chapter 8: data transmission in mobile communication systems schiller, voisard. location based...

Chapter 8: Data Transmission in Mobile Communication SystemsSchiller, Voisard. Location Based Services. Morgan Kaufman

Presented by: Fran Jarnjak

Software System Lab

[email protected]

Content

Introduction Basics of Wireless Communication Cellular-type Mobile Communication

Systems Wireless Local Area Networks Internet-Based Mobile Communication Ad-hoc Networking Location-Based Service Discovery Conclusion & QA

Introduction

In traditional tethered communication systems, the need for LBS was never felt.

As mobile environment was developed, LBS concept became necessary to be considered.

On top of wireless transmission, mobile communication systems can be constructed in many different ways: Goal: provide a large number of (highly) mobile users with

constant QoS at moderate data rates? Goal: provide high data rates for few stationary users?

LBS can be built on top of many different system architectures and LBS uses them to communicate (position information).

Introduction

A basic of most common architectures is introduced, where the focus is on how to communicate information between two entities.

Design decisions: Choice of transmission medium. Infrastructure support Type of mobility to be supported.

Two large families: Roots in telecommunication systems. Roots in data communication.

Introduction Choice of Transmission Medium

Different wireless media: Ultrasound Infrared light Electromagnetic waves in radio spectrum

For LBS considered, radio wave communication is the appropriate choice: Supports sufficiently high data rates Acceptable distance (meters or kilometers)

However, those properties can not be simultaneously maximized due to tradeoffs between them.

IntroductionAvailable infrastructure

Wireless communication has a limited range thus three different approaches can be taken to solve the problem.

Infrastructure based system: A mobile device communicates with a fixed device

(base station, access point) that is connected to a fixed, wired network. Data is transferred to another base station and transmitted wirelessly to the other mobile device.

Since, the areas around the base stations are in a cell-shape, those systems are also called: cellular systems.

IntroductionAvailable infrastructure

Introduction Available infrastructure

Ad-Hoc/Multihop Systems: Sometimes mobile terminals can talk directly to each

other (laptops in a conference room). Network between those terminals can be set-up in an

Ad-Hoc way. If direct connection is not available, a mobile terminal

in the middle can act as an relay (multi-hop system).

Examples: Disaster relief operations (firefighters, construction sites, etc.)

Common characteristic: Self-organizing network in set-up phase and

maintenance even when the members are moving.


Hybrid systems: A hybrid between infrastructure-based and

ad-hoc/multi-hop networks. In case a mobile terminal is far away from the

base station, it can use another mobile terminal as a relay and communicate.

It’s a powerful way to widen the cell’s coverage area or increase cell’s capacity.

IntroductionTypes of Wireless Communication and Mobility

Cable replacement – to replace cables or to reduce cost of installing cables in an old office building.

Nomadic mobility – a user goes from point A to a distant point B and he can successfully connect to either infrastructure-based or ad-hoc based network at a point B, using a well-known identity irrespective of its current location.

True mobility – a user goes from point A to point B and whilst traveling, ongoing communication sessions are ongoing and not disturbed Example: telephony calls in cellular communication

systems (mobile phone)

IntroductionArchitecture Families

Wireless communication was developed by telecommunication industry and data communication industry with different system architectures.

Telecommunication industry – focus on supporting a large number of users with a QoS for phone calls, covering a large geographic area with full coverage and has mechanisms for accounting, billing, etc. Examples: GSM, UMTS.

Data communication industry – focus on wireless counterparts of the wired networks supporting a small number of users, modest QoS, small geographic area but at high data rates and low infrastructure cost. Provides only limited mobility support.

IntroductionArchitecture Families

The primary goal is to point out what characteristics of those networks are relevant to the LBS: What are the consequences of a particular architecture

for the generation of location information ? Where is that information available ? How can it be accessed and how long does it take ? What types of communication service (bandwidth,

delay, dependability, cost) can a particular system offer ?

Do those communication parameters allow for certain types of location-based applications to be realized at all ?

Basics of Wireless Communication

Wireless communication has two essential problems: 1) How to communicate data between a

source and a destination2) How to organize multiple sources to send at

the same time Digital wireless communication is to transmit

data between a single sender and a single receiver.


Sender generates a sine-shaped electromagnetic wave.

Receiver detects such a wave and reconstructs its shape.

By choosing a proper shape, digital information (0 and 1) can be transmitted.

Shaping the form of a sine wave is called modulation. Amplitude Shift Key (ASK) Frequency Shift Key (FSK) Phase Shift Key (PSK)


Problem: How to decide when a bit actually starts (where to

look at high/low signal levels)

ASK


Source:

Stallings, W. [2003]

Data and Computer Communications (7th edition),

Prentice Hall, Upper Saddle River NJ

Subsequent figures in the presentation

Are taken either from the course textbook or the above reference.


First problem of wireless transmission is that the receiver will not see the identical shape a sender sent due to the: Electromagnetic noise Signal attenuation.

Electromagnetic noise arises due to random oscillations in the receiver’s circuitry.

Signal-to-noise ratio (S/N) – for a given modulation, the S/N ratio corresponds to the probability with which bits will be incorrectly received –bit error rate.

Strength of the signal depends on the power used by the sender. When the distance between sender and receiver is doubled,

arriving power is reduced to one-quarter - attenuation. Arbitrarily increasing the power is not possible due to physical

and legal limitations. Bit error rate determines the throughput – amount of data that

can be successfully transmitted per unit time.


Noisy signal at receiver’s end:

Left – small amount of noise

Right – large amount of noise

Attenuation


For a higher throughput one possibility is to use modulations that represent more than a single bit in a single time period. Tradeoff: better S/N ratio is required which requires

higher power More than one frequency can be used to send data

in parallel Tradeoff: Frequencies are limited and not all

frequencies ranges are equally suited for all application types.


Quadrature Amplitude Modulation (QAM)Source: www.physics.udel.edu/wwwusers/watson/ student_projects/scen167/thosguys/qam.html

Combination of ASK and PSK and uses one frequency. For transmitting at 3600bps we transmit 3 bits at the time – hence 8

combinations are possible.

Bit value Amplitude Phase shift

000 1 None

001 2 None

010 1 1/4

011 2 1/4

100 1 1/2

101 2 1/2

110 1 3/4

111 2 3/4


QAM:

Transmitting:

001010100011101000011110

First separate in three-bit triads:

001-010-100-011-101-000-011-110


A frequency is suitable for radio communication when: 1) Amount of data it can be transported (higher frequency –

more data) 2) Ability to reach the receiver that do not have straight line-of-

sight communication path Ability to reach receiver has 4 properties:

Shadowing of waves due to obstacles (e.g. car). Reflections of waves from those obstacles. The Scattering of waves at small edges or openings of the

objects. Effect of diffraction that makes waves bend at the edges of large

objects.


Multipath propagation is caused by those properties of wave propagation paths.


Those effects and properties are different for various frequencies. Frequencies above 5 GHz hardly penetrate walls. Mobile communication systems typically use a 800 MHz – 5GHz

band, where 900 MHz – 1800 MHz typically is used for outdoor communication and 2.4 GHz – 5 GHz for indoor communication.

Further, due to multipath propagation it is not easy to decode a receiving signal since the same signal arrives over different paths and hence different times thus interfering with itself.

Measuring distance (location) by calculating signal’s attenuation is also not precise since: Signal passing through a wall is more attenuated than passing

through air. Scattering and diffraction also cost power.


Link Layer for Multiparty Wireless Communication: When two senders send a signal at the same time,

they interfere with each other and receiver will see only noise.


Medium Access Problem: The wireless medium must only be accessed by a single entity at a time. 1) Which entities perform such organization? 2) What are the possibilities to organize the transmissions ?

Organization can be performed by a centralized scheme (e.g. cellular networks) or by a distributed scheme where all entities involved organize the access (e.g. ad-hoc networks).

Time Division Multiple Access (TDMA) – each entitiy transmits at a specific time slot.

Frequency Division Multiple Access (FDMA) – different senders use different frequencies for communication.

Space Division Multiple Access (SDMA) - Two sender/receiver pairs that are sufficiently apart can use the same frequency due to signal attenuation, so there is no interference. Used extensively in cellular mobile communication systems.


After communication is established it can happen in: One direction (simplex) Both directions (duplex)

Duplex can be achieved by: Time Division Duplex (TDD) – entities take turn in

sending and receiving data Frequency Division Duplex (FDD) – entities send the

entire time but using different frequencies.

Cellular-type Mobile Communication Systems

For mobile telephony two requirements are needed: Full coverage. Quality of (almost) fixed phone networks.

A first generation of analog mobile networks did not meet those requirements: 1981 Nordic Mobile Telephone (NMT) in Nordic

countries and some Europe/Middle East/Asia countries 1983 Advanced Mobile Phone System (AMPS) in USA

In 1982, development of a second generation digital communication system began, later to be called Global System for Mobile Communication (GSM), which was standardized by European Telecommunication Standards Institute (ETSI).


GSM design is cellular based to provide coverage by a sufficient amount of base stations.

Challenge: Fewer base stations yields cheap cost but also larger

cells that can support only limited amount of users. Smaller cells can support larger number of users but

yield a high cost. Solution: Use smaller cell sizes in urban areas

(more people) and larger cell sizes in rural areas (less people).

Membership of a mobile station within a cell is a rough indicator of the location and due to varying cell sizes location resolution is varying as well.


GSM is a circuit-switched network and existing circuit-switched architecture of fixed telephony was reused.

Hand-over: A user leaves the coverage of one base station area and goes to the other area – phone call should not terminate.

First GSM mostly concerned with voice communication. Data communication was possible using a modem-like data communication with slow data rates.

Later, a desire to better support data was needed and extensions were performed – 2.5G systems that charge the user on the amount of data transferred (not time that was needed for the transfer).

Third generation (3G) is targeted to integrate data and voice communication together – Universal Mobile Telecommunications System (UMTS).


The second generation (2G) – GSM GSM organizes the network in a hierarchy to contain

complexity. Mobile stations (MS) (e.g. phone). Base Transreceiver Station (BTS) – has radio and

signal processing equipment but does not have protocol functions.

Base Station Controller (BSC) – controls several BTSes and has protocol functions.

Mobile Service Switching center (MSC) Several BSC assigned to it Does mobility specific tasks It can act as a gateway to other networks (e.g. fixed

network) - called Gateway MSC or GMSC


GSM network also contains: Home Location Register (HLR) database And for each MSC: Visitor Location Register (VLR)


Two types of phone calls: Mobile-originated call (MOC): Mobile station calls a

terminal in the or fixed network Mobile-terminated call (MTC): Mobile station (or fixed

network phone) calls a terminal in the mobile network.

MOC: MS contacts MSC via BTS and BSC asking for a new connection. MSC check’s MS’s eligibility for a call by consulting terminal information in its VLR and network resources. If check is succeeded, connection to a fixed phone network is completed.


MTC: BTS serving a MS first needs to be looked up A moving terminal keeps its location in HLR and VLR

approximately up-to-date MS that is on (power on) registers with BSC that informs

MSC, who updates its VLR and HLR When gateway MSC receives a connection request for

some MS it contacts HLR to see where MS was last seen and contact that other MSC.

Since MS updates its information infrequently (saving on the signaling cost) MSC has to page all of its cells to find a MS and then connection can be made.


Handover: 1) User moves within to another BTS controlled by

the original BSC – re-routing of the connection is needed.

2) User moves to the region of another BSC which belongs to the same MSC, re-routing is done in MSC.

3) User moves to the region of another MSC – original MSC remains the part of the call: forwards the call and remains a stable anchor point for the outside.


For each MS a connection to the BTS is made which corresponds to a certain time slot in the TDMA structure.

Data rate for voice-communication is 13,000 bits/s because of advanced voice coding tehniques (fixed phone: of 64Kbit/s using 8000 samples per second in 8-bit resolution)

For data communication: 9600 bit/s with high delays (60-100 ms).

Frequently transmitting small amounts of data – useful for LBS but by the original GSM standard its either: Not well supported (time to set up the connection) Or Too expensive (keeping the channel open all the time)


2.5G: Between Generations – GSM Extensions Developed to support data communication via 3 main extensions. High-speed circuit-switched data (HSCSD) – better encoding techniques

for modem connections (14.4Kbits/s) and even higher data rates by combining several modem connections into one. Changes to the existing GSM infrastructure are small but new mobile

terminals are needed. Enhanced Data Rates for GSM Evolution (EDGE) – further improving of

channel coding and modulation techniques – single connection 59.2Kbits/s Require extensive changes to both mobile and base stations

General Packet Radio Service (GPRS) – GPRS amends GSM network with a packet-switched network so a mobile stations send data when available. Users are charged with amount of data transmitted. Different QoS profile (data throughput, 3 reliability classes, several

delay classes). Due to the delay classes, users will feel a difference between fixed and

mobile usage which needs to be considered for the LBS applications.


Third Generation (3G): UMTS In 1990s development for standardization of

the follow-up to GSM began International Telecommunications Union (ITU)

issued a call for proposals for International Mobile Telecommunications 2000 (IMT-2000) where Universal Mobile Telecommunication System (UMTS) was proposed.

UMTS vision is to build a uniform system for mobile multimedia communication.


UMTS is supposed to integrate various access technologies (existing GSM, WLAN, etc.) and to-be-developed radio access technologies and existing wired technologies.

Urban environments are provided with high data rates for slowly moving users (2Mbit/s) and rural areas with lower rates for highly mobile users (384Kbit/s @ 500 km/h). Realized with UMTS terrestrial radio access (UTRA)

Entire fixed UMTS network is based on the Internet protocol (all-IP network).

Several service classes exist: 1) conversational (voice), 2) streaming (real-time feeds) 3) interactive (low-bandwidth terminal-type) 4) background (non-critical e.g. email download).

Wireless Local Area Networks

Data communication via Internet is the second largest communication system used worldwide.

Internet community also went wireless with the first goal to replace cabling in the local area (building, etc.) Requirement: medium to high data rates over short to medium

distances & smooth integration Mobility support was a secondary concern.

Resulting system uses an access point (AP) to connect wireless terminals with the fixed network.

AP acts like an Ethernet switch, and mobile terminals are finding AP by themselves.

WLAN APs tend to be installed in a more ad-hoc fashion and frequency band it uses is also open for other usages (interference may happen). Thus, WLAN should be able to manage and organize itself.


IEEE 802.11 family: 802.11a 802.11.b Concerned about physical and link layers of the ISO/OSI model 802.11b uses direct sequence spread spectrum and 802.11a

orthogonal frequency division multiplexing in the physical level. 11MBit/s 802.11b and 54MBit/s for 802.11a but in practice

numbers are less Further data rate is adapted depending on the distance: Closer –

higher, far away from AP – slower. 802.11 systems offer two different link layers:

Point coordination function (PCF) – AP is in control when a certain terminal can access the medium. QoS properties can be controled.

Distributed coordination function (DCF) – solves medium access problem in a distributed fashion – every terminal listens if the medium is free before transmitting for some random time.

In current implementations of 802.11 the DCF is used.


IEEE 802.11 systems are naturally packet-switched and are a natural fit with Internet applications.

Their short range allows for deriving a rough estimate of terminal position from an AP.

Major shortcoming: Limited mobility support. Moving from one AP to another in the same

sub-net is good but:How to handle long-distance mobility in an IP

context ?

Internet-based Mobile Communication

Internet address: Uniquely identifies a terminal Used to route the data in the fixed network

In the mobile network, network topology changes as the mobile terminal moves and location information must be topologically correct so that the data is sent to the correct part of the IP network.

Mobile IP approach solves this problem by using two different addresses for a mobile terminal.


Home address: A terminal is known to others using this address and it never changes.

Care-of address: temporary address that is topologically correct and can be used to route the data.

Mapping between two are done by the home agent (proxy).

In Mobile IP, data is sent to the home address and home agent redirects data to the temporary care-of address.

Security and QoS support for Mobile IP are still active research areas.

Ad-Hoc Networking

Ad-hoc networks can solve communication needs within a group of terminals without the help of some infrastructure.

They typically use multi-hop radio communication to communicate and medium access control needs to be collaboratively solved.

Most research is concentrated on the routing problem: All terminal together have to ensure the data reaches its destination Environment is dynamic: terminals are mobile, can fail or simply be

turned off. A specific case are sensor networks: numerous, cheap nodes, with

simple processing, communication and sensing capabilities distributed randomly. Data rates are modest but the scaling requirement is extreme.

To determine location information, some nodes location needs to be known and then the idea is to iteratively spread such information to the neighbors. Location information can be used to solve routing problem.

Ad-Hoc Networking

Geographic routing: Forward the data to the node that minimizes the distance to the target.

Location information is more difficult to obtain and potentially more useful and is still an area of much active research.

Location-Based Services Discovery

Users location is sometimes not relevant but rather a context is more important. Example: A user wants to print out something on the close-by printer

but he doesn’t know how to access such a printer. In PAN, MP3 players wants to find earphones, etc.

Service Location Protocols: Providing information about what services exists and where and how they can be accessed.

A global solution is not possible (DB containing all world’s printers) so a localizing the search for such services is needed.

Localization fits well with ad-hoc networks and to a certain degree with WLAN’s.

Location-Based Services Discovery

Three main approaches can be distinguished: 1) Request for services are flooded through the entire

network. This approach is possible for a small number of devices – like PAN networks.

2) Introducing a dedicated service directory node which is well-know to other participants. Challenge is how to provide identity of such a node –

DHCP to provide node’s address, multi-cast requests to a well-known multicast address, or directory node periodically broadcasting its address.

3) Distributed service discovery – implementing on top of current sensor-networks primitives (Pub/Sub) is currently an active research area.

Conclusion and Q&A

The fundamental tradeoffs between distance, transmission power/interference and data rate determine many design decisions for mobile communication systems.

Two different families evolved: 1) cellular, wide area systems 2) short-to-mid range WLAN

When designing LBS for mobile communication systems one needs to keep in mind about the limited frequency a mobile terminal can update its location information (delay, cost, data rates particularly for fast-moving mobile terminals).

chapter 8: data transmission in mobile communication systems schiller, voisard. location based...

Documents