the basics of telecommunications international engineering consortium iec 2002

The Basicsof

Telecommunications

Presented by theInternational Engineering Consortium

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Table of Contents

Asymmetric Digital Subscriber Line (ADSL) .........................................1

Asynchronous Transfer Mode (ATM) Fundamentals ............................15

Cable Modems .......................................................................................41

Cellular Communications.......................................................................57

Fiber-Optic Technology .........................................................................81

Fundamentals of Telecommunications.................................................103

Intelligent Networks (INs) ...................................................................129

Internet Access .....................................................................................161

Internet Telephony................................................................................183

Intranets and Virtual Private Networks (VPNs) ..................................199

Operations Support Systems (OSSs)....................................................211

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Optical Networks .................................................................................235

Personal Communications Services (PCS) ..........................................265

Signaling System 7 (SS7) ....................................................................291

Synchronous Optical Networks (SONETs)..........................................321

Voice and Fax over Internet Protocol (V/FoIP) ...................................387

Self-Test Correct Answers....................................................................417

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1Asymmetric Digital Subscriber Line (ADSL)

DEFINITION

Asymmetric digital subscriber line (ADSL) is a new modem technology thatconverts existing twisted-pair telephone lines into access paths for high-speedcommunications of various sorts.

ADSL can transmit more than 6 Mbps to a subscriberenough to provideInternet access, video-on-demand, and LAN access. In interactive mode it cantransmit more than 640 kbps in both directions. This increases the existingaccess capacity by more than fifty-fold, enabling the transformation of theexisting public network. No longer is it limited to voice, text, and low-resolu-tion graphics. It promises to be nothing less than an ubiquitous system thatcan provide multimedia (including full-motion video) to the entire country.ADSL can perform as indicated in Table 1.

Table 1: ADSL Data Rates as a Function of Wire and Distance

Data Rate Wire Gauge Distance Wire Size Distance

1.5-2 Mbps 24 AWG 18,000 ft. .5 mm 5.5 km

1.5-2 Mbps 26 AWG 15,000 ft. .4 mm 4.6 km

6.1 Mbps 24 AWG 12,000 ft. .5 mm 3.7 km

6.1 Mbps 26 AWG 9,000 ft. .4 mm 2.7 km

TOPICS

1. A SHORT HISTORY OF ANALOG MODEMS . . . . . . . . . . . . . . . . . . . .3

2. THE ANALOG MODEM MARKET . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3. DIGITAL SUBSCRIBER LINE (DSL) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

4. XDSL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

5. THE MODEM MARKET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

6. ATM VERSUS IP TO THE DESKTOP . . . . . . . . . . . . . . . . . . . . . . . . . . .8

7. CAP VERSUS DMT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

8. THE FUTURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

9. SELF-TEST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

10. ACRONYM GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13


2 Web ProForum Tutorials

1. A SHORT HISTORY OF ANALOG MODEMS

The term modem is actually an acronym that stands for modulation/demodula-tion. A modem enables two computers to communicate by using the publicswitched telephone network (PSTN). This network can only carry sounds, somodems must translate the computers digital information into a series ofhigh-pitched sounds that can be transported over the phone lines. When thesounds arrive at their destination, they are demodulated or turned back intodigital information for the receiving computer (see Figure 1).

Figure 1. Analog Modem

All modems use some form of compression and error correction.Compression algorithms enable throughput to be enhanced two to four timesover normal transmission. Error correction examines incoming data forintegrity and requests retransmission of a packet when it detects a problem.

2. THE ANALOG MODEM MARKET

The dynamics of the analog-modem market can be traced back to July 1968when, in its landmark Carterfone decision, the FCC ruled that the provi-sions prohibiting the use of customer-provided interconnecting devices wereunreasonable.

On January 1, 1969, AT&T revised its tariffs to permit the attachment of cus-tomer-provided devices (such as modems) to the public switched networksubject to three important conditions:

The customer-provided equipment was restricted to certain output power andenergy levels so as not to interfere with or harm the telephone network inany way.

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Asymmetric Digital Subscriber Line (ADSL)

The interconnection to the public switched network had to be made througha telephone companyprovided protective device, sometimes referred to as adata access arrangement (DAA).

All network-control signaling such as dialing, busy signals, and so on had to beperformed with telephone-company equipment at the interconnection point.

By 1976, the FCC had recommended a plan whereby current protectivedevices would be phased out in favor of a so-called registration plan.Registration would permit direct switched-network electrical connection ofequipment that had been inspected and registered by an independent agencysuch as the FCC as technically safe for use on the switched network.

In the post-war era, heavy emphasis on information theory led to the pro-found and now famous 1948 paper by Claude Shannon, providing us with aconcise understanding of channel capacity for power and bandlimited gauss-ian noise channels (i.e., our analog telephone channel).

C = Bw * Log2(1+S/N)

This simply states that the channel capacity, C, is equal to the available chan-nel bandwidth, Bw, times the log base 2 of 1 plus the signal-to-noise ratio inthat bandwidth. It does not explain how to accomplish this; it simply statesthat this channel capacity can be approached with suitable techniques.

As customers started buying and using modems, speed and reliability becameimportant issues. Each vendor tried to get as close to the limit expressed byShannons Law as possible. Until Recommendation V.32, all modem standardsseemed to fall short of this capacity by 9 to 10 db S/N. Estimates of the chan-nel capacity used assumed bandwidths of 2400 Hz to 2800 Hz, and S/N ratiosfrom 24 db to 30 db and generally arrived at a capacity of about 24,000 bitsper second (bps). It was clear that error-correction techniques would have tobecome practical before this gap would be diminished.

Modems of the 1950s were all proprietaryprimarily FSK (300 bps to 600bps) and vestigial sideband (1200 bps to 2400 bps). These devices used orwere built upon technology from RF radio techniques developed during thewartime era and applied to wireline communications.



International standardization of modems started in the 1960s. In the 1964Plenary, the first CCITT Modem Recommendation, V.21 (1964), a 200bpsFSK modem (and now 300 bps), was ratified and is (still) used in the V.34/V.8handshake. The preferred modulation progressed to 4-Phase (or 2X2 QAM) in1968, and to 4X4 QAM with V.22bis in 1984. Additionally, in 1984, the nextmajor technological advancement in modem recommendations came withV.32 and the addition of echo cancellation and trellis coding.

Trellis codes, first identified by Dr. Gottfred Ungerboeck, were a major break-through in that they made it practical to provide a level of forward error cor-rection to modems, realizing a coding gain of 3.5 db, and closing over a thirdof the gap in realizing the Shannon channel capacity. RecommendationV.32bis built on this and realized improvement in typical-connection S/Nratios and increased the data rates to 14,400 bps.

As work on V.34 started in earnest (1989/90), a recognition of furtherimprovement in the telephone networks in many areas of the world was evi-dent. With this recognition, the initial goal of 19,200 bps moved to 24,000 bpsand then to 28,800 bps. The newer V.34 (1996) modem supports 33,600 bps.Such modems achieve 10 bits per Hertz of bandwidth, a figure that approach-es the theoretical limits. Recently, a number of companies have introduced a56.6kbps analog modem designed to operate over standard phone lines.However, the modem is asymmetrical (it operates at normal modem speedson the upstream end), and it requires a dedicated T1/E1 connection to the ISPsite to consistently reach its theoretical limits. For users without such a line,the modem offers, inconsistently at best according to reports, a modest gainin performance.

However, the bandwidth limitations of voice bandlines are not a function ofthe subscriber line but the core network. Filters at the edge of the core net-work limit voice-grade bandwidth to approximately 3.3 kHz. Without suchfilters, the copper access wires can pass frequencies into the MHz regions.Attenuation determines the data rate over twisted-pair wire, and it, in turn, isa function of line length and frequency. Table 1 indicates the practical limitson data rates in one direction compared to line length.

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3. DIGITAL SUBSCRIBER LINE (DSL)

Despite its name, DSL does not refer to a physical line but to a modem orrather a pair of modems. A DSL modem pair creates a digital subscriber line,but the network does not purchase the lines when it buys ADSL. It alreadyowns the lines; it purchases modems.

A DSL modem transmits duplex (i.e., data in both directions simultaneously)at 160 kbps over copper lines of up to 18,000 feet. DSL modems use twisted-pair bandwidth from 0 to approximately 80 kHz, which precludes the simul-taneous use of analog telephone service in most cases (see Figure 2).

Figure 2. DSL Modem

T1 and E1In the early 1960s, Bell Labs engineers created a voice multiplexing systemthat digitized a voice sample into a 64kbps data stream (8,000 voltages sam-ples per second) and organized these into a 24-element framed data streamwith conventions for determining precisely where the 8-bit slots went at thereceiving end. The frame was 193 bits long and created an equivalent datarate of 1.544 Mbps. The engineers called their data stream DS1, but it hassince come to be known as T1. Technically, though, T1 refers to the raw datarate, with DS1 referring to the framed rate.

In Europe, the worlds public telephone networks other than AT&T modifiedthe Bell Lab approach and created E1a multiplexing system for 30 voicechannels running at 2.048 Mbps.

Unfortunately, T1/E1 is not really suitable for connection to individual resi-dences. The transmission protocol they used, alternate mark inversion (AMI),required tranceivers 3,000 feet from the central office and every 6,000 feetthereafter. AMI demands so much bandwidth and corrupts the cable spec-trum so much that telephone companies could use only one circuit in any 50-pair cable and none in any adjacent cables. Under these circumstances, pro-



viding high-bandwidth service to homes would be equivalent to installingnew wire.

4. XDSL

High Data-Rate Digital Subscriber Line (HDSL)

HDSL is simply a better way of transmitting T1/E1 over copper wires, usingless bandwidth without repeaters. It uses more advanced modulation tech-niques to transmit 1.544 Mbps over lines up to 12,000 feet long.

Single-Line Digital Subscriber Line (SDSL)

SDSL is a single-line version of HDSL, transmitting T1/E1 signals over a singletwisted pair and able to operate over the plain old telephone service (POTS)so that a single line can support POTS and T1/E1 at the same time. It fits themarket for residence connection, which must often work over a single tele-phone line. However, SDSL will not reach much beyond 10,000 feet. At thesame distance, ADSL reaches rates above 6 Mbps.


ADSL is intended to complete the connection with the customers premises.It transmits two separate data streams with much more bandwidth devotedto the downstream leg to the customer than returning. It is effective becausesymmetric signals in many pairs within a cable (as occurs in cables comingout of the central office) significantly limit the data rate and possible linelength.

ADSL succeeds because it takes advantage of the fact that most of its targetapplications (video-on-demand, home shopping, Internet access, remote LANaccess, multimedia, and PC services) function perfectly well with a relatively lowupstream data rate. MPEG movies require 1.5 or 3.0 Mbps downstream but needonly between 16 kbps and 64 kbps upstream. The protocols controlling Internetor LAN access require somewhat higher upstream rates but in most cases canget by with a 10 to 1 ratio of downstream to upstream bandwidth.

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5. THE MODEM MARKET

Sales in the modem business started out slowly until customers started buy-ing PCs. Likewise, costs were high until the volumes picked up. When the14.4kbps modem was first introduced, it cost $14,400 or one dollar per bit.Today, a much faster consumer-level modem with many more features costsonly $100 to $300, making it unusual for a home PC today to be without amodem.

Over the years, customers watched modem vendors evolve their products ona standards basis. This technique, although somewhat time consuming, wasimportant and led to significant feature enhancement. Initially, several modu-lation schemes were in use, but by the time the V.34 modem came out, all ofthe major modem-modulation schemes were combined in that standardgiving the customer one modem that could be used in many applications. Asthe modem market matured, customers became less concerned with the inter-nals of standards and more concerned with features, size, and flexibility.

As a result of the progress in analog-modem technology and with the adventof mass-market consumer-level PCs, there are over 500 million modems inthe world today.

The xDSL modem market will follow similar market patterns. Today, thingslike modulation schemes, the type of protocol supported to the home orsmall business, and costs of the units are the main topics. As the xDSL marketmatures, most likely in a fashion similar to that of the analog modem, cus-tomers will become less concerned with modulation and protocols. On theother hand, they will look for vendors that provide plug-and-play interoper-ability with their data equipment, ease of installation, the best operating char-acteristics on marginal lines, and minimalist size and power requirements.

6. ATM VERSUS IP TO THE DESKTOP

There is a great debate raging among potential service providers as towhether there should be standard IP10t connections or ATM connections totheir customers PCs. The two are very similarthe difference is in thespecifics of the equipment and not in the amount of equipment required.

There are various advantages to each method of network access.



IP Advantages 10t Ethernet is basically self-learning. Inexpensive LAN PC cards already exist. 10t is an industry standard. LAN networks are proven and work today. There is much expertise in this technology. PC software and OS drivers already interface to IPbased LANs.

ATM Advantages Streaming video transport has already been proven. Mixing of services (e.g., video, telephony, and data) is much easier. Traffic speeds conform to standard telephony transport rates (e.g., DS3,

STS1). New PC software and drivers will work with ATM.

The issue actually gets more interesting because both architectures usuallyinterface to an ATM backbone network for high-speed connections over awide area. Therefore, the real issues are the costs of building the network, theservices that are to be carried over it, and the time frame for the implementa-tion. If the need is for data services (Internet connections, work-at-home, andetc.), the obvious choice is an IP network. The hardware and softwarerequired to implement this network is available and relatively inexpensive.

ATM would be the solution for multiple mixed QoS service requirements inthe near future. It is true that the IP technology is being extended to offertiered QoS with RSVP, and IP telephony is being refined to operate more effi-ciently. The paradox, however, is that these standards do not exist today.ATM standards are quite complete. However, not all may be easily imple-mentable. In spite of this, there are many ATM networks in existence or cur-rently under construction.

This leaves the issue of costs. The true costs of creating and operating a large-scale data-access network are not known. True, there are portions that areunderstood, but many others are only projected. This creates great debateover which technology is actually less costly. The only way for the costs tobe really known is to build reasonably large networks and compare costs. Ifone technology is a clear winnera somewhat doubtful hypothesisthenuse that technology. If there is no clear cost advantage, then build the network with the service set that matches the service needs of the potential

9


customers. The issue is to start the implementation phase where the realanswers will be determined and subsequently end the interminable discussionphase.

7. CAP VERSUS DMT

These are the two primary xDSL standards over which much debate hasensued. Although the debate continues, the real action is taking place in themarketplace. CAP demonstrated a clear lead in getting the product to market.Chips were available in quantity, and they worked. Numerous products thatincorporated these chips are installed in a number of locations by serviceproviders. Standards and interoperability issues between vendors and imple-mentations are now being addressed.

DMT, on the other hand, has been in the standards arena for some time andcontinues to evolve. It is now considered a standard by a number of serviceproviders. This technology featured some innovations that were not originallyin the CAP feature set such as rate adaptation. On the other hand, the chipsare just now finding their way into products. Trial activities are only nowbeginning, and advanced chip sets that match the features of CAP chips arenow being promised for 3Q97.

The issue is which will win the market. The service providers who are build-ing the xDSL network will select the technology that meets their needs.Many vendors are offering products that use either technology. Some newchips are being announced that allow adaptation between either technology.The point here is that the technology of xDSL chips is not a roadblock todeployment. Either appears to work well, and true interoperability remains inthe future, much like midspan meets for SONET equipment.

8. THE FUTURE

Look at the past of analog modems to crystal-ball the future of xDSL.Standards were an issue with modems and will be an issue with xDSL prod-ucts. However, it is not obvious to a technologist what technology will winout. Remember that in the VCR arena, Betamax had the better-quality pic-ture, but VHS eventually won out. In any event, only the marketplace andtime will answer these questions.



9. SELF-TEST

Multiple Choice

1. ADSL increases existing twisted-pair access capacity by .

a. two-fold

b. three-fold

c. thirty-fold

d. fifty-fold

2. A modem translates .

a. analog signals into digital signals

b. digital signals into analog signals

c. both of the above

3. The 1948 theorem, which is the basis for understanding the relationship of chan-nel capacity, bandwidth, signal-to-noise ratio, is known as .

a. The Peter Principle

b. The Heisenberg Uncertainty Principle

c. Shannons Law

d. Boyles Law

4. What appears to be the practical limit for analog modems over the standardtelephone network?

a. 33 kbps b. 28.8 kbps c. 24 kbps d. 19.2 kbps

5. Digital subscriber line (DSL) refers to .

a. a specific gauge of wire used in modem communications

b. a modem enabling high-speed communications

c. a connection created by a modem pair enabling high-speed communications

d. a specific length of wire

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Multiple Choice

6 What is the source of limitation on the bandwidth of the public switched network?

a. subscriber line

b. the core network

7. The practical upper limit of line length of ADSL lines is .

a. 6,000 ft.

b. 12,000 ft.

c. 18,000 ft.

d. 36,000 ft.

8. T1 and DS1 refer to the same multiplexing system. Which one is generally usedto refer to the raw data rate?

a. T1

b. DS1

True or False

9. T1/E1 and HDSL are essentially equivalent technologies.

a. true

b. talse

10. ADSL cannot handle Internet or LAN access.

a. true

b. false



10. ACRONYM GUIDE

13


ADSL asymmetric digital subscriber line

AMI alternate mark inversion

ATM asynchronous transfer mode

CAP cellular array processor

DAA data access arrangement

DMT discrete multitone

DSL digital subscriber line

FCC Federal Communications Commission

HDSL high data-rate subscriber line

IP Internet Protocol

LAN local-area network

Modem modulation/demodulation

MPEG Motion Pictures Expert Group

QoS quality of service

SDSL single-line digital subscriber line

SONET synchronous optical network

Asynchronous Transfer Mode (ATM)Fundamentals

DEFINITION

Asynchronous transfer mode (ATM) is a high-performance, cell-orientedswitching and multiplexing technology that utilizes fixed-length packets tocarry different types of traffic. ATM is a technology that will enable carriersto capitalize on a number of revenue opportunities through multiple ATMclasses of services; high-speed local area network (LAN) interconnection;voice, video, and future multimedia applications in business markets in theshort term; and in community and residential markets in the longer term.

TUTORIAL OVERVIEW

Changes in the structure of the telecommunications industry and market con-ditions have brought new opportunities and challenges for network operatorsand public service providers. Networks that have been primarily focused onproviding better voice services are evolving to meet new multimedia commu-nications challenges and competitive pressures.

Services based on ATM and synchronous digital hierarchy/synchronous opti-cal network (SDH/SONET) architectures provide the flexible infrastructureessential for success in this evolving market (see Figure 1).

15

Figure 1. ATM Technology, Services, and Standards Pyramid

ATM, which was once envisioned as the technology of future public net-works, is now a reality, with service providers around the world introducingand rolling out ATM and ATMbased services. The ability to successfullyexploit the benefits of ATM technology within the public network will pro-vide strategic competitive advantage to both carriers and enterprises.

In addition to revenue opportunities, ATM reduces infrastructure coststhrough efficient bandwidth management, operational simplicity, and theconsolidation of overlay networks. Carriers can no longer afford to gothrough the financial burden and time required to deploy a separate networkfor each new service requirement (e.g., dedicating a network for a single ser-vice such as transparent LAN or frame relay). ATM technology will allowcore network stability while allowing service interfaces and other equipmentto evolve rapidly.



TOPICS

1. DEFINITION OF ATM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

2. BENEFITS OF ATM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19

3. ATM TECHNOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

4. ATM CLASSES OF SERVICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

5. ATM STANDARDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

6. ATM LAN EMULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27

7. VOICE OVER ATM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

8. VIDEO OVER ATM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

9. ATM TRAFFIC MANAGEMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32

10. ATM APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

11. NORTEL NETWORKS ATM VISION . . . . . . . . . . . . . . . . . . . . . . . . . .36

12. SELF-TEST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37

13. ACRONYM GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39

17

Asynchronous Transfer Mode (ATM) Fundamentals

1. DEFINITION OF ATM

ATM is a technology that has its history in the development of broadbandISDN in the 1970s and 1980s. Technically, it can be viewed as an evolution ofpacket switching. Like packet switching for data (e.g., X. 25, frame relay,transmission control protocol/Internet protocol [TCP/IP]), ATM integrates themultiplexing and switching functions, is well suited for bursty traffic (in con-trast to circuit switching), and allows communications between devices thatoperate at different speeds. Unlike packet switching, ATM is designed forhigh-performance multimedia networking. ATM technology has been imple-mented in a broad range of networking devices:

PC workstation and server network interface cards

switched-Ethernet and token-ring workgroup hubs

workgroup and campus ATM switches

ATM enterprise network switches

ATM multiplexors

ATMedge switches

ATMbackbone switches

ATM is also a capability that can be offered as an end-user service by serviceproviders (as a basis for tariffed services) or as a networking infrastructure forthese and other services. The most basic service building block is the ATMvirtual circuit, which is an end-to-end connection that has defined end pointsand routes but does not have bandwidth dedicated to it. Bandwidth is allocat-ed on demand by the network as users have traffic to transmit. ATM alsodefines various classes of service to meet a broad range of application needs.

ATM is also a set of international interface and signaling standards defined bythe International Telecommunications Union (ITU) TelecommunicationsStandards Sector. The ATM Forum has played a pivotal role in the ATM mar-ket since its formulation in 1991. The ATM Forum is an international volun-tary organization composed of vendors, service providers, research organiza-tions, and users. Its purpose is to accelerate the use of ATM products andservices through the rapid convergence of interoperability specifications, pro-motion of industry cooperation, and other activities. It does this by develop-ing multivendor implementation agreements.



2. BENEFITS OF ATM

The benefits of ATM are listed in Table 1 below.

Table 1. Benefits of ATM

The Advantages of ATM

high performance via hardware switching

dynamic bandwidth for bursty traffic

class-of-service support for multimedia

scalability in speed and network size

common LAN/WAN architecture

opportunities for simplification via VC architecture

international standards compliance

The high-level benefits delivered through ATM services deployed on ATMtechnology using international ATM standards can be summarized as follows:

high performance via hardware switching with terabit switches on the horizon

dynamic bandwidth for bursty traffic meeting application needs and deliver-ing high utilization of networking resources; most applications are or can beviewed as inherently bursty; data applications are LANbased and are verybursty; voice is bursty, as both parties are neither speaking at once nor allthe time; video is bursty, as the amount of motion and required resolutionvaries over time

class-of-service support for multimedia traffic allowing applications withvarying throughput and latency requirements to be met on a single network

19


scalability in speed and network size supporting link speeds of T1/E1 toOC12 (622 Mbps) today and into the multiGbps range before the end ofthe decade; networks that scale to the size of the telephone network (i.e.,as required for residential applications) are envisaged

common LAN/WAN architecture allowing ATM to be used consistentlyfrom one desktop to another; traditionally, LAN and WAN technologieshave been very different, with implications for performance and interoper-ability

opportunities for simplification via switched VC architecture; this is particu-larly true for LANbased traffic, which today is connectionless in nature;the simplification possible through ATM VCs could be in areas such asbilling, traffic management, security, and configuration management

international standards compliance in central-office and customer-premisesenvironments allowing for multivendor operation

3. ATM TECHNOLOGY

In ATM networks, all information is formatted into fixed-length cells consist-ing of 48 bytes (8 bits per byte) of payload and 5 bytes of cell header (seeFigure 2). The fixed cell size ensures that time-critical information such asvoice or video is not adversely affected by long data frames or packets. Theheader is organized for efficient switching in high-speed hardware implemen-tations and carries payload-type information, virtual-circuit identifiers, andheader error check.



Figure 2. Cell Structure

ATM is a connection-oriented technology. Organizing different streams oftraffic in separate cells allows the user to specify the resources required andallows the network to allocate resources based on these needs. Multiplexingmultiple streams of traffic on each physical facility (between the end user andthe network or between network switches)combined with the ability tosend the streams to many different destinationsenables cost savingsthrough a reduction in the number of interfaces and facilities required to con-struct a network.

ATM standards defined two types of ATM connections: virtual path connec-tions (VPCs), which contain virtual channel connections (VCCs).

21


A virtual channel connection (or virtual circuit) is the basic unit, which carriesa single stream of cells, in order, from user to user.

A collection of virtual circuits can be bundled together into a virtual path con-nection. A virtual path connection can be created from end-to-end across anATM network. In this case, the ATM network does not route cells belongingto a particular virtual circuit. All cells belonging to a particular virtual path arerouted the same way through the ATM network, thus resulting in fasterrecovery in case of major failures.

An ATM network also uses virtual paths internally for the purpose ofbundling virtual circuits between switches. Two ATM switches may havemany different virtual channel connections between them, belonging to dif-ferent users. These can be bundled by the two ATM switches into a virtualpath connection. This can serve the purpose of a virtual trunk between thetwo switches. This virtual trunk can then be handled as a single entity by,perhaps, multiple intermediate virtual path cross connects between the twovirtual circuit switches.

Virtual circuits can be statically configured as permanent virtual circuits(PVCs) or dynamically controlled via signaling as switched virtual circuits(SVCs). They can also be point-to-point or point-to-multipoint, thus providinga rich set of service capabilities. SVCs are the preferred mode of operationbecause they can be dynamically established, thus minimizing reconfigurationcomplexity.

4. ATM CLASSES OF SERVICES

ATM is connection oriented and allows the user to dynamically specify theresources required on a per-connection basis (per SVC). Five classes of serviceare defined for ATM (as per ATM Forum UNI 4.0 specification). The QoSparameters for these service classes are summarized in Table 2:



Table 2: ATM Service Classes

Service Class

Constant bit rate (CBR) This class is used for emulating circuitswitching. The cell rate is constant withtime. CBR applications are quite sensitive to cell-delay variation. Examples of appli-cations that can use CBR are telephone traffic (i.e., nx64 kbps), videoconferencing, and television.

Variable bit ratenon This class allows users to send traffic at a real time (VBRNRT) rate that varies with time depending on the

availability of user information. Statistical multiplexing is provided to make optimumuse of network resources. Multimedia e-mail is an example of VBRNRT.

Variable bit rate This class is similar to VBR-NRT but is real time (VBRRT) designed for applications that are sensitive

to cell-delay variation. Examples for real-time VBR are voice with speech activity detection (SAD) and interactive com-pressed video.

Available bit rate (ABR) This class of ATM services provide rate-based flow control and is aimed at data traffic such as file transfer and e-mail. Although the standard does not require thecell transfer delay and cell-loss ratio to be guaranteed or minimized, it is desirable forswitches to minimize delay and loss as much as possible. Depending on the state of congestion in the network, the source is required to control its rate. The users are allowed to declare a minimum cell rate, which is guaranteed to the connection by the network.

Unspecified bit rate This class is the catch-all other class, and (UBR) is widely used today for TCP/IP.

23


The ATM Forum has identified the following technical parameters to be asso-ciated with a connection. These terms are outlined in Table 3:

Table 3: ATM Technical Parameters

Technical Parameter Definition

Cell-loss ratio (CLR) Cell-loss ratio is the percentage of cells notdelivered at their destination because they were lost in the network as a result of congestion and buffer overflow.

Cell-transfer delay (CTD) The delay experienced by a cell between network entry and exit points is called the cell-transfer delay. It includes propagation delays, queuing delays at various intermediate switches, and service times atqueuing points.

Cell-delay variation (CDV) Cell-delay variation is a measure of the variance of the cell-transfer delay. High variation implies larger buffering for delay-sensitive traffic such as voice and video.

Peak cell rate (PCR) Peak cell rate is the maximum cell rate at which the user will transmit. PCR is the inverse of the minimum cell interarrival time.

Sustained cell rate (SCR) This is the average rate, as measured over a long interval, in the order of the connection lifetime.

Burst tolerance (BT) This parameter determines the maximum burst that can be sent at the peak rate. This is the bucket-size parameter for the enforcement algorithm that is used to control the traffic entering the network.

Finally, there are a number of ATM classes of service. These classes are alloutlined in Table 4 below:



Table 4: ATM Classes of Services

Services CBR VBRNRT VBRRT ABR UBR

CLR cell-loss ratio Yes Yes Yes Yes No

CTD cell-transfer Yes No Yes No Nodelay

CDV cell-delay Yes Yes Yes No Novariation

PCR peak cell rate Yes Yes Yes No Yes

SCR sustained cell rate No Yes Yes No No

Burst tolerance @ PCR No Yes Yes No No

Flow control No No No Yes No

Its extensive class-of-service capabilities makes ATM the technology of choicefor multimedia communications.

5. ATM STANDARDS

The ATM Forum has identified a cohesive set of specifications that provides astable ATM framework. The first and most basic ATM standards are thosewhich provide the end-to-end service definitions as described in Section 4.An important ATM standard and service concept is that of service interwork-ing between ATM and frame relay (a fast-growing pervasive service), where-by ATM services can be seamlessly extended to lower-speed frame-relayusers. Frame relay is a network technology that is also based on virtual cir-cuits using variable-length frame transmission between users.

ATM user network interface (ATM UNI) standards specify how a user con-nects to the ATM network to access these services. A number of standardshave been defined for T1/E1, 25 Mbps, T3/E3, OC3 (1.55 Mbps), and OC12with OC48 (2.4 Gbps) in the works. OC3 interfaces have been specified foruse over single-mode fiber (for wide-area applications) and over unshieldedtwisted pair or multimode fiber for lower-cost-in-building applications.

25


Two ATM networking standards have been defined that provide connectivitybetween network switches and between networks:

Broadband intercarrier interface (BICI) and PNNI (the P being for publicor private, while NNI is for network-to-network interface or node-to-node interface).

PNNI is the more feature-rich of the two and supports class of servicesensi-tive routing and bandwidth reservation. It provides topology-distributionmechanisms based on advertisement of link metrics and attributes, includingbandwidth metrics. It uses a multilevel hierarchical routing model providingscalability to large networks. Parameters used as part of the path-computationprocess include the destination ATM address, traffic class, traffic contract,QoS requirements, and link constraints. Metrics that are part of the ATMrouting system are specific to the traffic class and include quality ofservicerelated metrics (e.g., CTD and CLR) and bandwidth-related metrics(e.g., PCR). The path computation process includes overall network-impactassessment, avoidance of loops, minimization of rerouting attempts, and useof policy (inclusion/exclusion in rerouting, diverse routing, and carrier selec-tion). Connection admission controls (CACs) define procedures used at theedge of the network, whereby the call is accepted or rejected based on theability of the network to support the requested QoS. Once a VC has beenestablished across the network, network resources must be held and qualityservice guaranteed for the duration of the connection.

All ATM traffic is carried in cells, yet no applications use cells. So, specificways of putting the data into cells are defined to enable the receiver to recon-struct the original traffic. Three important schemes are highlighted in Figure 3and discussed in detail later in the tutorial.

1. RFC1483 specifies how interrouter traffic is encapsulated into ATM usingATM adaptation layer 5 (AAL5). AAL5 is optimized for handling framed traf-fic and has similar functionality to that provided by HDLC framing in framerelay, SDLC, and X.25.

2. ATM LAN emulation (LANE) and multiprotocol over ATM (MPOA) aredesigned to support dynamic use of ATM SVCs primarily for TCP/IP. LANE isa current standard that is widely deployed and will be a subset of the MPOAstandard. It will be discussed later in the tutorial.



3. Voice and video adaptation schemes can use AAL1, which is defined forhigh efficiency (i.e., for traffic that has no natural breaks, such as a circuit car-rying bits at a fixed rate).

Figure 3. Three ATM Schemes

6. ATM LAN EMULATION

ATMbased Ethernet switches and ATM workgroup switches are beingdeployed by end users at various corporate sites. The most widely used set ofstandards in local ATM environments is ATM LAN emulation (LANE) (seeFigure 4). ATM LAN emulation is used to make the ATM SVC networkappear to be a collection of virtual-Ethernet/IEEE 802.3 and token-ring/IEEE802.5 LANs. The replication of most of the characteristics of existing LANsmeans that LAN emulation enables existing LAN applications to run overATM transparently, this latter characteristic leading to its wide deployment.In ATM LAN emulation, most unicast LAN traffic moves directly betweenclients over direct ATM SVCs, while multicast traffic is handled via a serverfunctionality. Bridging is used to interconnect real LANs and emulated LANsrunning on ATM, while routing is used to interconnect ATMemulated LANsand other WAN or LAN media for purposes of routing scalability, protocolspoofing, or security firewalls.

27


Figure 4. ATM LAN Emulation (LANE)

The ATM Forum LANE implementation agreement specifies two types ofLANE network components connected to an ATM network:

LANE Network Components in an ATM Network

1. LANE clients that function as end systems such as the following:

computers with ATM interfaces that operate as file servers

end-user workstations or personal computers

Ethernet or token-ring switches that support ATM networking

routers, bridges, and ATM ENS with membership in an emulated ATM LAN

2. LANE servers that support ATM LANE service for configuration management, multicast support, and address resolution

The LANemulation service may be implemented in the same devices asclients or involve other ATM network devices. The communications interface,LAN emulation user-network interface (LUNI), is the sequence and contentsof the messages that the clients ultimately use to transfer traffic of the typeexpected on IEEE 802.3/5 LANs. The component of the LANemulation



service that deals with initialization (i.e., emulates plugging the terminal intoa LAN hub), is the LAN emulation configuration server (LECS). It directs aclient to connect to a particular LAN emulation server (LES). The LES is thecomponent of the LANemulation service that performs the address registra-tion and resolution. The LES is responsible for mapping IEEE 48-bit MACaddresses and token-ring route descriptors to ATM addresses. One veryimportant MAC address for clients is the MAClayer broadcast address thatis used to send traffic to all locations on a LAN. In LAN emulation, this func-tion is performed by the broadcast and unknown server (BUS). ATM LANEis a comprehensive set of capabilities that has been widely deployed in ATMnetworks.

ATM LANE is an element of the multiple protocol over ATM (MPOA) archi-tecture that is being defined by the ATM Forum. This work is addressingencapsulation of multiple protocols over ATM, automatic address resolution,and the routing issues associated with minimizing multiple router hops inATM networks.

7. VOICE OVER ATM

As real-time voice services have traditionally been supported in the WAN viacircuit-based techniques (e.g., via T1 multiplexors or circuit switching), it is nat-ural to map these circuits to ATM CBR PVCs using circuit emulation and ATMadaptation Layer 1 (AAL1). However, there are significant disadvantages inusing circuit emulation in that the bandwidth must be dedicated for this type oftraffic (whether there is useful information being transmitted or not), providinga disincentive for corporate users to implement circuit emulation as a long-termstrategy. For example a T1 1.544 Mbps circuit requires 1.74 Mbps of ATMbandwidth when transmitted in circuit-emulation mode. This does not down-play its importance as a transitional strategy to address the installed base.

As technology has evolved, the inherent burstiness of voice and many real-time applications can be exploited (along with sophisticated compressionschemes) to significantly decrease the cost of transmission through the use ofVBRRT connections over ATM.

29


VBR techniques for voice exploit the inherently bursty nature of voice com-munication, as there are silence periods that can result in increased efficiency.These silence periods (in decreasing levels of importance) arise as follows:

when no call is up on a particular trunk; that is, the trunk is idle during off-peak hours; trunks are typically engineered for a certain call-blocking proba-bility; at night, all the trunks could be idle

when the call is up, but only one person is talking at a given time

when the call is up, and no one is talking

Work is just starting in the ATM Forum on ATM adaptation for VBR voice.

The addition of more bandwidth-effective voice coding (e.g., standard voice is coded using 64kbps PCM) is economically attractive, particularly overlong-haul circuits and T1 ATM interfaces. Various compression schemes havebeen standardized in the industry (e.g., G720 series of standards). Makingthese coding schemes dynamic provides the network operator with theopportunity to free up bandwidth under network-congestion conditions. Forexample, with the onset of congestion, increased levels of voice compressioncould be dynamically invoked, thus freeing up bandwidth and potentiallyalleviating the congestion while diminishing the quality of the voice duringthese periods.

A further enhancement to the support of voice over ATM is to support voiceswitching over SVCs. This entails interpreting PBX signaling and routingvoice calls to the appropriate destination PBX (see Figure 5). The advantagefrom a traffic management perspective is that connection admission controlscan be applied to new voice calls; under network congestion conditions, thesecalls could be rerouted over the public network and therefore not cause addi-tional levels of congestion.

The ATM Forum is currently focusing its efforts on voice handled on CBRSVCs. VBRRT voice is a future standards activity.



Figure 5. Interpreting PBX Singaling and Routing Voice Calls

8. VIDEO OVER ATM

While circuit-based videoconferencing streams (including motion JPEG run-ning at rates around 10 Mbps) can be handled by standard circuit emulationusing AAL1, the ATM Forum has specified the use of VBRRT VCs usingAAL5 for MPEG2 on ATM for video-on-demand applications, as thisapproach makes better use of networking resources.

MPEG is a set of standards addressing coding of video and surround-soundaudio signals and synchronization of video and audio signals during the play-back of MPEG data. It runs in the 2 Mbps to 15 Mbps range (with burstsabove these rates) corresponding to VCR and broadcast quality respectively.The initial MPEG standard (MPEG1) was targeted at VHSquality video andaudio. MPEG2 targets applications requiring broadcast-quality video andaudio and HDTV. MPEG2 coding can result in one of two modes:

(i) Program streams: variable-length packets that carry a single program ormultiple programs with a common time base

(ii) Transport streams: 188-byte packets that contain multiple programs (forexamples, see Figure 6)

31


Figure 6. Transport Streams with Multiple Programs

In both cases, time stamps are inserted into MPEG2 packets during the encod-ing and multiplexing process. MPEG2 assumes a constant-delay model acrossthe network, thus allowing the decoder to follow the original encoder sourceclock exactly. Due to the cost of coding, MPEG2 is primarily used in a nonin-teractive broadcast mode, as would be the case for a point-to-multipointbroadcast in residential video-on-demand applications and in a business TVapplication for training or employee communications.

9. ATM TRAFFIC MANAGEMENT

Broadly speaking, the objectives of ATM traffic management are to deliverquality-of-service guarantees for the multimedia applications and provideoverall optimization of network resources. Meeting these objectives enablesenhanced classes of service and offers the potential for service differentiationand increased revenues while simplifying network operations and reducingnetwork cost.

ATM traffic management and its various functions can be categorized intothree distinct elements based on timing requirements:

First are nodal-level controls that operate in real time. These are implementedin hardware and include queues supporting different loss and delay priorities,fairly weighted queue-servicing algorithms, and rate controls that provide



policing and traffic shaping. Well-designed switch-buffer architectures andcapacity are critical to effective network operation. Actual network experi-ence and simulation have indicated that large, dynamically allocated outputbuffers provide the flexibility to offer the best price performance for support-ing various traffic types with guaranteed QoS. Dynamically managing bufferspace means that all shared buffer space is flexibly allocated to VCs on an as-needed basis. Additionally, per virtual connection (VC) queuing enables trafficshaping and early and partial packet-level discard have been shown to signifi-cantly improve network performance.

Second, network-level controls operate in near real time. These are typically,but not exclusively, implemented in software, including connection admissioncontrol (CAC) for new connections, network routing and rerouting systems,and flow-control-rate adaptation schemes. Network-level controls are theheart of any traffic-management system. Connection admission controls sup-port sophisticated equivalent-bandwidth algorithms with a high degree ofconfiguration flexibility, based on the cell rate for CBR VCs, on average cellrate plus a configurable increment for VBR VCs, and on minimum cell rate forABR VCs. Dynamic class-of-service routing standards define support for fullydistributed link-state routing protocols, autoreconfiguration on failure and oncongestion, and dynamic load spreading on trunk groups.

Flow control involves adjusting the cell rate of the source in response to con-gestion conditions and requires the implementation of closed loop congestionmechanisms. This does not apply to CBR traffic. For VBR and UBR traffic,flow control is left as a CPE function. With ABR, resource management (RM)cells are defined that allow signaling of the explicit rate to be used by trafficsources. This is termed rate-based flow control. ABR is targeted at thoseapplications that do not have fixed or predictable bandwidth requirementsand require access to any spare bandwidth as quickly as possible while expe-riencing low cell loss. This allows network operators to maximize the band-width utilization of their network and sell spare capacity to users at a sub-stantial discount while still providing QoS guarantees. To enhance the effec-tiveness of network-resource utilization, the ABR standard provides for end-to-end, segment-by-segment, and hop-by-hop service adaptation.

Third, network engineering capabilities that operate in nonreal time supportdata collection, configuration management, and planning tools (see Figure 7).

33


Figure 7. Services Requested and Provided

10. ATM APPLICATIONS

ATM technologies, standards, and services are being applied in a wide rangeof networking environments, as described briefly below (see Figure 8).

Figure 8. ATM Networking Environments



ATM services: Service providers globally are introducing or already offer-ing ATM services to their business users.

ATM workgroup and campus networks: Enterprise users are deployingATM campus networks based on the ATM LANE standards. WorkgroupATM is more of a niche market with the wide acceptance of switched-Ethernet desktop technologies.

ATM enterprise network consolidation: A new class of product hasevolved as an ATM multimedia network-consolidation vehicle. It is calledan ATM enterprise network switch. A full-featured ATM ENS offers abroad range of in-building (e.g., voice, video, LAN, and ATM) and wide-area interfaces (e.g., leased line, circuit switched, frame relay, and ATM atnarrowband and broadband speeds) and supports ATM switching, voicenetworking, frame-relay SVCs, and integrated multiprotocol routing.

Multimedia virtual private networks and managed services: Serviceproviders are building on their ATM networks to offer a broad range of ser-vices. Examples include managed ATM, LAN, voice and video services(these being provided on a per-application basis, typically including cus-tomer-located equipment and offered on an end-to-end basis) and full-ser-vice virtual private-networking capabilities (these include integrated multi-media access and network management).

Frame relay backbones: Frame-relay service providers are deploying ATMbackbones to meet the rapid growth of their frame-relay services to use as anetworking infrastructure for a range of data services and to enable framerelay to ATM service interworking services.

Internet backbones: Internet service providers are likewise deployingATM backbones to meet the rapid growth of their frame-relay services, touse as a networking infrastructure for a range of data services, and to enableInternet class-of-service offerings and virtual private intranet services.

Residential broadband networks: ATM is the networking infrastructureof choice for carriers establishing residential broadband services, driven bythe need for highly scalable solutions.

Carrier infrastructures for the telephone and private-line networks:Some carriers have identified opportunities to make more effective use oftheir SONET/SDH fiber infrastructures by building an ATM infrastructureto carry their telephony and private-line traffic.

35


11. NORTEL NETWORKS ATM VISION

Nortel Networks believes that ATM is the only viable backbone networkingtechnology that can meet the objective of making multimedia calls as easy,reliable, and secure as voice calls are today.

ATM, coupled with SONET/SDH for fiber transport, sits at the core ofNortels long-term architectural vision. That vision embraces various residen-tial, business, and mobile access arrangements with a set of voice/data/videoand, ultimately, multimedia servers. There will be many ways of accessingATM networks including desktop ATM, switched Ethernet, wireless, andxDSL, to name a few. The vision includes extensive support of multiple class-es of service for native ATM, IPbased, frame-relay-based, and circuit-basedapplications. ATM accommodates the inherently bursty nature of data, voice,and video applications and the compressibility of these traffic types forincreased storage and bandwidth effectiveness. Nortel also believes that framerelay and ATM, both being virtual circuit based, provide a service continuumsupporting the broadest sets of speeds from sub64 kbps all the way to Gbps.Finally, Nortel envisages a family of application servers around the peripheryof this network to provide a range of data, image, video, and voice servicesthat take advantage of increasing insensitivity of the network to distance(see Figure 9).

Figure 9. Nortels ATM Architectural Vision



37


12. SELF-TEST

1. Which is better suited to handle bursty traffic?

a. circuit switching

b. ATM

2. Which organization is currently responsible for international signaling and inter-face standards?

a. ITU

b. CCITT

3. In ATM networks all information is formatted into fixed-length cells consisting ofhow many bytes?

a. 48 bytes

b. 64 bytes

4. The basic connection unit in an ATM network is known as the

a. virtual channel connection

b. virtual path connection

5. Which virtual circuit connection is the preferred mode of operation in an ATM net-work?

a. permanent virtual circuits

b. switched virtual circuits

6. CBR is used for .

a. multimedia e-mail

b. videoconferencing

7. Which ATM networking standard supports the greater range of features?

a. PNNI

b. BICI

8. In ATM LAN emulation, most unicast LAN traffic moves directly between clientsover .

a. servers

b. direct ATM SVCs

9. The initial MPEG standard (MPEG1) was targeted at .

a. VHSquality video and audio

b. broadcast-quality video and audio

10. ATM timing requirements feature how many elements?

a. 3

b. 4

c. 64k



39


AAL1 ATM adaption Layer 1





ABR available bit rate


ATM UNI ATM user network interface

BT burst tolerance

CAC connection admission control

CBR constant bit rate

CCITT Comit Consultif Internationale de Telegraphique andTelephonique

CDV cell-delay variation

CLR cell-loss ratio

CTD cell-transfer delay

IEEE Institute of Electrical and Electronic Engineers

ITUTSS International Telecommunications UnionTelecommunications Standards Sector


LANE LAN emulation

LES LAN emulation server

LUNI LAN emulation user-network interface

MPOA multiple protocol over ATM

PCR peak cell rate

P-NNI public network to network interface

PVC permanent virtual circuit

13. ACRONYM GUIDE



RM resource management

SAD speech activity detection

SCR sustained cell rate

SDH/ synchronous digital SONET hierarchy/synchronous optical network

SVC switched virtual circuit

TCP/IP transmission control protocol/Internet protocol

UBR unspecified bit rate

VBRNRT variable bit ratenonreal time

VCC virtual-channel connections

VPC virtual-path connections

41

Cable Modems

DEFINITION

Cable modems are devices that allow high-speed access to the Internet via acable television network. While similar in some respects to a traditional ana-log modem, a cable modem is significantly more powerful, capable of deliver-ing data approximately 500 times faster.

TOPICS

1. HOW CABLE MODEMS WORK . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43

2. CABLE DATA SYSTEM FEATURES . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3. CABLE DATA NETWORK ARCHITECTURE . . . . . . . . . . . . . . . . . . . .46

4. CABLE DATA NETWORK STANDARDS . . . . . . . . . . . . . . . . . . . . . . .49

5. SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52

6. SELF-TEST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52

7. ACRONYM GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55



1. HOW CABLE MODEMS WORK

Current Internet access via a 28.8. 33.6, or 56 kbps modem is referred to asvoiceband modem technology. Like voiceband modems, cable modems mod-ulate and demodulate data signals. However, cable modems incorporate morefunctionality suitable for todays high-speed Internet services. In a cable net-work, data from the network to the user is referred to as downstream, where-as data from the user to the network is referred to as upstream. From a userperspective, a cable modem is a 64/256 QAM RF receiver capable of deliver-ing up to 30 to 40 Mbps of data in one 6 MHz cable channel. This is approxi-mately 500 times faster than a 56 kbps modem. Data from a user to the net-work is sent in a flexible and programmable system under control of theheadend. The data is modulated using a QPSK/16 QAM transmitter with datarates from 320 kbps up to 10 Mbps. The upstream and downstream data ratesmay be flexibly configured using cable modems to match subscriber needs.For instance, a business service can be programmed to receive as well astransmit higher bandwidth. A residential user, however, may be configured toreceive higher bandwidth access to the Internet while limited to low band-width transmission to the network.

A subscriber can continue to receive cable television service while simultane-ously receiving data on cable modems to be delivered to a personal computer(PC) with the help of a simple one-to-two splitter (see Figure 1). The data ser-vice offered by a cable modem may be shared by up to sixteen users in alocal area network (LAN) configuration.

Figure 1: Cable Modem at the Subscriber Location

43

Cable Modems


Because some cable networks are suited for broadcast television services,cable modems may use either a standard telephone line or a QPSK/16 QAMmodem over a two-way cable system to transmit data upstream from a userlocation to the network. When a telephone line is used in conjunction with aone-way broadcast network, the cable data system is referred to as a telepho-ny return interface (TRI) system. In this mode, a satellite or wireless cabletelevision network can also function as a data network.

At the cable headend, data from individual users is filtered by upstreamdemodulators (or telephone-return systems, as appropriate) for further pro-cessing by a cable modem termination system (CMTS). A CMTS is a dataswitching system specifically designed to route data from many cable modemusers over a multiplexed network interface. Likewise, a CMTS receives datafrom the Internet and provides data switching necessary to route data to thecable modem users. Data from the network to a user group is sent to a64/256 QAM modulator. The result is user data modulated into one 6 MHzchannel, which is the spectrum allocated for a cable television channel such asABC, NBC, or TBS for broadcast to all users (see Figure 2).

Figure 2: Cable Modem Termination System and Cable HeadendTransmission

A cable headend combines the downstream data channels with the video,pay-per-view, audio, and local advertiser programs that are received by tele-vision subscribers. The combined signal is then transmitted throughout thecable distribution network. At the user location, the television signal isreceived by a set top box, while user data is separately received by a cablemodem box and sent to a PC.


Cable Modems

A CMTS is an important new element for support of data services that inte-grates upstream and downstream communication over a cable data net-work. The number of upstream and downstream channels in a given CMTScan be engineered based on serving area, number of users, data rates offeredto each user, and available spectrum.

Another important element in the operations and day-to-day managementof a cable data system is an element management system (EMS). An EMS isan operations system designed specifically to configure and manage aCMTS and associated cable modem subscribers. The operations tasksinclude provisioning, day-to-day administration, monitoring, alarms, andtesting of various components of a CMTS. From a central network opera-tions center (NOC), a single EMS can support many CMTS systems in thegeographic region.

Figure 3: Operations and Management of Cable Data Systems

2. CABLE DATA SYSTEM FEATURES

Beyond modulation and demodulation, a cable modem incorporates manyfeatures necessary to extend broadband communications to wide area net-works (WANs). The network layer is chosen as Internet protocol (IP) to sup-port the Internet and World Wide Web services. The data link layer is com-prised of three sublayers: logical link control sublayer, link security sublayerconforming to the security requirements, and media access control (MAC)sublayer suitable for cable system operations. Current cable modem systemsuse Ethernet frame format for data transmission over upstream and down-stream data channels. Each of the downstream data channels and the associ-

45


ated upstream data channels on a cable network form an extended EthernetWAN. As the number of subscribers increases, a cable operator can add moreupstream and downstream data channels to support demand for additionalbandwidth in the cable data network. From this perspective, growth of newcable data networks can be managed in much the same fashion as the growthof Ethernet LANs within a corporate environment.

The link security sublayer requirements are further defined in three sets ofrequirements: baseline privacy interface (BPI), security system interface (SSI),and removable security module interface (RSMI). BPI provides cable modemusers with data privacy across the cable network by encrypting data trafficbetween users cable modem and CMTS. The operational support providedby the EMS allows a CMTS to map a cable modem identity to paying sub-scribers and thereby authorize subscriber access to data network services.Thus, the privacy and security requirements protect user data as well as pre-vent theft of cable data services.

Early discussions in the Institute of Electrical and Electronic Engineers (IEEE)802.14 Committee referred to the use of asynchronous transfer mode (ATM)over cable data networks to facilitate multiple services including telephone,data, and video, all of which are supported over cable modems. Although cur-rent cable modem standards incorporate Ethernet over cable modem, exten-sions are provided in the standards for future support of ATM or other proto-col data units. IPtelephony support over cable data networks is expected tobe a new value-added service in the near term.

3. CABLE DATA NETWORK ARCHITECTURE

Cable data network architecture is similar to that of an office LAN. A CMTSprovides extended an Ethernet network over a WAN with a geographic reachup to 100 miles. The cable data network may be fully managed by the localcable operations unit. Alternatively, all operations may be aggregated at aregional data center to realize economies of scale. A given geographic or met-ropolitan region may have a few cable television headend locations that areconnected together by fiber links. The day-to-day operations and manage-ment of a cable data network may be consolidated at a single location, suchas a super hub, while other headend locations may be economically managedas basic hubs (see Figure 4).


Cable Modems

Figure 4: Basic Distribution Hub

A basic distribution hub is a minimal data network configuration that existswithin a cable television headend. A typical headend is equipped with satel-lite receivers, fiber connections to other regional headend locations, andupstream RF receivers for the pay-per-view and data services. The minimaldata network configuration includes a CMTS system capable of upstream anddownstream data transport and an IP router to connect to the super hub loca-tion (see Figure 5).

Figure 5: Super Hub

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A super hub is a cable headend location with additional temperature-con-trolled facilities to house a variety of computer servers, which are necessaryto run cable data networks. The servers include file transfer, user authoriza-tion and accounting, log control (syslog), IP address assignment and adminis-tration (DHCP servers), DNS servers, and data over cable service interfacespecifications (DOCSIS) control servers. In addition, a super hub may deployoperations support and network management systems necessary for the tele-vision as well as data network operations.

User data from basic and super hub locations is received at a regional datacenter for further aggregation and distribution through out the network (seeFigure 6). A super hub supports dynamic host configuration protocol (DHCP),DNS (domain name server), and log control servers necessary for the cabledata network administration. A regional data center provides connectivity tothe Internet and the World Wide Web and contains the server farms necessaryto support Internet services. These servers include e-mail, web hosting, news,chat, proxy, caching, and streaming media servers.

Figure 6: Regional Data Center

In addition to cable data networks, a regional data center may also supportdial-up modem services (e.g., 56kbps service) and business-to-businessInternet services. A network of switching, routers, and servers is employed atthe regional data center to aggregate dial-up, high-speed and business Internetservices.


Cable Modems

A super hub and a regional data center may be collocated and managed as asingle business entity. A super hub is managed by a cable television serviceprovider (TCI), while the regional data center is managed as a separate andindependent business (@home). In some regions, an existing Internet serviceprovider (ISP) may provide regional data center support for many basic andsuper hub locations managed by independent cable data network providers.

A regional data center is connected to other regional data centers by a nation-al backbone network (see Figure 7). In addition each regional data center isalso connected to the Internet and World Wide Web services. Traffic betweenthe regional networks, the Internet and all other regional networks is aggre-gated through the regional data center.

Figure 7: National Network

4. CABLE DATA NETWORK STANDARDS

A cable data system is comprised of many different technologies and stan-dards. To develop a mass market for cable modems, products from differentvendors must be interoperable.

To accomplish the task of interoperable systems, the North American cabletelevision operators formed a limited partnership, Multimedia Cable NetworkSystem (MCNS), and developed an initial set of cable modem requirements(DOCSIS). MCNS was initially formed by Comcast, Cox, TCI, Time Warner,Continental (now MediaOne), Rogers Cable, and CableLabs. The DOCSIS

49


requirements are now managed by CableLabs. Vendor equipment complianceto the DOCSIS requirements and interoperability tests are administered by aCableLabs certification program.

For further details see http://www.cablemodem.com

Some of the details of cable modem requirements are listed below.

PHYSICAL LAYER

Downstream Data Channel: At the cable modem physical layer, downstreamdata channel is based on North American digital video specifications (i.e.,International Telecommunications Union [ITU]T Recommendation J.83Annex B) and includes the following features:

64 and 256 QAM

6 MHzoccupied spectrum that coexists with other signals in cable plant

concatenation of Reed-Solomon block code and Trellis code, supports operation in a higher percentage of the North American cable plants

variable length interleaving supports, both latency-sensitive and latency-insensitive data services

contiguous serial bit-stream with no implied framing, provides complete physical (PHY) and MAC layer decoupling

Upstream Data Channel: The upstream data channel is a shared channelfeaturing the following:

QPSK and 16 QAM formats

multiple symbol rates

data rates from 320 kbps to 10 Mbps

flexible and programmable cable modem under control of CMTS

frequency agility

time-division multiple access

support of both fixed-frame and variable-length protocol data units


Cable Modems

programmable Reed-Solomon block coding

programmable preambles

MAC LAYER

The MAC layer provides the general requirements for many cable modemsubscribers to share a single upstream data channel for transmission to thenetwork. These requirements include collision detection and retransmission.The large geographic reach of a cable data network poses special problems asa result of the transmission delay between users close to headend versus usersat a distance from cable headend. To compensate for cable losses and delay asa result of distance, the MAC layer performs ranging, by which each cablemodem can assess time delay in transmitting to the headend. The MAC layersupports timing and synchronization, bandwidth allocation to cable modemsat the control of CMTS, error detection, handling and error recovery, and pro-cedures for registering new cable modems.

Privacy: Privacy of user data is achieved by encrypting link-layer data betweencable modems and CMTS. Cable modems and CMTS headend controllerencrypt the payload data of link-layer frames transmitted on the cable net-work. A set of security parameters including keying data is assigned to a cablemodem by the Security Association (SA). All of the upstream transmissionsfrom a cable modem travel across a single upstream data channel and arereceived by the CMTS. In the downstream data channel a CMTS must selectappropriate SA based on the destination address of the target cable modem.Baseline privacy employs the data encryption standard (DES) block cipher forencryption of user data. The encryption can be integrated directly within theMAC hardware and software interface.

NETWORK LAYER

Cable data networks use IP for communication from cable modem to the net-work. The Internet Engineering Task Force (IETF) DHCP forms the basis forall IP address assignment and administration in the cable network. A networkaddress translation (NAT) system may be used to map multiple computersthat use a single high-speed access via cable modem.

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TRANSPORT LAYER

Cable data networks support both transmission control protocol (TCP) anduser datagram protocol (UDP) at the transport layer.

APPLICATION LAYER

All of the Internet-related applications are supported here. These applicationsinclude e-mail, ftp, tftp, http, news, chat, and signaling network managementprotocol (SNMP). The use of SNMP provides for management of the CMTSand cable data networks.

OPERATIONS SYSTEM

The operations support system interface (OSSI) requirements of DOCSISspecify how a cable data network is managed. To date, the requirementsspecify a RF MIB. This enables system vendors to develop an EMS to supportspectrum management, subscriber management, billing, and other operations.

5. SUMMARY

Cable modem technology offers high-speed access to the Internet and WorldWide Web services. Cable data networks integrate the elements necessary toadvance beyond modem technology and provide such measures as privacy,security, data networking, Internet access, and quality of service features. Theend-to-end network architecture enables a user cable modem to connect to aCMTS which, in turn, connects to a regional data center for access to Internetservices. Thus, through a system of network connections, a cable data networkis capable of connecting users to other users anywhere in the global network.

6. SELF-TEST QUESTIONS

1. In addition to their other functions, cable modems function as 64/256 QAM receiversand QPSK/16 QAM transmitters.

a. true

b. false


Cable Modems

2.Cable modem users can modify the baseline privacy interface (BPI) to defeat data encryption.

a. true

b. false

3.A CMTS provides an extended Ethernet network with a geographic reach of up to ______.

a. 1 mile

b. 10 miles

c. 100 miles

d. none of the above

4.The upstream data channel specification requirements feature which of the following?

a. 64 and 256 QAM formats

b.data rates from 320 kbps to 10 Mbps

c. 6 Mhzoccupied spectrum that coexists with other signals in the cable plant

d.all of the above

5. If a subscriber is receiving or sending data on a cable modem, regular cable televisionservice is momentarily interrupted.

a. true

b. false

6.Current cable modem standards have completely rejected the ATM protocols as a data transmission method, as proposed by the IEEE 802-14 Committee.

a. true

b. false

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7.Cable modem operations can be managed by a local cable company or from a remote location.

a. true

b. false

8.Among other functions, the media access control (MAC) layer provides control of subscriber upstream transmissions such that no more than 25 percent of all such transmissions can collide (thereby requiring retransmission).

a. true

b. false

9.As the number of subscribers increases in a cable data network, all of the users will experience poor performance in their Internet access.

a. true

b. false

10. In a cable data network, Web television users can access all of their neighbors' data on television.

a. true

b. false

Cable Modems

6. ACRONYM GUIDE


BPI baseline privacy interface

CMTS cable modem termination system

DES data encryption standard

DHCP dynamic host configuration protocol

DNS domain name server

DOCSIS data over cable service interface specifications

EMS element management system

IP Internet protocol

ISP Internet service provider


MAC media access control

MCNS multimedia cable network system

NET network address translation

NOC network operations center

OSSI operations support system interface

QAM quadrature amplitude modulation

QPSK quaternary phase shift keying

RF radio frequency

SNMP signaling network management protocol

SSI security system interface

TCP transmission control protocol

TRI telephony return interface

UDP user datagram protocol

WAN wide-area network

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Cellular Communications

DEFINITION

A cellular mobile communications system uses a large number of low-powerwireless transmitters to create cellsthe basic geographic service area of awireless communications system. Variable power levels allow cells to besized according to the subscriber density and demand within a particularregion. As mobile users travel from cell to cell, their conversations are handedoff between cells to maintain seamless service. Channels (frequencies) used inone cell can be reused in another cell some distance away. Cells can be addedto accommodate growth, creating new cells in unserved areas or overlayingcells in existing areas.

TUTORIAL OVERVIEW

This tutorial discusses the basics of radio telephony systems, including bothanalog and digital systems. Upon completion of this tutorial, you should beable to accomplish the following:

1. describe the basic components of a cellular system

2. identify and describe digital wireless technologies

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TOPICS

1. MOBILE COMMUNICATIONS PRINCIPLES . . . . . . . . . . . . . . . . . . . . .59

2. MOBILE TELEPHONE SYSTEM USING THE CELLULAR CONCEPT . .60

3. CELLULAR SYSTEM ARCHITECTURE . . . . . . . . . . . . . . . . . . . . . . . . . .62

4. NORTH AMERICAN ANALOG CELLULAR SYSTEMS . . . . . . . . . . . . .66

5. CELLULAR SYSTEM COMPONENTS . . . . . . . . . . . . . . . . . . . . . . . . . . .68

6. DIGITAL SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .70

7. SELF-TEST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .76

8. ACRONYM GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .78



1. MOBILE COMMUNICATIONS PRINCIPLES

Each mobile uses a

the basics of telecommunications international engineering consortium iec 2002

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