, and a goodnetwork design will use protocols that achieve the highest network performance,for the specific application, while minimizing required satellite bandwidth.

2.1.4.2 Satellite Capacity Access Protocols

A satellite access protocol describes the way in which multiple VSATs share thesatellite bandwidth. There are four main techniques to access the satellite spacesegment among multiple users: Frequency Division Multiple Access (FDMA), TimeDivision Multiple Access (TDMA), Code Division Multiple Access (CDMA), andFrequency Time Division Multiple Access (FTDMA).

FDMA, the simplest access technique used by VSATs, allows the network to sharesatellite capacity by using a different frequency assignment for each carrier. Aspictured in Figure 2.8a, VSAT terminals share the allocated capacity by transmittingtheir carriers at different frequencies. The carriers need not have the same power orbandwidth, but their sum must be within the allocated capacity.

TDMA, the second access technique, allows users to access the allocated capacity ina time-shared mode. Each VSAT transmits in bursts during set time slots. Once theallocated burst time is finished, the VSAT will cease its transmission and yield thecapacity to other VSATs. As indicated in Figure 2.8b, at any given time, one user fillsthe entire allocated bandwidth and power.


Under CDMA, the third access technique shown in Fig. 2.8c, all VSATs transmitsimultaneously in the same allocated frequency, bandwidth, and power. In CDMA,a pseudo-random sequence encodes the original signal by spreading the signal overa larger bandwidth. To restore the original signal, the receiver correlates thecomposite input with the original encoding sequence stored in its memory.

FTDMA the fourth access technique employs a number of FDMA carriers, eachcomprising an N slot TDMA channel frame synchronized to a network-widereference, derived from a broadcast outbound channel. During each slot, themodem accesses a different FDMA carrier for the duration of the burst. (See figure2.8d.)

#1

#2

#3

#4

FREQUENCY

POWER

TIME

GUARDBANDSCARRIERS

ALLOCATEDBANDWIDTH

A) FDMA

FREQUENCY

POWER

TIME

ALLOCATEDBANDWIDTH

C) CDMA

3

P

C

X

R

ET

3

P

P

PCX

ET

3

P

P

X

DIFFERENTCODES

Composite Signal(Several Carriers with

Different Codes)

FREQUENCY

POWER

TIMETIMEGUARDS

BURSTS

ALLOCATEDBANDWIDTH

B) TDMA

#1

D) FTDMA

Figure 2.8 Basic Forms of Satellite Capacity Access Techniques

2.1.4.3 Satellite Network Access Protocols

Network access protocols assign capacity to a particular terminal based ontraffic demand. Capacity is requested by the VSATs and is assigned by the networkcontroller at the hub, either on-demand, at random, or permanently.

In an on-demand assignment protocol (DAMA), the VSAT requests the hub todynamically pre-assign capacity, either time slots or carriers, before transmitting.This process implies a slower initial response time, but is highly efficient during datatraffic transfer. In a random assignment protocol, each VSAT transmits its traffic


when it is received from one of its data ports. This mode offers a very shortresponse time, but the traffic handling capability of a carrier is limited to avoidoverloading the carrier. In a permanent assignment protocol, the VSAT haspermanent access to a small portion of the satellite space segment capacity. In thiscase, the carrier rate limits the traffic a VSAT can carry. Furthermore, when thecarrier is not used by the VSAT to which it is assigned, the capacity is wasted.

There are three commonly used satellite access protocols that use a combination ofon-demand assignments, random and permanent, assignments to improve themultiple-access efficiency. These are Time Division Multiplex /Time Division MultipleAccess (TDM/TDMA), FTDMA, and Single Channel per Carrier Demand AssignmentMultiple Access (SCPC/DAMA). TDM/TDMA uses a permanent TDM carrier for theoutbound traffic to transmit information from the hub to the VSATs. Informationfor many different VSATs is time division multiplexed onto a single outbound carrier.Multiple outbound carriers can be used for larger sized networks. The VSATs useTDMA to access shared inbound carriers. As depicted in Figure 2.9a, TDM/TDMA isa combination of FDMA and TDMA.

SCPC/DAMA uses a single channel per carrier to convey traffic. (See Figure 2.9b.)When traffic exists, carriers are assigned in pairs, one from the hub to the VSAT andanother from the VSAT to the hub, for the return channel.

TDM/TDMA and SCPC/DAMA handle voice and data with different efficiency. Bothcan operate with permanent or on-demand assignment, but only TDM/TDMA canaccess the satellite randomly.

FREQUENCY

POWER

TIME

INBOUND CARRIERS

INBOUND CARRIERS

OUTBOUNDCARRIER

FREQUENCY

POWER

TIME

INBOUNDCARRIERS

INBOUNDBURSTS

#3

#4

#2

A) TDM / TDMA CARRIERS B) SCPC / DAMA CARRIERS

Figure 2.9: Typical Multiple-Access Protocols from the Satellite Network AccessPerspective

2.1.4.4 TDM/TDMA Networks


TDM/TDMA protocols are very efficient and are used mostly in interactive dataapplications. Before data can be transported with these protocols, the data must bepacketized. Each packet contains an address that identifies a data terminal withinthe VSAT network domain. A receiver, either the VSAT or the hub, acknowledgessuccessful receipt of any packet. If noise, a collision or other impairment corrupts apacket, it will prevent the packet from reaching its destination. In this case, thereceiver will not send an acknowledgment (ACK), and the same packet will beretransmitted after a random time delay. The ACK mechanism ensures properdelivery and simplifies the data transport.

Hub-to-VSAT link: The outbound link is a single carrier, and is the result ofmultiplexing all the packets from different customers and directing them to thevarious VSATs in the network. The multiplexing is achieved at the Front-EndProcessor (FEP), which is connected to the customer’s host computers. Each VSATlistens to the entire traffic carried by the outbound carrier. However, each VSAT willonly decode those packets containing control information or traffic packetsaddressed to one of its terrestrial interfaces.

VSATs-to-hub link: Depending on the size of the network, there will be one orseveral inbound carriers. The inbound carriers convey traffic from the VSAT to thehub. If a VSAT needs to communicate with a peer, it will transmit to the hub thatwill relay the packet to the other VSAT on a second satellite hop.Inbound-access protocols: In a TDM/TDMA network, the access protocols areimplemented in the inbound link from the VSAT to the hub. The protocols mostcommonly used are known as “random” or “contention” protocols. The protocol israndom because no central control determines which VSAT will transmit. This lackof central control lets the inbound capacity open for contention among the VSATsin the network. Each VSAT transmits data as packets at random times and contendswith peers for capacity on the inbound carriers. The typical contention protocols areALOHA, Slotted ALOHA, Selective Reject ALOHA, and Demand Assignment TDMAwith slotted ALOHA reservation access. Further detailed information are provided inthe Intelsat published VSAT handbook.

2.1.4.5 SCPC/DAMA Networks

Demand Assignment Multiple Access (DAMA) is an access protocol that allows eachchannel to use one carrier pair in a Single Channel Per Carrier (SCPC) mode toestablish a link. (Refer to Figure 2.13) These networks are used primarily for voicecircuits.

An SCPC/DAMA network is composed of three blocks:


• Network Management and Control (NM&C);• Traffic terminal at the hub; and• Traffic terminal at the VSAT.

The NM&C is responsible for controlling the network operations, assigning thesatellite resources for each circuit, downloading channel configuration via controlchannels (CCs), and recording call records for billing.

The process of handling calls is as follows:• When a voice channel requests a circuit, by seizing a line, the VSAT willinform the hub of its identity and the dialed digits. The DAMA Network Controller(NCC) knows the origin and identifies the destination via the dialed digits. If thedestination circuits are busy, the NCC instructs the originator to produce the busytone. If the destination is not busy, the NCC provides the origin and destinationchannel units with the operating uplink and downlink frequencies. Once thechannel units (CUs) tune to the assigned frequencies, the circuit is ready. The dialeddigits are relayed over the satellite circuit to the PSTN at the destination CU for callcompletion. Upon termination of the call, the NCC is informed, the DAMA carriersare turned off, and the CUs return to an idle state to wait for a new call. Thesatellite frequencies return to a common pool of frequency for future use.

• DAMA can operate in either star or mesh topology. Once the connection isestablished, the CUs carry the traffic through the assigned traffic carriers withoutintervention from the NCC. All DAMA channel units in a terminal share a commonRF electronics and antenna facility.

NCC

FREQ.ASSIGNMENT

NCCCALL

REQUEST

ACK

NCC

DIRECTCOMMUNICATION

CALL REQUEST

CALL ASSIGNMENT

COMMUNICATIONS

Figure 2.13: Operation of SCPC/DAMA Protocol.

Additional features in a SCPC/DAMA protocol are:


• Voice compression. To minimize the bandwidth requirements, SCPC/DAMAsystems use voice compression. The modern compression algorithms operate at lowrates (4.8 to 9.6 kbps) while maintaining a good voice quality. Compression rates at4.8 kbps to 16 kbps per channel are available and provide bandwidth savings.

• Voice activation. SCPC/DAMA systems employ Voice Activation (VOX),which turns the carrier off during pauses of a conversation. VOX reduces therequired satellite power. In pools of 100 channels or more, VOX provides a netreduction of satellite power utilization of up to 2.2 dB.

• On-demand data channels. If required, DAMA can offer clear channels at 64kbps or higher on-demand. These clear channels can be used for data applications.

The user’s interface emulates the user protocols and locally terminates the user’sprotocol. This is the performance of the data throughput over satellite.

2.1.4.6 FTDMA/DAMA

FTDMA combines the benefits of FDMA and TDMA, especially for thin route,homogenous networks, where most of the remote stations are similar in terms ofcircuit capacity; this capacity being in the range of 4 to 16 circuits per single indoorunit.

FTDMA employs a number of FDMA carriers, each comprising an N slot TDMAchannel, frame synchronized to a network wide reference, derived from a broadcastoutbound channel. The network channel pool may, therefore, be described as atwo-dimensional time-frequency matrix, with every cell representing a time slot onone of the carriers, carrying one half of a full duplex circuit. The station modemsoperate at the bit rate of a single carrier, i.e. carry N channels each, N being equalto the TDMA multiplexing factor. During each slot, the modem accesses a differentFDMA carrier for the duration of the burst, and must therefore, be capable of fastfrequency switching between adjacent slots. The control channels are multiplexedonto the TDMA carriers, occupying a fixed set of slots.

FTDMA requires a single burst modem per station or per group of circuits in astation. The value of N is configurable at network set up time, and should typicallybe made just large enough so that the majority of stations in the network requiresingle modem. A reasonable multiplexing factor may be 8, yielding a net channel bitrate of 64 or 128 Kbps. For a station configuration comprising of a single modem,the HPA may be operated close to saturation without compromising either spurioussignal emission or power efficiency.

In order to be optimally efficient, the demand assignment procedure for thenetwork is required to handle channel allocation in two dimensions, i.e. in bothfrequency and time. Certain constraints are imposed on the DAMA procedure, in


terms of control channel assignment (e.g. a station modulator will not be able touse slot i if the inbound control channel occupies slot i), as well as circuitassignment (a certain station modulator will not be able to access more than onecircuit on slot j); assuming a Time Division Multiple Multiplexing factor of 8, theinefficiency brought about by these constraints is more than few percent in mostscenarios.

The major issue in FTDMA is clocking. An arrangement must be provided by whichnetworkwide synchronization is maintained between the various stations frameclocks as well as symbol clocks. On the transmit side, the station modulator shouldbase the transmit clocks on timing information derived from the control channelburst synchronization pattern. The receive clocks may be derived from the samesource, adjusting the source as required.

The major concern when using FTDMA for voice is delay. The delay involved may beestimated in the following manner: each transmission burst entails a certain amountof overhead, which is independent of the burst length. In order to keep the relativeoverhead reasonable, the burst should not be too short: for this case, a burst lengthof around 1,500 bits is reasonable. With careful buffering arrangements, theassociated delay can be kept to a single frame length, or at 16 Kbps, to less than100ms added to the 250ms one-way propagation delay and to other, smaller delaysassociated with the voice codec, etc., the total delay is smaller than the ITU-Trecommended maximum of 400ms.

The relatively high bit rate of the FTDMA carriers allows the use of QPSK modulationfor the burst modems, as opposed to FDMA/SCPC systems. The fact, however, isthat the HPA should be desirably driven as close to saturation as possible may makecontrolling of the spectral shape of the signal difficult and is taken intoconsideration.

2.1.4.7 Performance comparison between various access protocols

Maximizing throughput and minimizing delay are important characteristics in theselection of a network multiple-access protocol. Modern networks incorporate allthe protocols discussed to ensure that the most suitable technique is available foreach end user. This functionality allows the network to create Closed Users Groups(CUGs), where each group can use a different application and protocol withoutinterfering with the rest of the network. Table 2.1 presents a performance summaryfor the four protocols already discussed.

Table 2.1. Performance Comparison of Protocol Access Techniques.

TECHNIQUE MAX. TYPICAL APPLICATION REMARKS


THROUGHPUT DELAYALOHA 13 ~18% <0.5 sec. Variable

Lengthmessages.

Timing notrequired.

S-aloha 25 ~ 36% <0.5 sec. Fixed lengthmessages.

SREJ-ALOHA

20 ~ 30% <0.5 sec. VariableLengthmessages.

Capacitycompetitive with S-ALOHA.

DA-TDMA 60 ~ 80% <2 sec. VariableLengthmessages.

Generally attractivefor long messages(batch data, voice).

SCPCDAMA

50 ~75% <2sec

Fixed lengthmessage

Generally ideal forthin route traffic.Throughputdepends on thekind of servicebeing supported.

FTDMA 90~100%<2sec

Variablelengthmessage

2.2 WLL Technology

Wireless Local Loop is the provision of fixed telecommunications services usingterrestrial wireless technologies to connect subscribers to local exchanges. Examplesof wireless telecommunications solutions include cellular systems using GSM orCDMA based standards or wireless access systems such as DECT or PHS also knownas cordless systems. Cordless systems were originally designed to provide fixedtelecommunications services in high-density urban areas featuring small cell sitecoverage (<1 km). Both technologies have now developed into fixed wirelesstechnologies for all areas, urban and rural, and offering a wide range of fixedwireless services: voice, fax and data (IP applications).


The WLL solutions presented in this handbook are primarily narrowband solutionsproviding data rate up to 64 Kbps. Recent WLL developments in the upper portionof the spectrum (20-49 GHz) features broadband services to serve primarily carriersor business applications. Also the upcoming third generation cellular system willfeature 2 Mbps fixed services (See Table 2.2 below).

Table 2.2: Wireless Local Loop applications

Data Rates SpectrumUtilized

GeographicalFocus

SubstituteServices

CustomerFocus

Broad-band

FractionalT-1T-1T-3

24 GHz38 GHz28 GHzLMDS

MetropolitanDenseBusiness

T-1T-3FrameRelay/ATM

CarriersLarge andMulti-locationbusiness

Narrowband

Up to 64kbps

800, 900MHz1.8,1.9GHz2.3 GHz

ResidentialRural

WiredTelephony

Residentialand small business

2.2.1 WLL Hardware

The diagram in figure 2.15 shows a typical implementation of a Wireless Local Loopnetwork and its key network interfaces from the subscriber location out to the localPublic Switched Network. The heart of the entire WLL is the interface at point A;this is the link where the WLL eliminates the copper loop. Therefore, it must betransparent to the end user and it must also provide a connection to the centraloffice that's as reliable and clear as the copper version. The components thatestablish this wireless connection at interface "A" are the wireless subscriber unit(WSU), and the Base station transceiver system, or BTS. The WSU located in thehome or phone booth, is essentially a radio modem in a box that provides the RFinterfaces to the existing phone or modem usually via a RJ-11 plug. Some WLLequipment manufacturers combine the phone and the WSU into one integratedproduct. The companion to the WSUs is the BTS that is located in the field. As it iscommon in most wireless systems, the requirement for the BTS is to serve as theControl Station for the WSUs by providing over the designed pilot channel,synchronization and control information necessary to initiate and maintain two-waycommunication. In time division multiple access communication systems, which willbe discussed later, the BTS assigns the channel frequency and the specific time slotsto each of the WSUs in its coverage area.


Finally, all communications traffic from the BTS units are routed to the PSTN via aNetwork Interface Unit (NIU) or access controller which is shown as interface "B" inFigure 2.15 This connection between the BTS and the NIU can be made via wireline,fiber, microwave, or by satellite link, depending on the bandwidth, distance, and onthe environment in which the network is installed. For example, for rural areaswhich are usually isolated from the PSTN, long distances, a body of water ormountainous terrain, a satellite point-to-point or multipoint communications linkback to the NIU is usually the most cost effective and reliable means. In other ruralareas that are not as geographically challenged and not far from the PSTN, it maybe more cost effective to simply use a microwave point-to-point communicationslink or a wired T1/E1 connection.

Public SwitchedTelephone Network

Home or Office

NetworkInterface Unit

9/02/2581_Fig2_2

Microwave

T-1 / E-1

Fiber

B A

WirelessSubscriberUnit (WSU)

BasestationTranceiver

System

PSTN

Fax / Modem

Figure 2.15: Typical configuration of WLL

2.2.2 WLL network topology

WLL solutions are differentiated using not only air interface standards but also cellsite coverage. Various coverage options are available: macro-cell, micro-cell andhybrid mixture of macro and micro-cells. Macro-cellular site covers areas with atleast 5 to 10 km (typically up to 30 km) with antenna radiating more than 500mWatt. Conversely, micro-cellular site covers a region with a diameter of less than 5km with antennas radiating power of less than 250 mWatt. Traditionally, cellularsystems using GSM and CDMA have been featured to offer primarily macro-cellularcoverage, although recently a number of vendors have decided to offer bothoptions to provide macro and micro cell coverage using cellular standards. On theother hand, cordless standards are primarily offering micro-cellular coverage withlimited radiated power as shown in Table 2.3.


A macro-cell is what most people have come to know or expect to see for acommunications site for both cellular and WLL operations for rural areas. However,practical experience in the rural market has shown that many applications in theworld are characterized as having isolated pockets of subscribers associated with anumber of geographical constraints. In this vein, a macro system is sometimesdeemed to be unsuitable, in that, it is often not possible to uniformly cover a 30kmradius, implying that pockets of villages could go unserved. To solve this problem, amicro system is often cost-effective where small cells can be duplicated to makesure that all the areas are adequately served. Meanwhile, macro-cells can be used incertain areas with flat terrain, high population density and villages located close toeach other within 30km radius. Therefore, depending on the rural topography,population density, and required number of lines, an operator can configure a WLLnetwork accordingly to suit the traffic requirements.

2.2.3 Review of Wireless Standards

One of the most important concepts to any cellular telephone system is that of"multiple access", meaning that multiple, simultaneous users can be supported. Inother words, a large number of users share a common pool of radio channels andany user can gain access to any channel (each user is not always assigned to thesame channel). A channel can be thought of as merely a portion of the limited radioresource, which is temporarily allocated for a specific purpose, such as someone'sphone call. A multiple access method is a definition of how the radio spectrum isdivided into channels and how channels are allocated to the many users of thesystem.


Table 2.3: Common Wireless Air Interface standard Macro-cellular

Micro-cellular

GSM900 IS-136 IS-95DCS1800 (USPCS) USPCS/ PHS PACS DECTPCS1900 W-CDMA

Frequency 890-915 MHz(T)

Spectrum 935-960 MHz(R)

824-849 MHz(T)

824-849 MHz(T)

1895-1918MHz

1850-1910MHz (T)

1850-1920MHz

1710-1785MHz (T)

869-894 MHz(R)

869-894 MHz(R)

1930-1990MHz (R)

1880-1938MHz

1805-1880MHz (R)

1850-1910MHz (T)

1850-1910MHz (T)

1850-1910MHz (T)

1930-1990MHz (R)

1930-1990MHz (R)

1930-1990MHz (R)

Multiple TDM CDMAAccess TDMA TDM.CDMA TDM.CDMA TDMA TDMA TDMADuplexing FDD FDD FDD TDD FDD TDDChannel 200KHz 30KHz 1.25MHz/ 300KHz 300KHz 1728KHzSpacing 5MHzModulation

3,14/4 QPSK 3,14/4 3,14/4

GMSK DQPSK O-QPSK DQPSK DQPSK GFSKChannel 270kbps 48.6kbps 1.23Mbps 384kbps 384kbps 1152kbpsBit RateSpeech VSELP/ 8,13kbpsCoding RPE-LTP ACELP CELPT/ 32kbps 32kbps 32kbps

13kbps 8kbps 32kbps ADPCM ADPCM ADPCMADPCM

Tx Power 1W 0.6W 0.5/0.2W 80mW 200mW 250mW

2.2.3.1 Analog standards

Different types of cellular systems employ various methods of multiple access. Thetraditional analog cellular systems, such as those based on the Advanced MobilePhone Service (AMPS) and Total Access Communications System (TACS) standards,use Frequency Division Multiple Access (FDMA). FDMA channels are defined by arange of radio frequencies usually expressed in a number of kilohertz (kHz), out ofthe radio spectrum.


For example, AMPS systems use 30 kHz "slices" of spectrum for each channel.Narrowband AMPS (NAMPS) requires only 10 kHz per channel. TACS channels are25 kHz wide. With FDMA, only one subscriber at a time is assigned to a channel. Noother conversations can access this channel until the subscriber's call is finished, oruntil that original call is handed off to a different channel by the system.

The AMPS family of wireless standards were intended to be just another analogradiotelephone standard (e.g. Advanced Mobile Phone Service followed IMTS;Improved Mobile Telephone Service). However, due to the high capacity allowed bythe cellular concept, its lower power enabling portable operation and its robustdesign, AMPS has been a success. Today, approximately 20% of the cellular phonesin the world operate according to AMPS standards, which, since 1988, have beenmaintained and developed by the Telecommunications Industry Association (TIA).From its humble beginnings, AMPS has grown to accommodate TDMA and CDMAbased digital technology, narrowband analog operation (NAMPS), in-building andresidential modifications, and most recently, operation in the 1800-2000 MHz.

Given its wide availability resulting from serving high mobility markets, there issignificant momentum to use analog cellular for WLL. As a WLL platform, analogcellular has some limitations in regards to capacity and functionality. Due towidespread deployment, analog cellular systems are expected to be considered as awireless platform for WLL, at least in the short term. Given its characteristics, analogcellular is best suited to serve low-density to medium-density markets that don'trequire landline-type features.

2.2.3.2 Digital standards

2.2.3.2.1 TDMA

A common multiple access method employed in new digital cellular systems is theTime Division Multiple Access (TDMA). TDMA digital standards include NorthAmerican Digital Cellular (know by its standard number IS-54), Global System forMobile Communications (GSM), and Personal Digital Cellular (PDC).

TDMA systems commonly start with a slice of spectrum referred to as one "carrier".Each carrier is then divided into time slots. Only one subscriber at a time is assignedto each time slot, or channel. No other conversations can access this channel untilthe subscriber's call is finished, or until that original call is handed off to a differentchannel by the system.

For example, IS-54 systems, designed to coexist with AMPS systems, divide 30 kHzof spectrum into three channels. PDC divides 25 kHz slices of spectrum into threechannels. GSM systems create 8 time-division channels in 200 kHz wide carriers.The first implementation of AMPS digital cellular used TDMA, in the TIA IS-54


standard. This requires digitizing voice, compressing it and transmitting it in regularbursts. Following IS-54, which provided a TDMA voice channel, IS-136 the nextgeneration, also uses TDMA on the control channel. TDMA, as defined in IS-54 andIS-136, triples capacity of cellular frequencies, by dividing a 30 kHz cellular channelinto 3 timeslots, which supports 3 users in strict alternation. Future systems mayalso utilize half-rate voice coders, which will allow 6 users in one 30 kHz channel.GSM, Global system for mobile communications, the European 900 MHz digitalcellular system, has also expanded to many parts of the world and into the PCSband (where it is known as DCS1800 or PCS1900). These systems have seen rapidgrowth and are expected to outpace analog cellular over the next few years. GSMdominates the digital cellular market with approximately 71% of subscribers.

Digital cellular is expected to play an important role in providing WLL. Like analogcellular, digital cellular has the benefit of wide availability. Digital cellular cansupport higher capacity subscribers than analog cellular, and it offers functionalitythat is better suited to emulate capabilities of advanced wireline networks. Itsdisadvantage is that it is not as scalable as analog cellular. It is forecasted thatapproximately one-third of the installed wireless local loops will use digital cellulartechnology in the year 2000.

Although GSM currently dominates mobile digital cellular, there has been littleactivity in using GSM as a WLL platform. Since GSM's architecture was designed tohandle international roaming, it carries a large amount of overhead that makes itunwieldy and costly for WLL applications. In spite of these limitations, it is likely thatGSM WLL products will be developed over the next few years. The basicarchitecture is shown in Fig. 2.16.


Figure 2.16: GSM-based wireless access system architecture

2.2.3.2.2 CDMA

Though CDMA's application in cellular telephony is relatively new, it is not a newtechnology. CDMA has been used in many military applications, such as anti-jamming (because of the spread signal, it is difficult to jam or interfere with aCDMA signal), ranging (measuring the distance of the transmission to know when itwill be received), and secure communications (the spread spectrum signal is veryhard to detect).

With CDMA, unique digital codes, rather than separate RF frequencies or channels,are used to differentiate subscribers. The codes are shared by both the mobilestation (cellular phone) and the base station, and are called "pseudo-Random CodeSequences." All users share the same range of radio spectrum.

For cellular telephony, CDMA is a digital multiple access technique specified by theTelecommunications Industry Association (TIA) as "IS-95." In March 1992, the TIAestablished the TR-45.5 subcommittee with the charter of developing a spread-spectrum digital cellular standard. In July of 1993, the TIA gave its approval of theCDMA IS-95 standard.


IS-95 systems divide the radio spectrum into carriers which are 1,250 kHz (1.25MHz) wide. One of the unique aspects of CDMA is that while there are certainlylimits to the number of phone calls that can be handled by a carrier, this is not afixed number. Rather, the capacity of the system will be dependent on a number ofdifferent factors. CDMA is a "spread spectrum" technology, which means that itspreads the information contained in a particular signal of interest over a muchgreater bandwidth than the original signal. A CDMA call starts with a standard rateof 9600 bits per second (9.6 kilobits per second). This is then spread to atransmitted rate of about 1.23 Megabits per second. Spreading means that digitalcodes are applied to the data bits associated with users in a cell. These data bits aretransmitted along with the signals of all the other users in that cell. When the signalis received, the codes are removed from the desired signal, separating the users andreturning the call to a rate of 9600 bps.

In the final stages of the encoding of the radio link from the base station to themobile, CDMA adds a special "pseudo-random code" to the signal that repeatsitself after a finite amount of time. Base stations in the system distinguishthemselves from each other by transmitting different portions of the code at a giventime. In other words, the base stations transmit time-offset versions of the samepseudo-random code. In order to assure that the time offsets used remain uniquefrom each other, CDMA stations must remain synchronized to a common timereference. The Global Positioning System (GPS) provides this precise common timereference. GPS is a satellite based, radio navigation system capable of providing apractical and affordable means of determining continuous position, velocity, andtime to an unlimited number of users.

CDMA appears to be a standard well suited for WLL applications. (See Figure 2.17for the IS-95 CDMA -based system architecture). CDMA employs a spread spectrummodulation technique in which a wide range of frequency is used for transmissionand the system's low-power signal is spread across wide-frequency bands. It offershigher capacity than the other digital standards (10 to 15 times greater than analogcellular), relatively high-quality voice, and a high level of privacy. The maindisadvantage of CDMA is that it is just beginning to be deployed on a wide scale.


PSTN ( Local, Long Distance,

and International Calling)

Tandem Switch

Other BSC or wireline Switch

BSC

Switch LinksMicrowave or Fiber Backhaul

BTS

BTS

BTS BTS

BTS

BTS

BTS

BTS

Rural

Suburban

Figure 2.17: CDMA Wireless Local Loop Network Architecture

2.2.3.2.3 Cordless Telephones/Digital European Cordless (DECT)

The Digital Enhanced Cordless Telecommunications (DECT) standard provides ageneral radio access technology for wireless telecommunications, operating in thepreferred 1880 to 1938 MHz band using GFSK (BT = 0.5) modulation.

DECT has been designed to provide access to any type of telecommunicationnetwork thus supporting numerous different applications and services. The range ofDECT applications includes residential, PSTN and ISDN access, wireless PABX, GSMaccess, Wireless Local Loop, Cordless Terminal Mobility CTM, Local Area Networkaccess supporting voice telephony, fax, mode, E-mail, Internet, X.25 and manyother services in a cost efficient manner.

A DECT system comprises a DECT Fixed Part (FP), utilizing one or more base station(RFPs), and one or more portable parts (PPs). There is no limit to the size of theinfrastructure as far as the number of base stations and cordless terminals areconcerned. Infrastructures using the DECT technology can support traffic densities


up to 10,000 Erlangs/km2 , which is comparable to 100,000 users in officeenvironments.

In principle the DECT base standard (of which all parts are shown in Table 2.4) onlycovers the "air interface" between a DECT Fixed Part (FP) and a DECT Portable Part(PP), it provides toolbox with protocols and messages from which selections can bemade (profiles) to access any specific type of network. In addition to cordlessness,DECT makes available the network specific services and features (including mobility)to the user through the DECT common air interface transparently.

Table 2.4: Parts 1 to 8 of the DECT CI standard ETS 300 175Part Title Description1 Overview General introduction to the other parts of ETS 300 1752 Physical layer Radio requirements of DECT, e.g. carrier frequency allocation,

modulation method, transmission frame structure,transmitted power limits, spurious emission requirements etc.

3 Medium AccessControl layer

Description of procedures, messages, and protocols for radioresource management i.e. link set-up, channel selection,handover, link release and link quality maintenance etc.

4 Data link controllayer

Description of provisions to secure a reliable data link to thenetwork layer

5 Network layer Description of the signaling layer with call control andmobility management functions and protocols

6 Identities andaddressing

Description of the portable and fixed part identitiesrequirements for all DECT application environments

7 Security Aspects Procedures to prevent eavesdropping, unauthorized accessand fraudulent use

8 Telephony Telephony requirements for system supporting the 3.1 kHzspeech service to ensure proper interworking with publictelecommunications networks. Defines transmission levels,loudness ratings, sidetone levels, frequency response, echocontrol requirements etc.

Its Multi-Carrier, Time Division Multiple Access, Time Division Duplex(MC/TDMA/TDD) radio access method and continuous Dynamic Channel Selectionand Allocation capability enable high capacity, pico-cellular systems, being utilizedeven in busy or hostile radio environments. These methods enable DECT to offerexcellent quality of service. DECT makes efficient use of the assigned radiospectrum, even when multiple operators and applications share the same frequencyspectrum.

Standardized profiles have been defined for e.g. Generic Access (GAP; which ismandatory as a minimum requirement for all DECT voice telephony equipment asfrom October 1997), ISDN and GSM interworking (GIP). Standard profiles


encourage DECT equipment manufacturers to implement interworking with thenetwork in a harmonized way. This creates interoperability between DECTequipment from different manufacturers and directs competition towardsdifferentiation on non-technological features, providing consumers, and thenetwork operators with the luxury to choose from the variety of standard products.Standardization of interworking also allows for mass production of systemcomponents, which in turn provides significant costs benefits enabling highlyattractive price/performance ratios for DECT equipment. Figure 2.18 is a schematicdiagram of a DECT system.

PSTN (Local, Long Distance,

and International Calling)

Tandem Switch

Other Switches

Switch LinksMicrowave or Fiber Backhaul

Suburban

RCN

DAN DAN DAN

DAN

DAN

DAN

RCN DAN

DAN

DAN

DAN

DAN DA

DAN RCN DANDAN

DAN

DAN RCN DANDAN

DAN RCN DANDAN

DAN RCN DANDAN

DAN RCN DANDAN

RCN

RCN

RCN

RCN

RCN

RCN

RCN

RCN

RCN

RCN

RCN

RCN

RCN

RCN

Switch

Figure 2.18: DECT Architecture

The MC/TDMA/TDD principle

The DECT radio interface is based on the Multi Carrier, Time DivisionMultiple Access, Time Division Duplex (MC/TDMA/TDD) radio access methodology.Basic DECT frequency allocation uses 10 carrier frequencies (MC) in the 1880 to1938 MHz range. The time spectrum for DECT is subdivided into time framesrepeated every 10 ms. Each frame consists of 24 time slots each individuallyaccessible (TDMA) that may be used for either transmission or reception. For the


basic DECT speech service two time slots - with 5 ms separation - are paired toprovide bearer capacity for typically 32 kbps (ADPCM G.726 coded speech) fullduplex connections. To simplify implementations for basic DECT the 10 ms timeframe has been split in two halves (TDD), where the first 12 timeslots are used forFP transmissions (downlink) and the other 12 are used for PP transmissions (uplink).

The TDMA structure allows up to 12 simultaneous basic DECT (full duplex)voice connections per transceiver providing significant cost benefit when comparedwith technologies that can only have one link per transceiver (e.g. CT-2). Due to theadvanced radio protocol, DECT is able to offer widely varying bandwidth bycombining multiple channels into a single bearer. For data transmission purposeserror protected net throughput rates of n x 24 kbps can be achieved, up to amaximum of 552 kbps with full security as applied by the basic DECT standard.

2.2.3.2.4 PHS

The PHS is a Japanese standard for use in the 1895 to 1918 MHz frequency band. Ithas proved highly successful in telepoint applications in Japan, where the subscriberdensity is high and is generating much interest in the Asia-Pacific countries.

The PHS-WLL supports standard V5.2 open interfaces between any Access NetworkEquipment and Exchanges, giving vendor independence benefits. See Table 2.6below for key parameters of PHS.

Future evolution of PHS and PHS-WLL systems may include:

• Data services 32kbit/s;• Multi-slot operation: 64kbit/s and higher data rates e.g. 144kbit/s;• Support for multiple lines;• Higher power BTS, improved Rx sensitivity, use of interference cancellationtechniques and intelligent antennas; and• Integration of PHS-WLL and PHS-mobile for limited mobility or WLL/ cordlessapplications.

Table 2.5: Key Parameters of PHS

ServicesTelephony YesISDN NoFax YesData Up to 64 kbpsVideophone YesSupplementary services Good


Multiple lines NoPerformanceRange (radius) 5 kmCells per 100 km2 1.3Capacity per cell, 2x1 MHz 8

2.2.3.2.5 CT-2

CT-2 initially was developed in the United Kingdom as an alternative to analogcordless home phones. Before its development was complete, however, a numberof operators suggested that it should be deployed for telepoint. The subsequentfailure of the telepoint networks in most countries has tended to brand CT-2 as afailed standard. In practice CT-2 equipment is still manufactured and used forcordless applications around the world and has been suggested for WLL. Figure2.19 is the schematic diagram of CT-2 Architecture.

CT-2 operates in the 864.1 to 868.1 MHz band using FDMA with channelbandwidths of 100 kHz. Within the FDMA channel, TDD and DCA are used, as withDECT. Each channel can carry 32 kbps in both directions, providing ADPCM voicecoding. CT-2 shares the advantages of diversity gain that DECT achieves with TDD.However, because it cannot concatenate time slots, it does not have the bandwidthflexibility of DECT. It is that limited flexibility that generally makes CT-2 lessappropriate than DECT for WLL use. Table 2.7 lists the key parameters of CT-2.

Table 2.6: Key Parameters of CT-2

ServicesTelephony Yes, good voice qualityISDN NoFax NoData Up to 32 kbpsVideophone NoSupplementary services LimitedMultiple lines NoPerformanceRange (radius) 5 kmCells per 100 km2 1.3Capacity per cell, 2x1 MHz 7


Figure 2.19: CT-2 based wireless access system architecture

2.3 Choosing the appropriate WLL Technology

After considering a wide range of WLL systems, its appropriate at this stageto provide guidance to operators on how to select the most appropriate technologythat can match their requirements. Most operators will not be able to select fromthe full range of WLL technologies. Spectrum/frequency availability will immediatelylimit the range of WLL technologies that can be adopted, making the choiceconsiderably simpler. Other factors such as data transmission requirements will havea bearing on the capacity required, cost, and services that can be offered. Once allthese factors have been taken into account, the choice of technologies is likely to besignificantly reduced.

2.3.1 Coverage comparison

Comparing network cost requires the understanding of the range and capacity ofthe different systems. The number of base stations for coverage and for capacitycan be calculated as described in chapter 6. Table 2.7 compares the relative rangesof the different systems.

Table 2.7: A Comparison of System Ranges

Technology Range (km)DECT 5PHS 5


CT-2 5TACS 35GSM 30IS-95 30Nortel* 15Tadiran* 6DSC* 5Lucent* 4

*Proprietary technologies

In the cost calculation, a key parameter is the number of base stations required andthe cost per base station. As will be shown in Chapter 6, the number of basestations tends to drive the sizing of all the elements in the system. Given that, withthe exception of the cellular technologies, the WLL technologies have similar ranges,and the number of base stations will tend to be driven by whether additional cellsare required, over and above those required for coverage, to provide adequatecapacity. On average, the price of cordless base stations with limited power andlimited range is significantly lower than the price associated with a cellular solution(by a factor of 2 to 4).

2.3.2 Capacity comparison

Table 2.8 lists the number of voice channels that can be provided in a cell for eachof the different systems.

Table 2.8: A Comparison of System Capacity per cell.(Sectorization not included)

Technology Voice Channels/ 2x1MHz/CellDECT 5.2PHS 8CT-2 7TACS 3.3GSM 10CDMA (IS-95) 12Nortel* 10Tadiran* 8DSC* 8.5Lucent* 5.75

*Proprietary technologies

The information in Table 2.8 needs to be treated with caution. Some technologies,such as DECT or PHS, provide relatively few channels per cell but have inexpensive


and easy-to-deploy base stations that can be used with a short range. By usingmore base stations, a DECT network potentially could provide higher capacity thana system such as DSC or Tadiran at an equivalent cost. Also sectorization of a cellsite using directional antennas may multiply its capacity by 3 or 6.In general, the highest capacities per cell are provided by the proprietarytechnologies and the lowest by cordless technologies. That is a deliberate designdecision, because cordless is intended to be inexpensive to deploy hence its simpleand low cost but low-capacity base stations.

2.3.3 Functionality comparison

Table 2.10 compares the functionalities provided by the different systems. The tableshows that only cordless and the proprietary technologies could be classed asproviding a full range of services.

Table 2.9 Comparison of System FunctionalitiesTechnology ISDN Data VoiceDECT Yes 64 Kbps ADPCMPHS Yes 64 Kbps ADPCMCT-2 No 32 Kbps PCMTACS No No PCMGSM** No 9.6 Kbps 13.3 KbpsCDMA(IS-95)** No 9.6 kbps 13.3 KbpsNortel* Future release 64 Kbps 13.3 KbpsTadiran* Future release 32 Kbps 13.3 KbpsDSC* Yes 256 Kbps 13.3 KbpsLucent* Yes 128 Kbps 13.3 Kbps

*Proprietary technologies * *2nd generation cellular only. Third generation will provide data capability

up to 2 Mbps.

Selecting the most appropriate technology is also a process of understandingrequirements and using them as filters to derive a short list of technologies. Some ofthe following are the key filters:

• Available frequency bands;• Voice quality demanded; and• Data requirements (data rate, protocols).

Table 2.11 below shows which technologies can be adopted, depending on thefrequency band available.

Table 2.10: Equipment available in the different Frequency bandsFrequency Band Technology


800/900 MHz Analog cellular, GSM, IS-95, CT-21.5 GHz Tadiran and some other proprietary systems1.7 - 2 GHz DECT, PHS, GSM, and IS-95 variants2-2.5 GHz DSC, Tadiran3.4 - 3.6 GHz Nortel, Tadiran, Lucent10 GHz Emerging technologies> 10 GHz MVDS technologies

3 Rural Models and Micro- versus Macro-Cell Architecture

The appropriate coverage area of the WLL cell obviously depends on the terrain andpopulation distribution over the terrain. The following provides examples of ruralmodels, classified into eight rural telephony regions, which are representative of therural environments on various continents. These include jungles, mountains, islands,and flat terrain. The different rural models are summarized in Table 3.1 below.


Table 3.1: Overview of Rural ModelsRegion Rural

Popu-lation

RequiredCell Range(km)

Distancebetweenvillages (km)

General remarks on typeof terrain

Brazil - 30 7.5 Villages and towns foundalong rivers. The region isrelatively flat butcharacterized with highfoliage, limiting the range forMacro solution. Microsolution suitable for thisregion.

Mexico-1 <2,500 5 10 Villages and towns isolatedby mountains. Micro solutionsuitable for this region.

Mexico-2 <500 5 10 Villages isolated withintowns resulting in smallclusters of population inisolated areas.

Indonesia <9,000 5-10 20 Mountains separate villagesand towns. From thesubscriber perspective, bothmicro and macro can servethese region solutions.

Philippines(LubangIsland)

<3,000 7 5 Towns and villages isolatedby large bodies of water andmountains. The presence ofmountains limits line ofsight, making it difficult touse macro systems. Microsolution suitable for thisregion.

NorthernIndia

<2,000 5 20 Villages and towns isolatedby mountains. Terrain limitcell size to not more than10km. Micro solutionsuitable.

Central/SouthIndia

<1,000 30 5 Terrain is flat and subscriberdensity is high. Suitable formacro solutions.

Sudan <20,000

20 10-20 The region is relatively flatand, in this regard, is suitablefor solution.


An examination of the unique demographic patterns and geographical features inthree Asian countries reveals how difficult and dangerous generalizations can be.For example, Indonesia has many relatively large villages (1,500 to 9,000inhabitants) where the potential subscribers are clustered within 5 to 8 km. Indiahas at least two models, North and Central/South. The North is mountainous andcharacterized by isolated towns and villages with population of about 2,000. TheCentral/South model is generally flat with towns and villages separated by 5km orless. Other Asian prototypes can be similarly characterized; however, these diversescenarios demonstrate the problem in attempting to construct a single homogenousnetwork model for basic rural services. In Africa, the Sudan model is similar to theCentral India model in terms of terrain but the population density is lower thanIndia.

Analyzing the data in Table 3.1 led to the following three main conclusions: (1) ruraltopology and distribution of subscribers is not governed by one particular model interms of total population, population density and required number of lines, (2) thesubscriber distribution is not uniform due to the establishment of village clusters,separated by mountains or rivers, and (3) the villages appear to be served in a moreefficient and economical way by a microcell architecture, i.e. several cells with thecoverage area around the base station of no more than 5 km. This is in contrast towhat is commonly called a macrocell, where the power is uniformly broadcast in alldirections over a range of around 30km. Examples of microcell standards includecordless standards such as DECT and PHS. Examples of standards providing macro-cellular coverage are GSM, TDMA (IS-136) and CDMA. (IS-95).

The use of macro-solutions in rural areas that are often isolated, due to terrain orother factors, makes the coverage perspective of WLL incomplete. That is while amacro-solution could perhaps connect to a majority of subscribers within a cell(defined as 30km radius), a significant number of potential subscribers will stillremain isolated and unserved. Even when there are no terrain limitations, thesubscriber density in most of the rural areas is generally not uniformly distributedthroughout the cell, implying that a good portion of the area covered by a macro-cell is in fact unused. Most of the villages are often clustered in small groups of upto ten or more villages within a reasonably small area (5 to 10km radius). The villageclusters could however be served by a micro WLL/VSAT system. The terminal wouldonly be installed in areas where needed, thus servicing only the related subscribers.The terminal provides service between remote subscribers/PCOs and the PSTN.

The major driving force behind the development of a micro WLL/VSAT architectureand technology selection is the requirement of designing a terminal of less thanUS$1500 per subscriber that can cheaply and easily be implemented in a ruralenvironment.


4 Integrated WLL-VSAT Micro Terminal

4.1 Choice of WLL and VSAT solution.

At this stage, a wide variety of VSAT and WLL technologies has been discussed. Thischapter describes in more details the possible integration process of the VSAT andWLL technologies. Benefits from the integration concept are definitely many,especially for rural areas, where the choice of communication equipment is mainlydriven by the cost of the equipment and maintenance. So far several WLL standardshave been considered to serve the rural end-user. Rural models have beendeveloped based on the analysis conducted on rural requirements as discussed inchapter 3. Several characteristics pointed towards the DECT standard as the choiceamong the micro-cell technologies. First of all, a DECT micro-cell base station with12 channel appeared well suited for the medium population ranges (20 to 200subscribers) that are prevalent in rural areas.

This was further supported by the analysis summarized in Figure 4.2, which showsthe number of WLL channels required as a function of the number of subscribersand traffic intensity per subscriber. The figure shows three curves representingdifferent Erlangs per subscriber, all assuming a 2% blocking probability. We knowfrom the rural characteristics that traffic intensity of 0.06 Erlangs per subscriber lineand 100 subscribers is fairly representative for this environment, which results in arequirement of 12 WLL channels per cell. It is the case of the DECT base stationwhich supports a single DECT carrier and up to twelve duplex voice channels.Secondly, the "non-compressed" DECT voice information rate (32 kbps ADPCM)helps maintain excellent voice quality over the satellite link. In addition, a DECTterminal, more so than terminals for macrocell solutions, has low complexity andpower consumption, and can therefore be designed for low cost, fast deploymentand also requires minimum maintenance. Finally, the low power requirementassociated with DECT is more suited for rural environment where the availablepower infrastructure is often minimal or non-existent.


9/02/2581-5_2

Figure 4.1 Number of WLL Channels Required for Micro Terminal

The choice of VSAT technology is determined by many of the same requirementsthat drove the choice of the WLL standard, as well as by the requirements that theend user pays a low recurring cost per minute for his calls. The relatively lowintensity of use per subscriber line points to DAMA (Demand Assigned MultipleAccess) as the optimal access technology for a start-up system, because calls areestablished on demand and space segment resources can be shared among manysubscribers. Although the majority of the Intelsat space segment Users indicatedthat the number of calls leaving the cell could vary between 20 and 80%, thechoice of the microcell approach implies that the percentage will be closer to 80%,because there are fewer correspondents within the cell.

4.2 What needs to be integrated?

DECT Wireless Access

Ku or C-BandVSAT Antenna

Switch(Local Exchange)

9/02/2581Fig6_2

VSAT-WLLTerminal Controller

VSAT Radio(Up / Down Convert,Power Amp, LNA)

ODU

BaseStation

Controller(BSC)

DAMA-SCPCIDU

BaseStation

(BS)

Figure 4.2: Conventional chain of non-integrated DECT-VSAT components.

As indicated in figure 4.2, which shows the conventional chain of non-integratedDECT-VSAT components, there are many elements that could potentially serve asthe basis for integration. From left to right are illustrated: the DECT Base station,


which is the radio interface, the Base Station Controller (BSC, which assigns theWLL channels, monitors the base stations and does all processing and faultmanagement), the switch or local exchange (since DECT typically does not performlocal switching as part of the base station or base station controller), and the VSATelements, in this case the DAMA indoor unit and the outdoor unit, comprising ofthe up and down-converters, high power amplifier, and LNA.

After the analysis was performed regarding the relative cost of the individualfunctions performed by various components, it was concluded that the followingmodifications were essential to achieve any substantial cost reduction:

• Reduction or elimination of the function of the Base Station Controller;• Simplification and cost reduction of the local switching within the WLL/VSATterminal, in order to avoid the use of the satellite power and bandwidth for localcalls;• Elimination of channel banks and any digital to analog signal conversion;and• Ruggedization of the equipment in order to avoid the need for a costlyshelter and simplify terminal installation.

Based on the above modifications, STM Wireless engineered the WLL/VSATInterface Unit, the heart of the integration effort, which incorporates the followingfunctions:

• Assignment of WLL channels, WLL/VSAT call control and processing;• Voice compression for the satellite link;• Management of DAMA control link with the centralized networkmanagement system; and• Power distribution to the DECT base station and the VSAT outdoor unit.

The projected cost savings over what is currently available when buying non-integrated off-the-shelf components are substantial, on the order of 40%. Figure4.3 shows the corresponding network architecture. It is estimated that the cost ofthe WSIU-based terminal on a subscriber basis will be below US$1,000 (for asubscriber population of 180 with low calling rate) and generally less thanUS$1,500 (for around 50 subscribers, with high calling rate). This cost estimatesinclude a subscriber terminal cost, as well as per subscriber network cost, whichencompasses shared costs for the gateway and network management system.These also include the shared costs of the DECT Base Station, the WSIU and theDAMA VSAT.

4.3 Integrated DECT-VSAT terminal description


The terminal consists of three main electronic units, all of which are fully ruggedizedto avoid the need for a costly shelter. See Figure 4-3.

WLL BaseAntenna

StandardHandset #1

9/02/2581Fig6_3

WSIU(Indoor

4-ChannelVersion)

SingleChannelDECTBase

Station(DBS)

DECT Handset #1

DECT Handset #2

Multi-ChannelDAMA

DAMA 10000NCT

StandardHandset #2

DAMA/WLLNMS

Figure 4-3 Integrated DECT-VSAT Terminal Configuration

The units are as follows:

• Single channel DECT base station (DBS)- supports a single DECT carrier andup to 12 duplex voice circuits;• WLL-VSAT Interface Unit (WSIU)- provides WLL-VSAT call control andswitching functions; includes voice compression processing for the SATCOM link.May include additional 2/4-Wire interfaces for direct connection to PCOs; and• Multi-channel Subscriber Earth Station (MC-SES) - STM's integrated SCPC-DAMA terminal provides 8 full duplex SCPC satellite channels and a C or Ku-Bandradio in the single package.

The WSIU is the heart of the terminal providing the connection between the DECTand VSAT components. The WSIU is also responsible for complete terminal control-including the processing, radio control and maintaining a control channel link withthe centralized satellite network management system (NMS). The WSIU terminalconnects to the DBS while simultaneously providing for connection to local PCOs.This provision for connecting PCOs to the WSIU is a very important feature in ruralenvironment, where PCOs are heavily used.

The WLL-VSAT terminal characteristics can be summarized as follows:

• Number of WLL channels : 12• Number of VSAT channels: 8• Traffic supported: 4.53 erlangs


5 The integrated WLL-VSAT network

The combination of WLL and VSAT technologies is a perfect marriage to addresslarger population densities, thus offering a larger number of subscriber lines over awider, although still concentrated, coverage area served by the terrestrial wirelessloop. With the combination of VSAT and WLL, some of the key advantages andfeatures derived from both wireless technologies remains such as speed of networkdeployment, faster access to revenues, network optimization and flexibility, lowinstallation and maintenance costs and suitability for rugged terrain. Intelsat activelypromotes such solutions to address the needs of operators wishing to offer remoteaccess to telephony, Internet, education and healthcare.

In addition to selecting the optimum WLL technology and the proper level ofhardware integration to meet the desired subscriber line costs, an effective WLL-VSAT system must function as a completely seamless network while efficientlyutilizing the available space segment resources. The attributes of a WLL/VSATnetwork that meet these above objectives include the following:

• An effective DAMA satellite architecture that provides the desired level ofservice and connectivity but minimizes the equipment as well as recurring satellitelease costs.• An end-to-end call control scheme that provides the transparent level ofservice for all types of inter-network and intra-network calls scenarios.• A method for centralized collection of call details records (CDR) for all calltypes and the support of metering the pulses to enable public calling office (PCO)services.• An integrated and centralized network management scheme that provides asufficient level of remote monitoring, diagnostics and fault monitoring foroperations and maintenance purposes.

Each of these attributes, which all necessary for a truly integrated network solution,will be detailed in the subsections to follow.

5.1 DAMA – The rural telephony architecture of choice

A general introduction of GEO satellite multiple access architecture was provided inparagraph 2.1.4. All these multiple access solutions can be applied in varyingdegrees depending on the applications and services required as stipulated inparagraph 2.1.3. The purpose of this section is to answer the following questions asthey pertain to the use of GEO satellite DAMA in solving the rural telephonyproblem, and more specifically, when connected to the WLL solution:

1. What are the rural network requirements?2. Is DAMA necessary?


What are the rural network requirements?This first question will be addressed by examining rural telephony solutions from thenetwork perspective. The typical rural telephony situation consists of the followingelements:

• Remote VSATs directly connected to subscribers (e.g. public calling offices,other independent subscribers);• Remote VSATs connected to a WLL subscriber base with some form oftrunking concentration performed by the WLL segment;• Gateway sites connected to the PSTN.

Based on the above, there are really two types of network requirements that a givenservice provider expects, or more specifically, demands.

Case 1 (see figure 5-1)

PSTN

PSTN

PSTN

Area Code 123

Area Code 456

Area Code 789

Gateway

aaa-bbbb

xxx-yyyy

Dials aaa-bbbb

Dials 789-xxx-yyyy

Network Control

PSTN Routing to Destination Area Code

9/02/2581_Fig1

Figure 5-1: Case 1 –No space segment Routing

Each of the subscribers in the network (VSAT or WLL subscriber) always connects tothe same gateway (GW) site. In this scenario, the PSTN, typically for regulatorypurposes, does not expect nor want the satellite segment to perform any form ofcall routing.

Case 2 (see figure 5-2)


PSTN

PSTN

PSTN

Area Code 123

Area Code 456

Area Code 789

Gateway

aaa-bbbb

xxx-yyyy

Dials aaa-bbbb

Dials 789-xxx-yyyy

Network Control

Bypasses Terrestrial Network

9/02/2581_Fig2

Figure 5-2: Case 2 – Space Segment Routing

Each subscriber, based on the called party’s destination phone number, is routed tothe nearest GW site of the called party. In this case, the routing performed by thesatellite segment reduces the load on the PSTN infrastructure.

Regardless of which type of requirement apply to a given service provider, thenetwork should support a multi-star configuration if the rural telephony satellitenetwork is to be suitable for all applications. In the long run, very few of the largerural telephony networks will be built around a single GW. GWs are often co-located with a toll exchange that corresponds to given region (in a country) servingan area code or group area codes.

In the case 2 alternative, the goal of eliminating double satellite hops for voice call isalways preferable and more economical. Therefore, the opportunity for true (i.e.single hop) mesh communications is becoming a requirement for many ruraltelephony solutions.

Is DAMA necessary?

DAMA is mostly necessary from a business perspective. To establish an attractiverural telephony business case, operational costs must be minimized and the satellite-based approach contribute to adding recurring service charges. The ability to sharespace segment capacity among thin-route subscribers, on-demand, results insignificant cost reduction and, in most cases, the best non-DAMA approach withpre-assigned SCPC channels or TDMA time slots would require more than twice thebandwidth of a DAMA system.

The nature of the rural telephony requirements clearly has shown that individualremote VSAT sites only require a relatively small number of simultaneous voice


channels, all requesting connectivity to possibly different area codes, regions, orGateway sites. It is generally a universal goal in providing satellite services tominimize the recurring space segment costs. If this monthly cost is too high, thecost of the service becomes prohibitive and the following conclusions can be drawn:

• In rural telephony, call intensities per subscriber rarely exceed 0.2erlangs/subscriber, even for public call centers. Therefore, providing a pre assignedchannels for each voice channel, which is not used 80% of the time (best case) isnot economical• In the trunking case, even a 180-user subscriber base at 0.25erlangs/subscriber (80% of the traffic via satellite) requires 8 satellite trunks (2%blocking grade of service) at 0.56 erlangs/trunks. Even in this trunk case, the circuitremains unused almost 45% of the time, still by no means economical• In addition, given the fact that subscribers may desire connectivity todifferent PSTN points-of-presence (POP) at different times (Case 2 in the networkrequirements discussion), attempting to pre-assign trunks to different gatewaylocation increases the level of trunking inefficiency, and thus, a degraded quality ofservice.

In conclusion, it is often difficult to provide a competitively priced service offering(i.e. cost/minute) without employing a DAMA architecture. The competitionpresented by completely wireless/microwave solutions or the new breed ofLEO/MEO offerings will not permit a non-DAMA solution.

The VSAT trade-off discussion is summarized as follows:

• A multi-star and mesh architecture is a pre-requisite to satisfy the specifiedrural telephony requirements.• Based on the nature of call activity in telephony, a DAMA approach is anecessity for rural VSAT applications to reduce the space segment costs.

5.2 The Distributed Call Processing Approach

Network models have indicated a percentage of the traffic, typically on the order of20 %, is being established between the WLL subscribers within the same cell. Basedon this requirement, WLL-VSAT architecture may be designed to handle these localcalls in either of two ways:

1. Perform neither digit analysis nor switching locally and route all calls over thesatellite.2. Perform the necessary digit analysis and switching locally and only route the longdistance calls over the satellite.


An analysis of available systems indicates that switch vendors looking at adding awireless access network component to their local exchange product line developedmost of these products. By acting only as a Wireless Access Network (WAN), thetypical WLL systems do not provide any level of switching. In the typical WLLmarkets, the Base Station Controller/Central Office interface is always co-locatedwith the local exchange, so this apparent shortcoming is really not a liability andmore than adequately serves its intended applications.

In the WLL-VSAT case, the DAMA network, as a complete entity, actually mayperform the functions of a local exchange (Case 1 above) and in some cases,provides tandem exchange long distance routing (Case 2 above). However, theexecution of this switching task is not performed locally at the VSAT site, but via thesatellite DAMA. The Gateway terminal then provides trunk access to the PSTN,usually to toll exchanges (or equivalent) for PSTN routing to the called party’sdestination.

If the WLL component and the local VSAT component are both not capable ofperforming this local switching function (as in the case above), then all calls must berouted over satellite regardless of destination. See figure 5.3 for a description of thenetwork switching options. Assuming double satellite hops are not acceptable froma voice quality perspective, the method for achieving connectivity between twosame-cell, WLL subscribers is to create a full duplex circuit between two separatechannel cards on the same VSAT.

Rural WLL-VSAT Basestation Network Backbone(DAMA Satellite)

Option 1 - Switch at theLocal VSAT Node

Subscribers Option 2 - Switch at theSatellite - Mesh(2 channel cards

at the VSAT)

Option 3 - Switch at thePSTN - Double Hop

(2 channel cards at the VSATand the GW site)

PSTNSwitch

Gateway / NCC

Master NMS

9/02/2581_Fig3

Figure 5-3: Network Switching Options


5.2.1 Subscriber management

The combination of WLL and VSAT networks leads to two functional NetworkManagement System (NMS) entities at a centralized Gateway site, one for theDAMA network management and one for the WLL network Management. Theseare shown as two individual computers/workstations at this juncture for illustrationpurposes. As deemed necessary and often preferable by the operators, these twofunctional management entities may reside on the same workstation/computer.However, regardless of physical implementation, both network managemententities perform their own monitor, control, and management functions for theirrespective WLL and DAMA VSAT components from a centralized site. If not residingon the same physical machine, both the WLL and DAMA management segmentsare linked for the exchange of relevant management information.

There are two types of subscribers in this type of satellite and WLL networks:

1. Subscribers that interface to the VSAT directly and;2. Subscribers that are part of the WLL network, which interfaces to the VSATsystem via the WLL base station controller interface.

In the first case, the subscriber’s profile, which includes items such as phonenumber, connection type, location, allowed services, restrictions, public/privatephone etc., are entered via the DAMA NMS. In the second case, due to the inherentconcentration function of the WLL air interface, there is clearly not a 1:1correspondence between the WLL channel and a given subscriber. Even from theperspective of the DAMA NMS, each VSAT channel interface is a trunk interfacethat is available to any WLL subscriber, part of the local WLL community. Therefore,in this case, the subscriber profile is generated as part of the WLL NMS anddownloading call data records (CDR) from the local WLL NMS site to the centralizedgateway site may occur in one of two ways (see figure 5-4):

A- The WLL NMS features a data interface at the Gateway site and eachremote VSAT with the WLL extension also contains a dedicated data interface. In around robin fashion, a DAMA data circuit is set-up (i.e. 9600 BPS) between the WLLNMS and each remote VSAT (with a WLL component). Via the aforementioned linkbetween the respective management entities, the WLL NMS will initiate a requestfor the desired data circuit from the DAMA NMS to each WLL BSC. Followingallocation of the requested satellite resources, the required subscriber datainformation is then downloaded.

B- Based on further level of integration between the two respectivenetwork management entities, this subscriber profile can be downloaded via theexisting DAMA control channel between the DAMA NMS and each remote VSAT


(with a WLL extension). In this manner, no separate dedicated data channel isrequired and the need for data channel cards is eliminated.

Figure 5-4: Subscriber Management

5.2.2 Call scenarios for integrated VSAT/WLL with local switching

Given that the necessary subscriber database information is present locally at thelocal WLL site, the next question to address is: what are the different call scenariosand how does the integrated network actually set-up and create these end-to-endconnections?

With a WLL system integrated with a remote VSAT component, there are threebasic call scenarios; all of, which require some form of, distributed call processingbetween the local WLL site and the centralized DAMA NMS. As soon as the abovesubscriber profiles are completely downloaded from the central WLL NMS, thismanagement entity is no longer involved in the real-time call processing functions.All DECT WLL real-time processing is performed locally. The three call scenarios andaccompanying descriptions are as follows:

5.2.2.1 WLL subscriber-PSTN

Based on the network model described earlier, this scenario is the most prevalentand consists of a WLL subscriber attempting to place a call to a PSTN subscriber. Inthis case, for an outgoing call to the PSTN, the local WLL site authenticates thecalling subscriber, provides a limited dial digit analysis to validate the fact that the


called party is not a local subscriber, and translate the WLL call request message intoa suitable DAMA call control format (the WLL to DAMA protocol interworking). Thelocal DAMA controller composes a call request message to the DAMA NMSrequesting satellite bandwidth to connect the call. The centralized DAMA NMSrecognizes the called number is an external call and performs the required allocationfor both satellite endpoints (Gateway, remote VSAT). The calling subscriberinformation is, in parallel, passed on to the PSTN switch for further call routing atthe designated Gateway location.

For an incoming call from PSTN, the Gateway site formulates a call request messageto the NMS indicating, at a minimum, the called subscriber’s number. The DAMANMS, which contains a mapping of the block of subscribers numbers allocated toeach VSAT site, allocates the circuit for both satellite endpoints (Gateways, remoteVSAT). The destination phone number is then passed to the WLL interface card viathe DAMA controller. At that point, the DECT subscriber authentication/pagingtakes place and the remaining call control is completed via the WLL networkprotocol.

5.2.2.2 WLL subscriber-WLL subscriber (intra-cell)

In this case, the local WLL authenticates the calling subscriber, processes the callrequest via standard WLL protocol processing, and provides the limited dial digitanalysis to validate the called party is indeed a local subscriber. The WLL calledsubscriber’s authentication/paging takes place and the remaining call control to thiscalled subscriber is performed via the WLL network protocol. Following thesuccessful page, the communications path is performed via the internal switchingfunction of the WLL. Note that in this scenario, the complete call control process isperformed locally by the WLL, which is a key attribute of distributing thisfunctionality to the remote WLL-VSAT.

5.2.2.3 WLL subscriber-WLL subscriber (inter-cell)

This call scenario includes segment of both the outgoing and incoming PSTN calltypes detailed in the first call scenario. In this case, as in the outgoing call type, thelocal WLL determines the call requires satellite routing and passes the call request tothe NMS via the DAMA controller. The NMS recognizes the called subscriber asinternal and assigns a circuit between the two remote VSATs. While the two VSATssynchronize, the WLL subscriber authentication/paging takes place (as a function ofthe destination phone number) and the remaining call control is completed via theWLL network protocol (between the remote WLL and the called subscriber).

Note that all WLL protocols are terminated locally and are utilized only on the WLLsegment for this call type. Following the called subscriber WLL’s analysis to ensuresatellite routing is necessary, the satellite call control scheme employs the


centralized approach with all satellite requests handled, in real time, by the DAMANMS.

5.3 The Call Detail Record (CDR) Collection/Public Calling Office Support

A key component of the telephony network solution is to accurately maintain adatabase of the calls taking place in the VSAT/WLL network. Based on the databaseinformation, also termed CDRs (call data records), an external billing system is thencapable of performing the required processing to generate the telephonebills/charges for each subscriber in the network. When a rural telephony networkincludes both a WLL and satellite segment, it is essential that these CDRs becollected at one central site.

As described in the previous section, there are three main call types that can takeplace in this WLL-VSAT network architecture. In this WLL/VSAT network, thecentralized collection point is the DAMA NMS. The DAMA NMS CDR collectionprocess will now be analyzed for each of these cases. See figure 5-5 for a pictorialview of this CDR collection architecture.

Calling / Called party info passed to DAMA NMS

(call ctl)After call complete,only the local WSIU

has the CDR data


(call ctl)

DAMA NMS has all CDR info immediately

after call


(call ctl)

WSIU DigitAnalysis

WSIU DigitAnalysis

WSIU DigitAnalysis

Solution 1DCU Method(Batch Mode)

Solution 2FOW / ROW Method

“pseudo real-time”

Call Not Local Call is Local, Callset / up, teardownperformed locally

Call Not Local

Call Length(call cleardown)

Call Length(call cleardown)

Case 1: WLL Subscriber - PSTN

Case 2: WLL Sub - WLL Sub(Intra-cell)

Case 1: WLL Sub - WLL Sub(Inter-cell)

Figure 5-5: The Call Detail record (CDR) Collection Architecture

5.3.1 WLL subscriber-PSTN


In the case of outgoing calls from a WLL subscriber to the PSTN, the WLL stationcontroller has both the calling and called party’s number available as part of the callprocessing/authentication process. This information is forwarded to the DAMAcontroller and then to the DAMA NMS as part of the normal calls control sequence.Once the call is completed, the DAMA NMS is again notified of the call duration bythe normal call tear down process. Therefore, in this case, no additional mechanismis required since all the necessary information (calling party, called party, callduration) is available at the DAMA NMS following call completion.

For an incoming call from the PSTN, the calling subscriber is not part of the satellitenetwork and a CDR record for billing purposes is not required. However, as part ofnormal DAMA NMS operation, at a minimum, the called party information and callduration is compiled.

5.3.2 WLL subscriber – WLL subscriber (intra-cell)

In this case, the local WLL station controller performs all call processing and routingfunctions. Following completion of the call, the WLL processor contains the relevantCDR information. The question then is how to forward the CDR information to theDAMA NMS?

Various solutions, similar to the subscriber management case, are described below:

• Solution # 1- The simplest method (although not necessarily the mostelegant) is to have both NMS entities (DAMA, WLL) connected to a data interfacecard (see subscriber management discussion above), called Data Channel Unit(CDU), at the central Gateway Site. Then once per day (or more often as deemednecessary), a data circuit is established between the DCU at the GW site and one ofthe available satellite channels at the VSAT/WLL site. All call detail records for thecollection period are then downloaded to the Gateway. After the batch modeprocess is complete, the data call is torn down, and the NMS (WLL, DAMA) can nowset up a data call with the next VSAT/WLL site in the network. In this round robinfashion, the local CDRs are collected for the day’s activity for the entire network ofWLL base stations.• Solution # 2 – Rather than setting up overnight batch mode data circuits,the alternative solution is to have the local WLL send the CDR for each local call inpseudo real-time fashion. In this case, the normal DAMA control channels are usedto transmit the CDR information in the next available status message following callcompletion. The advantage of this solution is 1) no additional data interface cardand associated bandwidth are required, and 2) no overnight batch modetransmissions and part time use of one of the channels are required.

5.3.3 WLL subscriber –WLL subscriber ( inter-cell)


In this case, as in the WLL - PSTN discussion above, the local WLL has both callingand called party’s number available as a part of its call processing/authenticationfunctionality. This information is passed to the DAMA controller and then to theDAMA NMS as a part of the normal calls control sequence. The only difference isthat the call is not routed to the PSTN, but to another WLL-VSAT, which is actuallyirrelevant from the CDR perspectives. When the call is completed, the DAMA NMSis again notified of the call length by the normal call tear down process. Therefore,no special mechanism is required since all the necessary information (calling party,called party, call duration) is available at the DAMA NMS following call completion.

The issue of metering pulses for public calling offices (PCOs) or payphones deservessome attention. In calls to the PSTN (case 1), the PSTN provides 12 kHz or 16 kHzmetering pulses to the Gateway site VCU (voice channel unit) as a function of R2signaling information (calling, called party etc.) exchanged with the PSTN exchange.The VCUs recognize this tone content and send this information over the satellitelink to the VSAT as a part of the voice packet structure. The WLL protocol should becapable of passing this information to the end subscriber where the analogmetering pulses are reconstructed for payphone debiting purposes.

In case 2 (WLL intra-cell calls), the local WLL is responsible for having a look-up tableof metering pulse rates as a function of the called and calling subscribers. Thisinformation is downloaded to the WLL from the NMS as a part of the subscribermanagement function. In this scenario, the metering pulses are generated, asnecessary, locally and sent to the calling payphones during the actual call.

PSTN provides meteringpulses to GW VCU

Sent to PCO wallsetvia DECT messages

WSIU DAMAInterworking

to DECT protocol

WSIU DAMAInterworking

to DECT protocol

Calling / Called Partyinfo to PSTN (R2-MF)

Based on calling,called party, WSIUtable look-up

MP rate sentto

MC-SES(CA message)

MP sent to MC-SESvia satellite(voice packets) MP sent to

PCO WSvia DECTmessages

Case 1: WLL Subscriber -PSTN

Case 2: WLL Sub - WLL Sub(Intra-cell)

Case 3: WLL Sub - WLL Sub(Inter-cell)

NMS Local TariffTable Download

(Sub Mgmt)

DAMA NMScall Set-upComplete

NMS Local TariffTable Download

(Sub Mgmt)

Metering Pulsesfrom WS to PCO



MP sent to PCO WSvia DECT messages

Figure 5-6: The PCO Metering Pulse Process


In case 3 (WLL inter-cell calls), the DAMA NMS has a look-up table of meteringpulse rates as a function of the called and calling subscribers. The metering rateinformation is included as part of the channel assignment message to the callingWLL. The WLL generates the metering pulses sent to the calling payphone duringthe actual call.

5.4 The Centralized Network Management (and O&M) Design

As mentioned earlier in this chapter, the solution is to have two functional NetworkManagement System (NMS) entities at a centralized Gateway site, one for theDAMA network management and one for the WLL network management. Thesetwo NMSs, as well as the GW Network Control Terminal (the access point to thenetwork) are connected to the same LAN to enable communication between thethree units. If deemed necessary, the two functional management entities mayreside on the same workstation/computer. In this case, the DAMA NMS willincorporate the additional screens and applications software to interact directly withthe remote WLL components. Regardless of the number of physical computers, thenetwork management design concept is identical. In the discussion to follow, theemphasis is on the collection of fault, alarm, and performance management datafrom the WLL components for O &M purposes.

The second issue, related to the network management design, is how to implementthe physical data path to communicate WLL related management information. Thecurrent VSAT-only architecture uses the DAMA control channels, not only for callcontrol purposes, but also for the passage of VSAT monitor and control messages.With the addition of the WLL component to the architecture, there are two options:1) use an external DCU for a round robin status polling approach or 2) use theexisting DAMA channels, and integrate the WLL management and controlapplication messages into the DAMA architecture. See the subscriber managementand CDR discussion for more details of these respective communication approaches.

In the O&M case, however, the situation is not quite analogous to the CDR case. Bynature of CDR collection, the batch mode design (i.e. CDR overnight data loading) isa reasonable, although not optimum, design approach because 1) the nature of thedata content is not time sensitive and 2) the data link is only enacted overnight sonetwork performance is not impacted. In the O&M case, fault/alarm andperformance data from a WLL site is certainly not appropriate for this type of batchmode design. If the DCU approach is absolutely necessary (in the short term), thisround polling mechanism must occur on the order of every 30-45 minutes to makethe WLL O&M data collection meaningful. The other disadvantage to this approachis that one of the VSAT channels is required to service this polling design.

The optimum approach, which may even be construed as a necessary approach(compared to the CDR case), is the use of already existing DAMA control channels


between each VSAT and the DAMA/WLL NMS. In the current architecture, eachVSAT in the network is allocated a dedicated slot for transporting status messagesto the NMS for display to O&M personnel. The solution, in the integrated case, is toalter the data structure for these status messages to incorporate data field from theWLL unit and the Base Station (BS). These messages will include information on thestate of the BS (e.g. RF faults, modem status, burst mode controller status, etc.) andWLL base station controller (processor status, switch status, etc.).

6 Planning and implementation of rural communicationnetworks, via the Intelsat system.

In Chapter 6, we will examine the full range of issues related to the deployment of arural telephony network including technical, operational and economical tradeoffsconcerning rural communications systems. Central to these discussions is anunderstanding of the nature of the rural environment: its features and obstacles.

Our aim is to present a general guideline that can be used by operators planning toimplement a rural communications network via the Intelsat satellite system.

6.1 Sizing and implementation of a VSAT DAMA SCPC network

The following processes apply to the planning and implementation of a VSATDAMA network for rural areas:

• Traffic estimate;• Definition of the network performance;• Network size;• Evaluation of investment and associated costs (power, installation … );• Implementation plan; and• Post-implementation issues.

6.1.1 Traffic estimation

In a DAMA system, resources are assigned to a call on demand (i.e. call by call) andproper traffic estimation is important to ensure that the Grade of Service (GOS)objectives (call blocking probability) are met.

6.1.1.1 The DAMA Network Channel Pool

The DAMA network system is assigned bandwidth that is divided into Traffic andControl channels pools. The Control channels provide communications between the


Network Control System (NCS) and the Remote DAMA sites. There are two types ofControl channels:

• The Outbound Control Channels (OCC), provide continuous data betweenthe Network Control System and the remote DAMA sites; and• The Inbound Control Channels (ICC) carry data from the remote VSAT tothe NCS.

6.1.1.2 Estimation of the number of Channel Units

The DAMA systems differ from pre-assigned point-to-point systems in the sensethat traffic to different destinations in the DAMA system can be aggregated into asingle stream resulting in the efficient utilization of the transmission facilities. Thegeneral reduction in the number of circuits is in the range of 40 to 60% dependingupon the traffic distribution.

The traffic carrying capacity of the terminal in terms of the total number oforiginating and terminating paid minutes per year (or per month) can be estimatedby applying the ITU-T Rec. E-506 formula, as follows:

Erlang = (Paid Min/Year)/12*(1/25*8*60*0.83)Where 25=25 working days; 8=8 working hours; 60=60minutes; and0.83=efficiency.

Table 6.3 shows the results of the calculations done based on the above formula;the number of Traffic Channels Units versus Traffic Levels (in+out) by using theErlang B Table and assuming a probability of loss between 1% and 3%.


Table 6.1 Number of Traffic Channel Units vs.Traffic level (in + out)

Number of TrafficChannel Units

Paid Minutes/Month (1% GOS)

Paid Minutes/Month (3% GOS)

4 8,694 12,600 8 31,300 39,900 12 58,700 71,400 16 88,800 105,100 20 120,300 140,000 24 153,000 175,800 28 186,400 212,200 32 220,500 249,100 36 255,100 286,500 40 290,100 321,100 44 325,400 362,000 48 361,100 400,200 52 397,000 438,500 56 433,100 477,000

In order to dimension the number of Traffic Channels per site, the following stepsare recommended:

Step 1: Determine the paid minutes per year for each site.Step2: Treat traffic to/from ALL destinations as a SINGLE trunk group and determinethe pbh (peak busy hour) in Erlangs using the above formula.Step 3: Determine the number of Traffic Channel Units using Table 6.1 above.

Table 6.2: Configuration of a basic DAMA system, Network Size

One OCC, 16kbit/s /25 kHz spacing 25 kHzTwo ICC, 16 kbps 2 x 25 kHz = 50 kHzAll Channel Units are 16 kbps R 1/2 QPSK CELP 25 kHzBandwidth available 18 MHz 18,000 kHz25 kHz for the OCC50 kHz for 2 ICCsBandwidth available for circuits 17,925 kHzNumber of corresponding channels 717 channelsNumber of Circuits 358 circuits

6.1.2 Space segment


Sizing the space segment depends upon the number of the channel units requiredor the call utilization the network is supposed to handle. Based on the callutilization, the DAMA platform can be subdivided into several pools and the spacesegment sized accordingly.

Table 6.3: Example of Bandwidth sizing

One OCC 25 kHzTwo ICCs 50 kHz100 voice channel units with 50% usage (peak)100 voice channel units with 30% usage (peak)100 voice channel units with 20% usage (peak)100 VCU x 2VCU/circuit x .50 x 50 kHz/circuit - pool1 1,250 kHz100 VCU x 2VCU/circuit x .30 x 50 kHz/circuit - pool2 750 kHz100 VCU x 2VCU/circuit x .20 x 50 kHz/circuit - pool3 500 kHzTotal Bandwidth 2,575 kHz

Note: The power equivalent bandwidth or the satellite bandwidth leased isdetermined by the link budget.

6.1.3 Earth Segment

The basic configuration of a remote earth station combined with a WLL equipmentis shown in Figure 5.1 of chapter 5. The results of the link budget will determine thesize of the VSAT antenna and the corresponding High Power Amplifier.

The Network Planner will seek to balance the requirements of the earth stationequipment and satellite resources in order to find the overall cost effective solution.Optimum WLL/VSAT network design will minimize the capital and operating costswhile meeting the service requirements and involves also a number of tradeoffanalysis regarding the satellite capacity, antenna sizes, proposed network topology,connectivity, availability, quality and growth over time.

6.1.4 Intelsat Satellite and frequency band

After identifying the sites and their physical locations, the network planner has todetermine which Intelsat satellite covers the desired network geographical area. Theplanner will determine also whether the appropriate Intelsat satellite offers C- orKu-band, and the associated transponder characteristics. If both frequency bandsprovide suitable coverage, tradeoff analyses should be undertaken to determinewhich band better accommodates the network. The analysis takes into accountboth satellite characteristics, and propagation characteristics for the network'sgeographical area. In general, Ku-band antennas are smaller and less costly than C-band. In either case, Intelsat offers satellites providing continental and


intercontinental services in every part of the world. Capacity is leased under flexiblecombinations of bandwidth and satellite power, and can be tailored to suit anyVSAT service provider's needs.

6.1.5 Topology and access alternatives

The goal in defining the network topology is to balance the Earth segment costswith the required satellite resources. A star architecture allows use of less expensiveremote terminals. However, the connection between two remote nodes will requiretwo hops (node A via satellite-to-hub station, and hub via satellite-to-node B). Directconnection between nodes, as in a mesh configuration, will not be possible. Fornode-to-node connection, the star architecture will result in 0.5 second ofadditional delay. If node-to-node connections are infrequent, double hop can beused because it reduces the cost of the VSAT equipment and satellite capacity.

Developing the most cost-effective network requires defining an initial topologythat can be tested with link budgets. Various scenarios and iterations will helpdetermine if the topology fits the service requirements (current and future). Somearchitecture options will fall out naturally from the traffic requirements.

6.1.6 Link budget

The link budget is used to optimize the network parameters. The objective is toreach the most economical compromise between antenna size and power amplifierat the remote VSAT sites and Hub, and space segment required. Several iterationswill be needed to find the best combination of antennas, satellite, and carriercharacteristics for a given network. To assist its customers, Intelsat has prepared theLST program that can be used to compute transmission plans. The LST program canbe used to test various configurations and topologies and includes almost everypossible modulations, coding, and the satellites available in the Intelsat's system.Users are able to check on the impact of rain attenuation on transmissions and totest other operating conditions in the Link Budget. Once a various of link budgetscenarios have been conducted, the network planner needs to compare the resultswith the available equipment. This must be done in terms of traffic design, antennasize, High Power Amplifier, carrier rates, coding, and modulation schemes.

6.1.7 Network implementation costs

The cost analysis usually starts with a summary of the capital costs. The followingelements must be included:

• Equipment costs: With the current level of competition amongmanufacturers, equipment costs are somewhat volatile and negotiable. The VSATunits are likely to represent the highest cost, if there are many VSATs in thenetwork. Each remote site should include not only the VSAT alone (antenna and


RF), but also the interfacility link and indoor unit as well as the con of power supplyequipment if needed (solar, generators… ). The cost of the hub facility and ancillaryequipment should be included as well. (The level of redundancy at the hub alsoneeds to be assessed by the operator.)

Table 6.4 Equipment list

Hub StationKu- or C-band antennaAntenna, HPA, up/down converter, LNA, trackingIF equipmentDAMA NMS: Network Management SystemWLL NMS (if needed)DAMA chassisDAMA Control ChannelsDAMA traffic ChannelsCabling

Remote VSAT Terminal with WLL addition (if required)VSAT Ku- or C-band antennaOutdoor unit-ODU (SSPA, Up/Down Converter, LNA)Multi-channel (DAMA)WLL interface Unit (WSIU) (if required)WLL base station Controller (if required)WLL Base station Radio (if required)Switch (local exchange) (if required)DAMA chassisDAMA Control ChannelsDAMA traffic ChannelsCablingPower supply (if needed: rectifier, converter, generator,solar system)

• Operational Costs: The operational costs include satellite resources, staff,and facilities. Facilities operational costs include power, heat, and air conditioningcosts.• Staff and training: Additional staff will be required to operate and maintainthe DAMA VSAT network, in particular the Network Management System. Stafftraining on the new VSAT equipment will also be required. In addition it can beassumed that one routine maintenance visit will be required each year for eachVSAT site.


• Local facilities: Each site will require site survey, local permit approval, andpossibly frequency coordination, followed by civil works, access to power, WLLantenna tower if needed and air conditioning installations.• Spare parts: At the hub, it is typical to have 1 spare for each 10 items of theonline equipment. For the remote VSAT terminals, it is typical to have 1 spare forevery 20 items of the common equipment (such as SSPA and ODUs). The cost rangeof the spares would be 5 to 10 percent.• Space Segment lease cost: Intelsat has flexible lease tariffs, and a client canchoose to either pay per carrier or to lease a portion of a transponder. Leasesprovide the client with the highest flexibility possible. The client is free to define theservice quality, availability, and any parameter affecting the network performance.The lease can start with bandwidth as small as 100 kHz. Prices are available fromany Intelsat Sales Representative. In carrier-based tariffs, Intelsat charges the clientby carrier size. The advantage of carrier-based tariff is that the service is pre-engineered and all the parameters are defined in the IESS documents. Capacity andquality of service are guaranteed by Intelsat.• Cost of upgrades: Given the competitive nature of the VSAT equipmentbusiness, the costs of upgrades are negotiable. Some vendors provide softwareupgrades freely (or at nominal charge). In doing so, they expect to keep the linesopen for future business. Most, however, charge for updates, which can bepurchased in packages or individually. It is important that the network has up-to-date software that can be supported by the vendor. Hardware upgrades may alsorequire new hardware modifications to existing modules and should be negotiated.

6.1.8 Pre-implementations activities

Implementation plan: The implementation plan must cover several key aspects toensure successful completion of the project. The implementation plan must includethe preparation of the following documents:

(a) The Statement of Work (SOW): The SOW includes the objectives of theproject, a brief description of the work, the financing, the technical constraints, ifany, the specifications, and the schedule. The SOW must use plain language andmust avoid the use of imprecise language, like "optimum" and "approximately".(b) Project Specifications and the Request for Proposal (RFP): The specificationsmay be separately identified or called out as part of the SOW. The specificationsshould stress the targeted quality.(c) The Milestone schedule: Must contain information such as, project startdate, project end date, major milestones, expected deliverables, installation,commissioning, acceptance tests, training, warranty, etc. The Work BreakdownStructure (WBS), which defines the detailed tasks, effort required, and projecttimeline, is normally prepared by the contractor. The WBS establishes schedules forthe accomplishment of a task and describes how a contractor has assigned theresponsibilities. It will specify tasks to be subcontracted or outsourced.


6.1.9 Post-implementation issues

A new VSAT network represents a significant capital investment. This investmentneeds to be protected by developing and implementing a maintenance plan evenbefore the network becomes fully operational. At the beginning of the operation,little maintenance will be required. However, it is advisable to regularly inspect allequipment and perform routine maintenance to prevent future problems. Forexample, while changes in voltage or signal levels may not be noticeable enough toregister a service complaint, it is advisable to monitor these issues to detect signs ofperformance deterioration. Equipment located outdoors should be inspectedperiodically for weather-related deterioration or other damages. Spare partsinventory system needs to be planned so that parts can be located and used whenneeded, and reordered as necessary. The manufacturer may recommend aminimum set that will support daily network operation.

Training should be provided to all staff who will be involved in operation andmaintenance of the new equipment. This area is often overlooked. Training mayrequire several days, and should be included in post-implementation plans. A planfor periodic training may also be necessary to account for staff turnover and newhardware/software releases.

6.2 Deploying WLL

This section describes the areas that must be carefully addressed in planning thedeployment of wireless access systems. Each important factor in the deployment ofWLL is considered and briefly outlined.

Successful deployment of a proposed new wireless access system requires athorough understanding of the proposed business plan, and the intendedcustomer/subscriber base. It also requires a good knowledge of demography andtopology of the territory where service is supposed to be deployed. A detailed sitesurvey should be conducted at the area of possible equipment installations toensure that the site meets all the geographical, geological, logistical andinterference requirements.

WLL networks are based around cells that deliver signals to receivers and as suchare broadly similar to cellular systems. Nearly all aspects of deploying WLL systemsare closely related to those of a cellular system. The information hereafter providegeneral guidance in the implementation of WLL systems.


6.2.1 Choosing a service offering

Different technologies have different capabilities as is indicated in Table 2.9 ofchapter two. Though for most of the rural areas basic access to the telephony is themain objective. Nevertheless, it is necessary to define the appropriate mix ofcapabilities and services demanded. This definition must include voice and dataservices, the interfaces that are to be supported, and the type of mobility that isrequired and other needed features.

6.2.2 The Network build-out costs

The network build-out costs are divided into subscriber equipment costs andnetwork equipment costs.

6.2.2.1 Subscriber equipment costs

The cost of a Subscriber unit can be obtained from the manufacturer. A number ofdifferent subscriber units with different functionality and different costs will likely berequired, and each need to be treated separately. The following information needsto be gathered:

• The projected number of units required each year;• The projected cost of the subscriber units over the investment period; and• The installation cost with associated civil works expenses will also beincluded in thecalculation (power supply included if needed).

6.2.2.2 Network costs

The key components of the network costs are the following:

• Base station radios;• Base station interconnection;• Base station controllers;• Base station controller interconnection;• Switching equipment and Network Management costs;• Operation, maintenance, and billing system costs; and• Installation costs, civil work and transportation included with the provisionof powersupply if needed


6.2.3 The recurring costs

Once the network is operational, a host of on-going costs arise, including thefollowing:

• Rental: Site rental will vary dramatically from country to country;• Leased Lines costs: The costs are dependent on the capacity, length, and thePTO;• Maintenance costs: These costs vary depending on the technology selected.As anestimate, the industry average for the mobile telecommunications industry isbetween 1% and 2.5% per year of the capital costs. That compares favorably withthe 5% cost typical of the wired access network;• Radio Spectrum Costs: These costs vary significantly. In some countries therewill be nofee payable to the regulator for the use of the radio spectrum. In other countries, anauction fee, payable upfront and so becoming part of the capital cost may belevied. Finally, annual fees may be required.• Subscriber management costs: These costs are associated with submittingbills tosubscribers and managing any problem they can encounter;• General management costs: These costs include management salaries; thecost of theHeadquarters Building, power and water bill, fleet-vehicle costs, and auditors andconsultants fees.• Marketing, Sales, and Customer retention costs: These costs include directmarketingand sales expenses, as well as the cost of incentives.

6.2.4 Rolling Out the Network

The process of rolling out a WLL network is similar to that followed by cellularoperators.

6.2.4.1 Selecting the number of cells

The total number of cell sites is a critical parameter for the network. It is one of thekey cost drivers in the total network cost.


The following inputs are used to determine how many cells are required to provideadequate capacity:

• The number of subscribers;• The expected traffic per subscriber (erlang); and• The area to be covered given the topography and geographical constraints;

Calculations are then made to determine the number of cells. The number of cellsrequired for a particular coverage area is given by:

Number of Cells = Size of Area *I Π*r2

Where r is the expected cell radius in km, and I is the factor that represents theinefficiency of tessellating cells.The expected radius can be obtained from the manufacturer or from trial results. Inthe case of DECT system r = 5 km.The number of cells required for a particular capacity is given by:

Number of Cells = Traffic Channels required Traffic Channels per cell

It is necessary to calculate both of those factors. The number of traffic channelsrequired is given by:

Number of Channels = E [number of subscribers* penetration (%)*busy-hour Erlangsper subscriber]

Where E[x] represents the conversions from Erlangs to traffic channels using theErlang formula. That is simply a formula that describes how many radio channels arerequired to ensure that blocking is no worse than required for a given amount oftraffic. The Erlang B formula is given by:

PB = AN/N!N

Σ An/n!n=0

where PB is the probability of blocking, A is the offered traffic in Erlangs, and N isthe number of traffic channels available.

6.2.4.2 Connecting the cells to the PSTN


As already pointed out in the chapter one, there are three general ways to connectthe Rural cell sites to the PSTN:

• Using a link leased from a telecommunication organization;• Using a microwave point-to-point link; and• Using a satellite link.

Important factors in making the decision include the relative cost, which is affectedby the distance of the link, the capacity required for the link, the presence ofavailable infrastructure and the availability of radio spectrum;

7 Rural Telephony via Intelsat: summary

Rural Telephony via Intelsat is the service that enables the provision oftelecommunications to developing or rural areas of the world via Intelsat satellites.These VSAT-based solutions can service a wide range of population densities andare, therefore, flexible enough to grow as user requirements change. Varioussolutions include:

• VSAT connectivity to subscriber lines to serve scattered populations (1 to 20lines);• VSAT connectivity to wired or wireless/cordless local loop for clusteredpopulations (20 to 300 lines); and• VSAT connectivity to macro-cellular networks for medium densitypopulations uniformly distributed (> 300 lines). VSATs offer a very cost-effective means of implementing high quality, reliablecommunications to regions that are not well served by terrestrial networks. Advantages of VSAT networks include: • Costs that are independent of distance and terrain;• Expansion costs that are predictable;• Customized solutions to meet varying customer requirements and differentapplications;• A high level of security, control, and network management;• Rapid installation and relocation; and• Unattended and maintenance-free operation. As a result of these advantages, VSATs provide customers with significant gains inproductivity, efficiency, cost control, and profitability.


7.1 VSATs directly connected to Subscriber Lines

As an initial entry to a low-density area (<0.1 inhabitant per sq. km), a VSAT stationconnected to a few phone lines (public, business or residential) or a tele-centerproviding access to voice, fax and data services represents the most economicalsolution. (Figure 7.1 shows VSATs connected to subscriber lines). In a ruralenvironment, most of these lines are initially shared among subscribers using publicpayphones (approximately $3,000) or community phones (regular phone with ametering unit) located in a store (Peru, Venezuela) or an administrative building(Indonesia). Rural telephony subscribers spend between 10% to 20% of theirdisposable income on telecommunications. Calls are placed primarily for socialreasons (50% to family and friends), then business. Subscribers may not have theability to pay individually for their own phone, nevertheless revenue per line remainsthe same as in urban areas once the line is shared. Therefore, VSAT networksconnected to a small number of telephone lines are an ideal solution for serving theinitial demands of remote telephony. Such networks: • Offer a cost-effective solution for implementing high quality, reliablecommunications inlocations that terrestrial facilities cannot economically accommodate;• Can be installed in as few as two days per site;• Provide customers with low cost operations;• Enable hub facilities to be shared among multiple uses and applications;• Operate with very low power requirements (30-50 watts per channel);• Use mature technologies assuring stability and reliability; and• Operate in an extremely competitive market environment and, therefore,provide customerswith numerous customized equipment options at very low cost. In addition, VSATs support both high quality narrow and widebandcommunications. They also have limited power requirements and allow the use ofalternative power sources such as solar energy. And because VSAT networks can beconnected to pay telephones or a small number of lines, they serve widely dispersedsubscriber bases. Stations can be located at an individual home or co-located with apublic telephone. The public telephone network can be composed of individualpayphones connected to a VSAT station that is typically a 1.8m C-band or a 1.2mKu-band antenna. Or it can consist of telephone shops (telecenters) where multiple“community” lines, telephones, and/or facsimile machines are connected to a singleVSAT station.


PSTN

HubVSAT

VSAT

Payphones

Telecenter

VSAT

9/04/2641-4c

Figure 7.1: VSATs Directly Connected to Subscriber Lines

7.2 VSAT connected to Local Loop

VSAT associated with a local loop solution targets a second segment of the ruraltelephony subscriber market. This combination is not only seen as a way to expandthe first application (VSAT alone) but also as a solution to serve an immediatedemand from higher, although still rural, population densities where the requirednumber of lines typically ranges from 20 to 500 per local loop. This new segment isderived from the traditional local loop market which is at least a fifty times largerthan the VSAT segment (3 million lines projected in 1999 for the WLL market alonecompared to 60,000 lines for the VSAT market).

7.2.1 VSAT connected to Wired Local Loop

This solution is well suited to serve concentrated populations in rural areas,especially clusters of populations located within a 5km range. VSAT networksconnected to a local switch may provide high data rate transmission for local traffic.Wired local loop areas can be co-located with a VSAT station or interconnected tothe VSAT using a microwave solution. Because this configuration relays more trafficat higher data rates than single lines, antenna and transceiver sizes increase(minimum 2.4m C-Band or 1.8m Ku-Band). Main advantages are summarized asfollows:

• Seamless connectivity via VSATs between wired local loop networks andPSTN.• Service offerings include voice, fax and broadband data.• Alternative power supply such as solar energy can be used, with no separatepowerrequirement for subscriber terminals wired to the local switch.


• Optional battery reversal and 12/16 kHz signaling available for pay phoneconnections. VSAT networks with local switching provide high data rate transmission for localtraffic. Wired local loop areas can be co-located with a VSAT station orinterconnected to the VSAT using point-to-multipoint microwave solutions. Whenthis configuration is used, billing records are gathered locally and downloaded atthe hub site. 7.2.2 VSAT connected to Cordless/Wireless Local Loop (DECT/PHS)

VSATs and microcell/cordless WLL technology (small wireless cell site coverage ofless than 5 km and limited mobility) provide communication solutions for subscriberdensities greater than 20 lines. In this configuration, VSATs are used for longdistance communication, while microcell/cordless WLLs are used for localcommunication. WLL microcell standards include DECT (Digital Enhanced CordlessTelephone) and PHS (Personal Handyphone System). DECT operating frequencyranges from 1880 to 1938 MHz while PHS frequency ranges from 1895 to 1918MHz. Cordless telephony was originally designed to provide wireless access toresidential and business areas and has recently developed into a cost-effective WLLsolution for low density areas. Cordless telephony has significant advantages interms of scalability and functionality.

Compared to cellular, cordless telephony is capable of carrying higher levels oftraffic (more adapted to fixed telephony as opposed to mobile traffic), providesbetter voice quality (32 kbps ADPCM) and can transmit data at higher rates(currently 14.4 kbps with migration to 64 kbps by end of 1999). Other advantagesare summarized as follows:

• Non-compressed WLL voice call processing maintains voice quality over theVSATnetwork.• Power requirements are low for both WLL network equipment (< 700 watts)andsubscriber terminal (5 Watts)• Cost per line is significantly reduced by integrating WLL and VSAT hardwareandnetwork management systems (refer to later chapter “VSAT/WLL – what tointegrate?”)• Optional battery reversal and 12/16 kHz signaling available for pay phoneconnections.


Hub VSAT

VSAT BSC

Payphones

Switch/CO

CordlessWireless

Local Loop

WiredLocal Loop

Switch/CO

5 km

BTSBSC = Base Station ControllerBTS = Base Transceiver StationCO = Central Office

PSTN

9/04/2641-5c

Figure 7.2: VSAT and Local Loop (Wired and Cordless/Wireless)

WLL cordless cell sites may be co-located with a VSAT station or inter-connectedusing microwave Solution. (Figure 7.2 shows a VSAT connected to a Local Loop -Wired or Cordless). 7.2.2.1 Case Study: Intelsat Trial Project in Senegal-DAMA VSAT/DECT WLLconfiguration

During the first quarter of 1999, Intelsat began a trial with the cooperation ofSONATEL in Senegal to serve multiple villages with two VSAT/WLL DECT stations.The objective of the trial is to demonstrate the technical feasibility of providingsatellite interconnection for WLL installations through Intelsat satellites, includingbackhauling telephony services to the public switched network. To conduct the trial,Intelsat is using a DAMA network provided by STM Wireless operating over theIntelsat 603 satellite. Each VSAT/WLL site houses a single chassis with the VSAT andDECT WLL controller equipment co-located. This pre-integrated solution is aprecursor to the fully integrated version being developed by STM Wireless that willbe made available mid-99.

Figure 7.3 contains the hardware configuration deployed in Senegal. As shown inthe middle of the Figure, a Southeast Zone Beam transponder on the Intelsat 603satellite connects the Standard A station in Gandoul, which is used here as theDAMA hub station to two remote sites in Senegal: Tivaouane and Kafrine.

Each remote site is equipped with a 2.4m antenna, and a 5W HPA. In addition, eachindoor unit holds three Voice Channel Units (VCUs), one Data Channel Unit (DCU)dedicated to the transfer of the WLL call data records (CDR), and a PC-basedterminal to run the WLL Network Management System (NMS). The VSAT trafficterminals are connected via a multiplexer (E&M to E1) to the DECT WLL Base StationController, which is also incorporated in the same chassis as the VSAT basebandequipment. Two DECT/WLL antennas (provided for space diversity) are mounted on


an already existing microwave tower, which transmits and receives to a maximum of20 pay phones, installed in villages surrounding the WLL/VSAT site. The DECT radiopower and antenna gain specifications limit the coverage radius to 3 to 5 km. Asolution has been established to gather all CDR at the DAMA hub site in Gandoul.First, the WLL Call Detail Records are collected locally by the WLL NMS and thentransferred to the central DAMA NMS using an Ethernet connection via a dedicateddata channel.

At the Hub site, the DAMA Network Control Terminal (NCT) consists of 4 VCUs, aDCU and Control Processors to transmit and receive the DAMA signaling and trafficinformation from the remote sites. The DAMA NMS resides on a Sun Microsystemscomputer which is located at the premises of the Standard A earth station inGandoul. The E&M leads from the VCUs at the hub connect via a multiplexer to theSwitching Center in Dakar, where normal call processing and routing proceduresare applied. Communications between the remote sites and the hub occur via 9.6kbps channels; a continuous TDM carrier is used in the outbound direction (to thesites), while the remote sites use slotted ALOHA TDMA for DAMA network login,logout, status and call setup and tear down in the inbound direction.

DAMA NetworkControl Terminal

Hub Station32m Standard A

Gandoul, Senegal

2.4m RemoteTerminalMbour

WLLOMC

Ethernet DECTBase Station Controller

DECTBase Station Controller

3 VCUs1 DCU

E-1Multiplexer

8 PublicPayphones

INTELSAT 603

8/12/2532-01

2.4m RemoteTerminal

Tivaouane

WLLOMC

Ethernet

3 VCUs1 DCU

E-1Multiplexer

7 PublicPayphones

VCUCards

DCU ControlProcessor

forDAMA NMS

MUXandPBX

MainSwitching

Centerin Dakar

Domesticand

InternationalPSTN

Figure 7.3: Functional configuration for the WLL/VSAT trial in Senegal.

7.3 VSAT connected to Wireless Macro-cellular Networks

VSAT networks combined with wireless macrocellular networks provide costeffective solutions for more than 300 lines to populations uniformly distributed overa 30-km range. Macrocellular standards include AMPS (FDMA), GSM (TDMA) andCDMA. Given its wide availability resulting from serving approximately 20% of thehigh mobility market, the analog cellular AMPS is best suited to serve economicallylow density to medium density markets (> 150 lines). (Figure 7.4 shows a Macro-cellular network connected to a VSAT). Digital cellular such as GSM also benefits


from wide availability. However these systems have been designed to serve highercapacity subscriber lines (> 300 lines). Another digital cellular solution, CDMA,employs a spread spectrum modulation technique in which wide ranges offrequencies are used for transmission and the system low power signal is spreadacross wide frequency bands. Like GSM, CDMA becomes more cost-effective forlarger population densities (> 500 lines). Advantages are summarized as follows:

• Supports large coverage area with single base station transceiver (more than30 km).• Provides seamless connectivity via VSATs between local loop networks andthe PSTN.• Supports FDMA (AMPS), TDMA (GSM, IS-54) or CDMA (IS-95).• Service offerings include voice, narrowband data and mobile telephony.

HubVSAT MTSO BSC

30 km

BSC = Base Station ControllerBTS = Base Transceiver StationMTSO = Mobile Telephone Switch Office

BTS

BTS

30 km

PSTN

9/04/2641-6c

Figure 7.4: VSAT and Wireless Macrocellular Networks

7.3.1 Case Study: Intelsat trial in Peru

In 1998, Intelsat and Telefonica del Peru conducted a rural telephony trial using acombination of a VSAT station and a macrocell wireless local loop to serve threerural locations in Peru: Chuambra, Santa Rosa de Tiestes and Succlla. The installedVSAT/WLL network included three major components: the subscriber telephones toaccess the WLL network, the WLL equipment to provide local wireless access andthe VSAT network to ensure interconnection to the main PSN center in Lima. Thenetwork features a 2.4m DAMA VSAT station with three voice channels using theIntelsat 603 satellite at 335.5oE and an AMPS WLL cell site with 3 voice channels.The network used coin operated pay phones. This trial has demonstrated Intelsatsatellites’ feasibility for interconnection of WLL and backhauling telephony services


into the PSN. It also provided valuable statistical and operational information aboutcommunication service in a rural environment.

For instance, the percentage of incoming (versus outgoing) calls was quite high andincreased steadily after the trial began. In the beginning, more than 80% of thecalls made with the pay phones were outgoing. After 3 months, the split was about60%/40% for outgoing versus incoming calls.

Overall, both organizations gained a better understanding about technical andoperational challenges faced by an operator in a rural environment, especially whenimplementing a combination of technical solutions such as VSAT and WLL. Newoperational issues such as the power requirement for multiple equipment and thecentralization of call data records via various communications links (satellite andterrestrial wireless) emerged and required special attention. In addition, bothorganizations gained a better understanding about the rural telephony subscribermarket in this region and what factors drive the demand for such a service. Many ofthese statistics, such as the percentage of dropped calls and the percentage ofincoming calls, reflect a general trend seen in many emerging markets in LatinAmerica and elsewhere (Bangladesh, for instance). Finally, this particular experiencealso revealed an instance where the combination of VSAT and WLL can serveeconomically rural areas by combining wireless thin-route services offered tosuburban and rural subscribers under one umbrella. This particular solution will alsostand on its own in remote areas with larger population densities where there is arequirement for at least 200 lines. A detailed trial report is available from Intelsat.

7.4 Generic features of rural telephony solutions

The technical features of a rural telephony solution are summarized in the followingtable.

Table 7.1: Technical Features of Rural Telephony solutionVariables VSAT Alone VSAT & Wired

LoopVSAT & WirelessLoop / CordlessAccess Solution

VSAT & WirelessMacrocellularSolution

PopulationDistribution

Scattered Concentrated &clustered

Clustered Uniform

SubscriberDensity

Very low(<0.1/sq. km)

Low to medium Low to medium Medium(<0.1/sq. km)

TrafficCapacity(erlang)

Low Low to high Low to high Low to medium

Applications Voice, data,fax

Voice, data, fax Voice, data, fax Voice, data, fax


Data Rate Broadband Broadband Up to 64 kbps Narrowband,14.4 kbps

Mobility None None Limited YesArea ofCoverage

< 300 m < 5 km < 5 km < 30 km

Power Supply– Equipment

Low(< 250 Watts)

Medium(< 600 Watts)

Medium(< 700 Watts)

High(~2000 watts)

Power Supply–User Terminal

None None Low(< 5 Watts)

Medium(< 30 Watts)

VoiceCompression

Selectable(4.8 to 32kbps)

Selectable 32 kbps 8 to 13 kbps

Access toSwitchingFacilities

Required Not required Optional Not required

Terrain Insensitive Sensitive Insensitive(No tower required)

Insensitiveexcept towerinstallation

Installation Rapid(2-3 days)

Lengthy(wired network)

Rapid(2-3 days per site)

Rapid, excepttowerinstallation

Maintenance Very Low Medium Low LowSecurity issues Antennas and

shelterWire theft andshelter

Antennas andshelter

Antennas,tower andshelter

RegulatoryIssues

VSAT license(C or Ku band)

VSAT license Cordless and VSATlicenses

Cellular andVSAT licenses

7.5 Summary

In summary, a VSAT-based network can be the ideal solution for providingtelecommunications services, including basic telephony, to regions of the world withinsufficient terrestrial facilities. Such a network has the flexibility to grow, ascustomer needs change and therefore, can meet a wide range of customerdensities. Each of the solutions described in this article may exist by itself or mayco-exist within the network. For example, a VSAT connected to a few lines, a VSATplus wireless local loop and a VSAT plus wired local loop may be implemented in alarge region to serve the disparate needs of both residential customers. See figure7.5 for the roadmap graph on VSAT based rural telephony networks.


VSAT& Wired

Local Loop

VSAT &Cordless

WLL

VSAT &Cordless

WLL

VSAT& Wired

Local Loop

ScatteredPopulation

Clusters ofPopulation

VSAT & Macrocellular

Clusters ofPopulation

Low Density(<300 Lines)

Medium Density(>300 Lines)

VSAT's withPayphones/Phone Shops

30 km 30 km

5 km

5 km

PopulationUniformly

Distributed 5 km

5 km

B2

B1

B2

A

C

B1

9/04/2641-8c

Figure 7.5: Roadmap graph on VSAT based rural telephony network

The above roadmap provides some preliminary indications on how differentsolutions can serve economically the needs of specific population based upon thefollowing two factors: population distribution (assuming a coverage area with radiusof 30 km) and a number of lines required (breakpoint is approximately 300 lines).

If less than 300 lines are required within the coverage area (30-km radius), then twoconfigurations are described (upper half of chart):

• VSATs connected to a few lines to serve a dispersed/scattered subscriberpopulation.• (B1&B2) VSATs connected to a small local loop area (wired or cordless microcell)if the subscriber population can be grouped into small clusters/cells whose coveragearea does not exceed 5 km.

If more than 300 lines are required within the coverage area (30-km radius), twomain options become available (lower half of chart):

• (C) VSAT connected to a macro-cellular network (coverage up to 30 km per cellsite) if the population is evenly distributed throughout the area.


• (B1&B2) VSAT connected to a small local loop area (wired or cordless microcell)if the subscriber population can be grouped into small clusters/cells whose coveragearea does not exceed 5 km.

The final decision between wired (B1) and wireless (B2) local loop is based upon anumber of issues such as geographical constraints and type of service required.Finally, these solutions may coexist: a combination of single VSATs, VSATs pluswired and Wireless local loop may be implemented domestically to serve the variousneeds of residential and business subscribers.

8 Intelsat Rural communications service portfolio

8.1 The Intelsat value added services

Intelsat is committed to supporting the customer in all aspects of their serviceneeds. Intelsat will:

• Help identify the most appropriate satellite capacity;• Assist in the network design; and• Provide technical assistance in the implementation of the ground segment.

Some of the unique value added resources include:

• Intelsat's Application Support & Training. This department offers in-houseconsultancy, training and technical expertise in emerging services. It also providesaccess to cutting-edge technologies, including advanced rural telephonyapplications.• Intelsat Workshops and Seminars. Intelsat sponsors events in many parts ofthe world to bring customers, suppliers, and service providers together to sharetheir knowledge on the latest developments in their fields.• Earth station type approval program. Customers can ensure high quality,lower cost and faster implementation of rural telephony solutions by purchasingtype-approved ground equipment.• Contacts with Suppliers. Intelsat has excellent contacts with the VSATindustry and can provide a list suppliers of VSAT systems and services worldwide.• Service Development Initiatives. Intelsat's service development programsbring cost-effective and cutting edge solutions to areas of high growth. The resultsof these programs are available to customers interested in developing newer andbetter services using Intelsat capacity.8.2 Type of service offered


Intelsat offers rural telephony operators a number of voice and data serviceplatforms that accommodate a variety of architectures and applications.

8.2.1 Intelsat Business Service (IBS) via VSATs

IBS is a digital carrier service available for point-to-point rural telephoneapplications. Customers can order IBS services in 64Kbps increments using earthstation equipment standardized to meet Intelsat's specifications. IBS offers:

• Pre-engineered service parameters for easy implementation; and• Simple pricing structure as a function of earth station standards.

Intelsat Business Services (IBS) are defined for distinct offerings: IBS and VSAT IBS.

(a) Intelsat Business Service (IBS) employs digital carriers, which use QuadraturePhase-Shift-Keying (QPSK) modulation with Frequency Division Multiple Access(FDMA) technique.IBS is designed for communications between Standard A, B, C, E and F earthstations, which may function as national gateways, urban gateways and/orcustomer premise installations.

(b) VSAT IBS (IBS to/from Small Earth Stations) are defined as those whichterminate at or originate from Standard E-1, F-1, H and K earth stations only. VSATIBS employ digital carriers, which use Quadrature Phase-Shift-Keying (QPSK) orBinary Phase-Shift-Keying (BPSK) modulation, with Frequency Division MultipleAccess (FDMA) technique.

IBS and VSAT IBS can be offered as Channel/Carrier-based or leased services. Forchannel or carrier-based services, Intelsat provides a guaranteed, specified quality ofservice at the receive earth station. To achieve this specified quality, subscribers useearth stations and other equipment conforming to Intelsat's technical specifications.

8.2.2 Leases

The space segment resources are utilized through fractional or full transponderleases which are accessed by earth stations approved in categories A, B, C, D, E, F,H, K and G.

Leased transponder definitions


Non-Preemptible Leases: A non-preemptible lease supports any type of service,without restriction, on a domestic, regional or international basis. A non-preemptible lease offers the highest level of service protection. In the unlikely eventof failure or significant degradation of service, non-preemptible services have thehighest priority and will be restored on the backup transponder on either the samesatellite or on another satellite in the same region. Non-preemptible leases areavailable in any bandwidth from 100 kHz to a full transponder in integer multiplesof 100 kHz.

Preemptible Leases: Preemptible leases may be used to provide any international ordomestic service except international public switched telephony services and IBS.Service may be interrupted or displaced by a non-preemptible or higher priorityservice in the unlikely event of a satellite contingency. Preemptible capacity isoffered at lower rates than non-preemptible services. Preemptible leases areavailable in any bandwidth from 100 kHz up to a full transponder in integermultiples of 100 kHz.

Leased services allow customers to design networks with whatever equipment andquality specifications they desire and find acceptable. The only Intelsat criterion forleased networks is that they cannot interfere with other communications networkon the satellite system.

Intelsat offers rural telephony customers complete flexibility in defining theirnetwork characteristics:

• Capacity allotment ranging from 100 kHz to a full transponder;• Both C-band and Ku-band capacity; and• Star, Mesh and Demand-assigned VSAT networks.

8.2.3 Intelsat DAMA

Intelsat's Thin Route-on-Demand DAMA (Demand Assignment Multiple Access)service is a usage-based, state-of-the-art digital solution for Public SwitchedTelecommunications Network (PSTN) applications. It provides thin route operatorsinstant dial-up global connectivity.

Thin Route-on-Demand is one of the most flexible, cost-effective multiple accesstechnologies available today, particularly for carrying the following types of traffic:

• Replacement of transit routes;• Overflow traffic; and• Replacement of analog circuits.

Toll Quality Telephony


The service offering is 16 kbps telephony for speech, fax and voice-band dataapplications. Intelsat has adopted the ITU standard G.728 speech coding techniqueto deliver internationally accepted voice quality. Voice-band data at speeds of up to64 kbps are supported.

Pay-as-You-Use

The service is charged on a per minute basis for the duration of answered calls.There are no sign-up or minimum monthly fees.

Wide range of Earth Station Sizes

Intelsat's Thin Route-on-Demand service is supported by a wide range of earthstations, from large gateway standard A down to Very Small Aperture Terminals(VSATs). Thin Route-on-Demand can provide full mesh connectivity on global beamswith antennas as small as 4.5m (standard F1). VSATs as small as 2.4m cancommunicate in star connectivity with large earth stations (standard A or B). Thecharges for small stations are attractive for remote access applications.

Remote Access Applications: While the service has been mainly established forinternational PSTN applications, attractive costs for Very Small Aperture Terminals(VSATs) have stimulated demand for other applications, especially those extendingbasic telecommunications to hard-to-reach rural areas. Customers will be able torapidly expand services with low cost earth stations that are easy to install, maintainand redeploy. Existing gateway earth stations may be used to connect remote usersinto the PSTN. Users will immediately be able to offer this service in a closed usergroup arrangement. This eliminates the expense of implementing dedicatednetwork management facilities for small networks.

rural com

Documents