39
Chapter 2 Technical Basics: Layer 1 Methods for Medium Access: Layer 2 Chapter 3 Wireless Networks: Bluetooth, WLAN, WirelessMAN, WirelessWAN Mobile Networks: GSM, GPRS, UMTS Chapter 4 Mobility on the network layer: Mobile IP, Routing, Ad-Hoc Networks Mobility on the transport layer: reliable transmission, flow control, QoS Mobility support on the application layer Wireless Telecommunication Systems GSM as basis of current systems Enhancements for data communication: HSCSD, GPRS, EDGE UMTS: Future or not?

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Page 1: Lehrstuhl für Informatik 4 Kommunikation und verteilte …

Lehrstuhl für Informatik 4

Kommunikation und verteilte Systeme

1Chapter 3.4: Mobile Networks

Chapter 2• Technical Basics: Layer 1• Methods for Medium Access: Layer 2

Chapter 3• Wireless Networks: Bluetooth, WLAN,

WirelessMAN, WirelessWAN• Mobile Networks:

GSM, GPRS, UMTS

Chapter 4• Mobility on the network layer:

Mobile IP, Routing, Ad-Hoc Networks

• Mobility on the transport layer: reliable transmission, flow control, QoS

• Mobility support on the application layer

Wireless Telecommunication Systems

• GSM as basis of current systems• Enhancements for data communication:

HSCSD, GPRS, EDGE

• UMTS: Future or not?

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2Chapter 3.4: Mobile Networks

Mobile Telephony

A-Netz• 1958 introduced by Federal Post Office• Analogous and connected by operator• No handover between base stations

• 1977 stopped

B-Netz• Introduced 1972• Caller had to know in range of which base station the called resided (using a

region dialing code!)

• Partly roaming agreements with Austria, The Netherlands, Luxemburg• 1994 stopped

C-Netz• No region dialing code necessary

• Cellular system with large number of base stations• Also data and fax connections

• 2000 stopped

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3Chapter 3.4: Mobile Networks

Standardization of Networks

In the 70th and 80th: analogous, cellular mobile systems in most European countries (1st generation networks)

• Incompatibility of the mobile systems• 1982: Foundation of Groupe Spéciale Mobile (GSM) for solving interoperability issues

• Goal: digital network (D-Netz, also called “2nd generation, 2G” because of change in technology)

• 1990: first specification of GSM: GSM900 (900 MHz)

• 1991: specification of GSM1800 as E-Netz• 1992: 13 networks in 7 countries, D1 and D2 in Germany

• 1994: E-Plus• 1995: GSM1900 in the USA

• 1998: E2-Netz, VIAG Interkom• 2000: auctioning of UMTS licenses (Integration of voice and data: 3rd generation, 3G)• 2001 Start of GPRS as enhancement to GSM for packet-oriented data transfer

(also called “2.5G”)

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4Chapter 3.4: Mobile Networks

GSM – Basis of Current Mobile Systems

• GSM today means Global System for Mobile Communications• Introduction by the European telephone exchange offices (Germany: D1 and D2)

→ seamless roaming within Europe possible• Today many providers all over the world use GSM (more than 210 countries in in

Asia, Africa, Europe, Australia, America)• More than 747 million subscribers in more than 400 networks• More than 10 billion SMS per month in Germany, > 360 billion worldwide (more

than 10% of the sales of the operators)• Uses the frequency ranges of 900, 1800, and 1900 MHz• Voice and data connections with up to 9.6 KBit/s (enhancement: 14.4 KBit/s)

• Access control by chip-cards• Cell structure for a complete coverage of regions (100 – 500 m Ø per cell in cities,

up to 35 km on country-side)

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5Chapter 3.4: Mobile Networks

Performance Characteristics of GSM

Most important technical aspects:

• Communication: mobile, wireless communication; support for voice and data services

• Total mobility: international access, chip-card enables use of base stations of different providers

• Worldwide connectivity: only one number, the network handles localization• High capacity: good frequency efficiency; relatively small cells to allow for a high

number of customers

• High transmission quality: high audio quality and reliability for uninterrupted wireless phone calls also at higher speeds (cars, trains, …)

• Security functions: access control and authorization via chip-card and PIN

GSM offers three types of services:• Bearer Services• Telematic Services

• Supplementary Services

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6Chapter 3.4: Mobile Networks

Bearer Services

• Basic telecommunication services to transfer data between access points

• Specification of services up to the terminal interface (corresponding to OSI layers 1 – 3)

• Different data rates for voice and data (original standard)

� Data service (circuit switched)• synchronous: 2.4, 4.8 or 9.6 KBit/s

• asynchronous: 300 – 1200 Bit/s� Data service (packet switched)

• synchronous: 2.4, 4.8 or 9.6 KBit/s• asynchronous: 300 – 9600 Bit/s

• Additionally: signaling channels for connection control (used by telematicservices)

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7Chapter 3.4: Mobile Networks

Telematic Services

• Telecommunication services that enable voice communication via mobile phones

• All services have to obey cellular functions, security measurements, etc.

• Offered services:� Mobile telephony

Primary goal of GSM was to enable mobile telephony offering the traditional bandwidth of 3.1 kHz

� Emergency number

Common number throughout Europe (112); mandatory for all serviceproviders; free of charge; connection with the highest priority (preemption of other connections possible)

� Multinumbering

Several phone numbers per user possible

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8Chapter 3.4: Mobile Networks

Telematic Services

Non-Voice-Teleservices

• Fax• Voice mailbox (implemented in the fixed network supporting the mobile

terminals)

• Electronic mail (MHS, Message Handling System, implemented in the fixed network)

• ...

• Short Message Service (SMS)

Alphanumeric data transmission to/from the mobile terminal using the signaling channel, thus allowing simultaneous use of basic services and SMS

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9Chapter 3.4: Mobile Networks

Supplementary Services

• Services in addition to the basic services, cannot be offered stand-alone• Similar to ISDN services besides lower bandwidth due to the radio link

• May differ between different service providers, countries and protocol versions

• Important services

� Identification: forwarding of caller number� Suppression of number forwarding� Automatic call-back

� Conferencing with up to 7 participants� Locking of the mobile terminal (incoming or outgoing calls)

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10Chapter 3.4: Mobile Networks

Cellular Network

• Signal attenuation restricts distance between sender and receiver (~ d² in line of sight, d5.5 within buildings)

• Frequency range very limited and not suited for high number of subscribers� Frequency re-use by SDMA: divide the whole area in cells

� Intentionally restriction of a cell by lowering the transmission power� Frequency ranges can be re-used in a larger distance without problems of

interference

� Two subscribers in distant cells can use the same channel simultaneously

technical possible transmission range

Zelle1

Zelle1

intentionally restriction of transmission range

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11Chapter 3.4: Mobile Networks

Cellular Network

• The size of a cell is determined by a maximum given transmission power and a minimum receiver signal strength for a good voice quality

• Hexagonal cell pattern is idealized (Cells overlap irregularly)

• No uniform cell size, size depends on attenuation as well as expected traffic amount (inner city vs. unpopulated regions)

• Cell change of mobile user during a phone call

→ Passing the connection to the neighbor cell: handover

16

7

45

2

16

73

45

2

16

73

45

2

16

73

Distance depends on remaining signal strength

Cluster

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12Chapter 3.4: Mobile Networks

Cell Concept

Cluster: Area in which all frequencies are used. Each cell in the cluster at least is assigned one frequency, but also several frequencies per cell are possible

• More cells per cluster:� Less channels per cell

� Lower system capacity� Less co-channel interference (co-channel cells have larger distance in

between)

• Less cells per cluster:� More channels per cell

� Higher system capacity� More co-channel interference (co-channel cells are nearby)

Cell planning:• Optimize the luster size N in a way to maximize capacity and minimize

interferences

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13Chapter 3.4: Mobile Networks

e-plus (GSM-1800)

T-Mobile (GSM-900/1800)

O2 (GSM-1800)

Vodafone (GSM-900/1800)

Coverage of GSM Networks (www.gsmworld.com)

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14Chapter 3.4: Mobile Networks

Architecture of the GSM System

The GSM system is a so-called PLMNs (Public Land Mobile Network). Several providers setup mobile networks following the GSM standard within each country

• The GSM system consists of several components:� MS (mobile station)� BS (base station)

� MSC (mobile switching center)� LRs (location register)

• Different subsystems are defined:� RSS (radio subsystem): covers all radio aspects

� NSS (network and switching subsystem): call forwarding, handover, switching

� OSS (operation subsystem): management of the network

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15Chapter 3.4: Mobile Networks

GSM - Architecture

Base Station Subsystem

Cell Cell Base Station Subsystem

Location Area

Base Station Subsystem

Location Area

Region with Mobile Switching Center (MSC)

GSM Network

Location Area

MSC Region

MSC Region

GSM networks are hierarchical structured:

• At least one administrative region with Mobile Switching Center

• An administrative region consists of at least one location area

• A location area consists of several Base Station Subsystems

• A Base Station Subsystem consists of one Base Station Controller (BSC) and several Base Transceiver Stations (BTS, cells)

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16Chapter 3.4: Mobile Networks

GSM – Architecture

EIRAUC

HLR VLR

PLMN, international

PSTNISDN

OMC

MSC

ISC

GMSC

BSC

BSC4

4

4

ISC: International Switching CenterMSC: Mobile Switching CenterOMC: Operation and Maintenance CenterPLMN: Public Land Mobile NetworkVLR: Visitor Location Register

AUC: Authentication CenterBSC: Base Station ControllerEIR: Equipment Identity RegisterGMSC: Gateway Mobile Switching CenterHLR: Home Location Register

OSS

NSS

RSS

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17Chapter 3.4: Mobile Networks

Radio Subsystem

• The radio subsystem is the cellular network up to the switching centers

• It comprises several components:� Base Station Subsystem (BSS):

• Base Transceiver Station (BTS): radio components including sender, receiver, antenna. A BTS can serve one cell or, if directed antennas are used, several cells.

• Base Station Controller (BSC): The BSC performs the switching between BTSs and the control of BTSs. It manages the network resources, mapping of radio channels onto terestrial channels. The complexity of BTSs only is low by that separation.

• BSS = BSC + Sum(BTS) + interconnection

� Mobile stations (MS) are seen as mobile network components.

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18Chapter 3.4: Mobile Networks

Base Transceiver Station und Base Station Controller

Functions BTS BSCManagement of radio channels XFrequency hopping (FH) X XManagement of terrestrial channels XMapping of terrestrial onto radio channels XChannel coding and decoding XRate adaptation XEncryption and decryption X XPaging X XUplink signal measurements XTraffic measurement XAuthentication XLocation registry, location update XHandover management X

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19Chapter 3.4: Mobile Networks

Base Transceiver Station

Fx – Frequency range of a cellF1 F2 F3 F5 F7

F7,F6

F8 F4 F6 F1

F9 F3

BSC BSC

BSC

Base Station ControllerBSC

Base Station Subsystem

A BTS controls all transmission in a cell. Communication only is possible between a mobile station and its BTS

Problems: • Cell changes (Handover to another BTS), combined with a frequency change

• Location of a mobile station (HLR/VLR)

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20Chapter 3.4: Mobile Networks

Mobile Station

Terminal for the use of GSM services; it comprises several functional groups:

MT (Mobile Terminal):• Offers common functions used by all services the MS offers

• Corresponds to the network termination (NT) of an ISDN access• End-point of the radio interface

TA (Terminal Adapter):• Terminal adaptation, hides radio specific characteristics

TE (Terminal Equipment):• Peripheral device of the MS, offers services to a user• Does not contain GSM specific functions

SIM (Subscriber Identity Module):• Personalization of the mobile terminal, stores user parameters

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21Chapter 3.4: Mobile Networks

Network and Switching Subsystem

• The network subsystem is the main component of the public mobile network GSM. It interconnects the BSSs with other networks and performs switching, mobility management, and system control

• Components are:� Mobile Services Switching Center (MSC)

Controls all connections via a separated network to/from a mobile terminal within the domain of the MSC - several BSC can belong to a MSC

� Databases

• Home Location Register (HLR)Central master database containing user data, permanent and semi-permanent data of all subscribers assigned to the HLR (one provider can have several HLRs)

• Visitor Location Register (VLR)Local database for a subset of user data, including data about all user currently in the domain of the VLR

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22Chapter 3.4: Mobile Networks

Mobile Switching Center

• The exchange central of a GSM network is the Mobile Switching Center: path choice, signaling and processing of service features

• Administration of and access to radio resources

• Additional functions for location registration and handover when a cell change occurs (support of subscriber mobility)

• Certain gateways to other fixed or mobile telephony networks (Gateway-MSC; GMSC)

• Most important functions of a MSC:� Specific functions for paging and call forwarding� Mobility specific signaling

� Location registration and forwarding of location information� Provision of new services (fax, data calls)

� Support of short message service (SMS)� Generation and forwarding of accounting and billing information

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23Chapter 3.4: Mobile Networks

Home and Visitor Location RegisterTwo types of databases are used for subscriber registration and location

management:

Home Location Register (HLR)• Central location management, a subscriber can be searched for here, not the

whole network has to be searched• Contains all static subscriber data (number, access rights, subscribed services,

service features) as well as a raw location information• MSCs use HLR to get information about rights, services and current (raw) location

of subscribers

Visitor Location Register (VLR)• Locale database for a subset of subscriber data, most important the current

(detailed) subscriber location; is assigned a MSC• Only stores information about subscribers which are in range of the corresponding

MSC• Contains dynamic data which are updated by information exchange with HLR and

the mobile stations• Data from a VLR “follow” the subscriber when he comes into range of another

VLR

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24Chapter 3.4: Mobile Networks

Connection Establishment

POTSGateway

MSC BSSDestination

MSC

VLRHLR

934 71 9

1 - Call for a mobile station2 - POTS forwards call to the GMSC connecting the GSM network3 - GMSC uses HLR to request currently responsible MSC 4 - Response with switching information to the current subscriber location5 - Forwarding of the call to the destination MSC6 - MSC requests exact position of the subscriber in its VLR7 - VLR checks service profile and availability of the MS and gives back the current BSS8 - Paging of the mobile subscriber (broadcast in the whole BSS)9 - MS answers, call can be established

4

6 8

2 5 8

9 9 9

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25Chapter 3.4: Mobile Networks

Handover

• Automatic change of the responsible BTS without influence on the quality of a connection – a caller should not be able to notice the change.

Process:

1. Measurement• During a transmission permanently measurements in the signaling channel are

performed to detect the necessity of a handover (receiving power, bit error rates, distance to base station, participants in the cell, narrow-band interference)

2. Initiation of handover• Establishment of a connection from the responsible MSC to the new base

station• Selection of a new channel with the new base station

3. Switching to new BTS• Network-controlled handover (e.g. C-Netz), MS-supported handover (e.g. GSM)

or MS-controlled handover (e.g. DECT)

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26Chapter 3.4: Mobile Networks

Handover Decision

BTSA

MS MS

BTSB

receiving power

movement

Last point of switching

Signal strength ofsignal A

Signal strength ofsignal B

handover range

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27Chapter 3.4: Mobile Networks

Handover Procedure

HO access

BTSold BSCnew

measurementresult

BSCold

Link establishment

MSCMSmeasurementreport

HO decision

HO required

BTSnew

HO request

resource allocation

ch. activation

ch. activation ackHO request ackHO commandHO commandHO command

HO completeHO completeclear commandclear command

clear complete clear complete

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28Chapter 3.4: Mobile Networks

Operation Subsystem

• The OSS performs some central tasks for the provision of the whole GSM network as well as maintenance of that network

• Components are:� Authentication Center (AUC)

• Creates on demand of a VLR the access right parameters for a subscriber• These parameters serve for security and protection of subscriber

information in the GSM system

� Equipment Identity Register (EIR)• Registers serial numbers of GSM mobile stations as well as the assigned

usage right• Devices which are registered in the AUC can be locked and maybe located

if stolen

• Not a mandatory component in the GSM architecture� Operation and Maintenance Center (OMC)

• Control centers for the maintenance of all other GSM architecture parts

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29Chapter 3.4: Mobile Networks

GSM900 vs. GSM1800

GSM900 GSM1800

Frequency range (Uplink) 890 MHz - 915 MHz 1710 MHz - 1785 MHz

Frequency range (Downlink) 935 MHz - 960 MHz 1805 MHz - 1880 MHz

Duplexing distance 45 MHz 95 MHz

Bandwidth Up- and Downlink 2 x 25 MHz 2 x 75 MHz

Bandwidth of a channel 200 kHz 200 kHz

Access method FDMA & TDMAFDMA & TDMA

Number of carrier frequencies 124 372

Timeslots per carrier frequency 8 8

Channels 992 2976

Bit rate 270,833 KBit/s 270,833 KBit/s

Modulation method GMSK GMSK

Cell size (radius) 2 - 35 km 0,2 - 8 km

Transmission power of a MS max. 20 Watt max. 1 Watt

Criterion

Net bit rate for voice 13 KBit/s 13 KBit/s

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30Chapter 3.4: Mobile Networks

GSM Protocol

• Access method: combination of:

� Frequency multiplexing (FDMA/FDD)• Sending on 124 channels of 200 KHz each between 890 and 915 MHz• Receiving on 124 channels of 200 KHz each between 935 and 960 MHz

� Time multiplexing (TDMA) with a shift of 3 time slots between sending and receiving time by to avoid the need for duplex-enabled transceiver units

f

t

124123122

1

124123122

1

20 MHz

200 kHz

890.2 MHz

935.2 MHz

915 MHz

960 MHz

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31Chapter 3.4: Mobile Networks

TDMA Frames Fr

eque

ncy r

ange

Time

GSM TDMA Frame

GSM Timeslot

4,615 ms

546,5 µs577 µs

3

935-960 MHz 124 channels with 200 kHz eachDownstream

890-915 MHz

Higher GSM Frame Structures

124 channels with 200 kHz eachUpstream

1 2 3 4 5 6 7 8

guard time

tail payload S training S payload tailguardtime

3 bit57 1 26 1 57

GSM timeslot: Burst und guard times• Tail (000): define start und end

of a Bursts• Training: synchronization

sequence with well-known bit pattern for adapting the receiver to the current signal propagation characteristics, e.g. calculating the strongest signal part in case of multipathpropagation

• S (Signaling): what is the content of the payload field: user or control data

(optional: slow frequency hopping after each TDMA frame to avoid frequency-dependent signal fading)

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32Chapter 3.4: Mobile Networks

Frame Hierarchy

0 1 2 2045 2046 2047...

Hyperframe

0 1 2 48 49 50...

0 1 24 25...

Superframe

0 1 24 25...

0 1 2 48 49 50...

0 1 6 7...

Multiframe

Frame

burst

slot

577 µs

4,615 ms

120 ms

235,4 ms

6,12 s

3 h 28 min 53,76 s

One MS can use one slot per frame as a channel. But there are also other possibilities:• Sharing of one channel with

other devices (by voicecompression)

• Control channels formaintenance, requestsof new stations to getassigned a channel, …

• Thus: different typesof channels: TCH/F (full)and TCH/H (half) as wellas control channels

• Result: complex framehierarchy to come to acommon structure

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33Chapter 3.4: Mobile Networks

Data Services in GSM

Data transmission in GSM with only 9.6 kBit/s• Advanced channel coding allows 14.4 kBit/s

• Still not enough for Internet access or even multimedia applications

Thus: UMTS as “3G network”: Integration of data and voice in one network• But: new network infrastructure, new software, new devices, …

• Development of other enhancements of GSM as interim solutions

“2.5G networks” as interim solution• HSCSD as software solution

• GPRS as hardware solution• EDGE as 3G solution in a 2G network

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34Chapter 3.4: Mobile Networks

HSCSD

HSCSD (High-Speed Circuit Switched Data)• Put together several time slots for one AIUR (Air Interface User Rate, up to 57.6

kBit/s with 4 Slots of 14.4 kBit/s)• Symmetrical (2 time channels each for up- and downlink) and asymmetrical (3 + 1

channels) communication are supported

• Mainly software update for the realization of the putting together• Advantage: fast availability, continuous quality, simple

• Disadvantage: connection-oriented, 4 channels are blocked the whole time, signaling for several channels necessary

AIUR [kbit/s] TCH/F4.8 TCH/F9.6 TCH/F14.44.8 19.6 2 1

14.4 3 119.2 4 228.8 3 238.4 443.2 357.6 4

Three possible data rates for a full channel depending on the used coding and error correction

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35Chapter 3.4: Mobile Networks

GPRS

� Needed infrastructure: GSN (GPRS Support Nodes) - GGSN and SGSN- GGSN (Gateway GSN): translation between GPRS und PDN (Packet Data

Network)- SGSN (Serving GSN): support of the MS (location, accounting, security)- GR (GPRS Register): Management of user addresses

GPRS (General Packet Radio Service)

• Packet-oriented transmission, usable also for multicast• Usage of up to 8 time slots of a TDMA frame on demand

• Usage of time slots only when data are available for sending (e.g. 50 kBit/s with short usage of 4 slots)

• Advantage: step towards UMTS, flexible

• Disadvantage: expensive because some new infrastructure is needed to handle the new transmission mechanism, wireless transmission becomes a bottleneck for high traffic amount

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36Chapter 3.4: Mobile Networks

GPRS – Infrastructure Components

HLRGR VLR

PSTNISDNMSC GMSC

4

4

4 NSS

RSS

GPRS

EIRAUC

OSS

BSC

PCU

SGSNPDNGGSN

AUC: Authentication CenterBSC: Base Station ControllerEIR: Equipment Identity RegisterGMSC: Gateway Mobile Switching CenterHLR: Home Location RegisterMSC: Mobile Switching CenterVLR: Visitor Location Register

GGSN: Gateway GPRS Support NodeGR: GPRS RegisterPCU: Packet Control UnitSGSN: Serving GPRS Support Node

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37Chapter 3.4: Mobile Networks

GPRS Data Rates [kBit/s]

171,2149,8128,410785,664,242,821,4CS-4

124,8109,293,67862,446,831,215,6CS-3

107,293,880,46753,640,226,813,4CS-2

72,463,3554,345,2536,227,1518,29,05CS-1

8 time slots

7 time slots

6 time slots

5 time slots

4 time slots

3 time slots

2 time slots

1 time slot

(error-)coding

CS-1 to CS-4: decreasing error protection

• Dynamic choice of coding

• Basing on measurements of signal quality (and the needed QoS)

• The user is assigned the highest possible data rate

Position

Data rate

CS-4

CS

-3

CS

-2

CS-1

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38Chapter 3.4: Mobile Networks

Advantages of GPRS

“Always connected”• Long duration for connection establishment are eliminated

• Transmission of data on demand• Accounting by data volume, not by

connection duration

• Robust connection� Coding of data bases

on current signal quality

� Even the BSS checks the data correctness and initiates – if necessary – a transmission repeat

Page 39: Lehrstuhl für Informatik 4 Kommunikation und verteilte …

Lehrstuhl für Informatik 4

Kommunikation und verteilte Systeme

39Chapter 3.4: Mobile Networks

EDGE

EDGE (Enhanced Data Rates for GSM Evolution)

• Up to 384 kBit/s by enhanced modulation (8PSK instead of GMSK)• Transmission repeat:

Change of coding to adapt to the current channel quality

• Is build upon the existing GSM/GPRS system:

� New transceiver are needed (hardware upgrade in the BSS)

� Software-Upgrade BSS und BSC� New devices (8PSK)

� No changes in the core network!� Cheap alternative to UMTS?