1Antti Siitonen

GSM & GPRS

Antti Siitonen

Technologist MSc (EE), Radiolinja

Antti.Siitonen@radiolinja.fi

›T-110.300 Telecommunications architectures

›Lectures on 14.11.2001

›Introduction to GSM & GPRS

2Antti Siitonen

Contents1. Short introduction to mobile telephony / mobile telecommunication

1.1. Radio interface

1.2. Mobility management

1.3. Technology generations

2. GSM

2.1. Architecture overview BSS+NSS

2.2. Mobile handset and SIM

2.3. GSM services

2.4. GSM data enhancements

3. GPRS

3.1. Architecture overview

3.2. GPRS terminals

3.3. GPRS services

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Advantages of wireless access› Speed of deployment and easiness of reconfiguration

› In certain cases cost efficiency

› Independence of wired infrastructure etc..

› BUT THE MAIN ADVANTAGE IS

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Frequecy spectrum and spectrum allocation› Who? What band and what bandwidth? What technology?

› Globally CEPT, Europe ERC» World Radio Conference (WRC) every fourth year

› Nationally local regulators (Ficora in Finland)

› Licenced frequencies

› Requires licence, typically for operator use» Co-ordination, frequency planning, interference regulation» Mobile networks, LMDS, Broadcasting TV and radio, links» T.ex. 3G spectrum allocation 155 MHz (2*60+35) in 2 GHz band

› Unlicenced frequencies

› Free for all, but within regulated boundaries (maximum power,technology)

» WLANs, cordeless phones, DECT, ..» 2,4 GHz, 5 GHz, 17 GHz, ..

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Wireless environment› Path loss

› Radio signals decrease exponentially with distance

› The wide range of path loss associated with different radio linksresults in the Near-Far problems

› Shadow Fading and Multipath Fading

› Blockage of radio paths by buildings and terrain causes shadowfading. Signals may drop 20 to 30 dB by turning a building corner

› Radio signal received from different paths may destructivelycombine and result in multipath fading

› In broadband systems the delay spread caused by multipathpropagation may result in inter symbol interference

› Traffic loading

› Mobile speed and users density are elusive in nature

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Multiple access methods

› FDMA – Frequency Division Multiple Access(NMT, TACS, AMPS)

› One frequency per user

› Filtering!

› CDMA – Code DivisionMultiple Access (IS-95, UMTS)

› Same frequency, same timeslots,but different codes

› Power Control!

› TDMA – Time Division MultipleAccess(GSM, D-AMPS, PDC)

› Same frequency, different timeslots

› Timing advance!

Picture: Ericsson

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Mobility management› Status of the mobile (on/off) - pageing

› Location of the mobile - Location update

› Moving mobile

› Change the active cell - handover (handoff)» Network optimization» Movement prediction

› Change the active operator - roaming» Typically when crossing the country border

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Handover

1. Call in progress cell 1

2. Signal cell 2 for handover3. Switch to cell 2 = handover complete

cell 2 cell 1

› BSC

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Soft handover

› 1. Call in progress cell 12. Cell 2 has good signalstrength

3. Call in progress cell 1 and cell 2 = soft handover

cell 2 cell 1

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Satellite

GSM + Evolution

- GPRS (2000)(General Packet Radio Service)

- EDGE (2002)(Enhanced Development for GSM

Evolution)

Universal

Mobile

Telecommunication

Service

= UMTS

(> 2002)

Wireless systems in generalkBit/s

Coverage areaRoom House Campus City Country Continent

10

100

1000

10 000

Wireless Local

Area Networks

= WLANs

(Available)

Broadband FixedWireless Access

(Available)

100 000 FUTURE

SYSTEMS

( >2003)

BLUETOOTH

SCOPE

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1G, 2G, 3G…….

1980 1990 2000 2010 2020

1 G• Analog• Voice services• ARP, NMT, AMPS,TACS 3 G

• Advanced digital• Voice, mediumspeed data, multi-media messaging• Convergence ofwireless and internet• IMT 2000 = UMTS, CDMA2000

Beyond 3 G (4G)• Voice over IP• High speed data, real-timemultimedia services

2 G• Digital• Voice, messaging and low speed dataservices• GSM (TDMA), cdma one (IS-41), D-AMPS (TDMA), PDC (TDMA)

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GSM› 1982 Group Special Mobile was established by CEPT

› 1987 Air interface TDMA was chosen as access method and InitialMemorandum of Understanding was signed by telecom operators (from 12countries)

› 1989 GSM specification responsibility is passed to ETSI

› 1990 GSM First Phase specification

› 1991 First commercial GSM service (Global System for MobileCommunications)

› 1992 More operators signed the MoU

› 1995 GSM Second Phase specification

› check http://www.gsmworld.com/

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GSM radio technology basics› Frequency bands

› GSM400» 450.4 - 457.6 MHz paired with 460.4 - 467.6 MHz or 478.8 - 486 MHz paired with 488.8 - 496 MHz

› GSM900

» 880 - 915 MHz paired with 925 - 960 MHz

› GSM1800

» 1710 - 1785 MHz paired with 1805 - 1880 MHz

› GSM1900

» 1850 - 1910 MHz paired with 1930 - 1990 MHz

› Multiple access method

› TDMA

› Modulation

› GMSK

› Radio transmitted power (Mobile Station)

› 1 W Handset on GSM1800, 2 W Handset on GSM900, 8W Fixed installation (Car)

› Channels per carrier

› 8 timeslots

› Channel bit rate

› 22,8 kbit/s

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PLMN Architecture overview

BSCBTS

BTS

MS +SIM

TRAU

MSC

HLR

EIR

BSS

BSS

BSS

MSC area

BTS

MSC area

MSC area

MSC areaHLR

HLR

VLR

Customer care & Billing system

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Base station subsystem› BTS = Base Transceiver Station

› Radio-related tasks, Air interface

› Standard configuration (cell size 300 m - 35 km)

› Umbrella configuration

› Sectorized configuration

› BSC = Base Station Controller

› Controls several BTSs and takes care of all central functions of BSS

› TRAU = Transcoding Rate and Adaptation unit

› Bitrate adaptation and voice coding/decoding

› OSS = Operation and support subsystem (also for NSS)

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Network switching subsystem› MSC = Mobile-Services Switching Center

› Basically ISDN telephone exchange + mobility modifications» Modifications: BSS channel assignments and inter-MSC handover

› Gateway MSCs interface other networks

› HLR = Home Location Register / Authentication Center (AuC)

› Database for subscriber identity - Home

› AuC provides triplets for authentication and ciphering

› VLR = Visitor Location Register

› Database for temporary subscriber identity - Visited

› Integrated into each MSC

› EIR = Equipment Identity Register

› GSM phone IMEI register (Lists: White, Black, Gray)

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Mobile Station› Voice encoding Voice decoding

› Channel encoding Channel decoding

› Interleaving De-interleaving

› Burst generation Re-formatting

› Ciphering Deciphering

› Modulation Demodulation

› Amplifier Receiver

filter + Antenna

› Burned in IMEI = Individual Moblie Station Equipment Identity

› Central processor, clock and tone, internal bus system, keyboard

› SIM interface

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SIM› Subscriber Identity Module

› Data storage and algorithm executions

› Memory 8 kB / 16kB / 32 kB /64 kB

› Administrative data: PIN/PUK, SIM service table..

› Security related data: Algorithms A3 and A8, Ki, Kc, CKSN

› Subscriber data: IMSI (International mobile subscriber identity),MSISDN, Access control classes

› Roaming data: TMSI (Temporary MSI), NCCs (Network Color Codes)

› PLMN data: Network identifier and home network frequencies

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GSM authentication and ciphering- Algorithms A3, A5/X ja A8 . Algorithms are secret.

- SIM parameters:Parameter Need:

Algorithms A3 ja A8 (mandatory, fixed) Authentication, Key Kc calculation

Key Ki (individual) (mandatory, fixed) Unique key, stored in SIM+HLR

Key Kc (cipher) (mandatory, changeable) Result of A8 algorithm, calculated from Ki and RAND

CKSN (mandatory, changeable) Ciphering key sequence number, referred to Kc

- Ciphering:MS has at least A5/1 and A5/2 (There are seven different cipheringalgorithms, MS informs the network of it´s capabilities). Cipher is only for air interface.

- Note! Ciphering is optional!

- Network parameters:

› Every subscription has unique Ki stored in HLR

› AuC provides authentication triples. For one user five triplets may be generated at one time. HLRforwards triplets to VLR. VLR negotiates authentication and ciphering with MS

› Triplet:» Signed Response (SRES), Random Number (RAND) and Kc

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GSM authentication processNetwork challenges the user. User must prove to have correct identity.

BSCBTS

VLR

MSCAuC/HLR

2. Terminal (SIM)- Receives RAND- A3(Ki&RAND) = SRES- Returns SRES to VLR

3. VLR compares SRES

1. Triplet to VLR:lle- Key Kc- RAND- A3(Ki&RAND) = SRES

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GSM ciphering process

BSCBTS

VLR

MSCAuC/HLR

2. Cipher key A8(Ki&RAND)=Kc

3. VLR provides BTS information of chosenA5/x algorithm and key Kc4. BTS transfers the information to MS5. Traffic is ciphered and decipheredin 114 bit sequences

1. Cipher key A8(Ki&RAND) =Kc

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GSM Services› Bearer services: Bit transmission, 3,1 kHz voice, sync./async.data

› Voice

› Full Rate codec 13 kbit/s (+ 9,8 kbit/s for error correction)

› Enhanced Full Rate codec 13 kbit/s (+ 9,8 kbit/s for error correction)

› Half Rate codec 6,5 kbit/s (+ 4,9 kbit/s for error correction)

› Emergency call

› Suplementary services: Call forwarding, Calling line identity..

› Fax

› 9,6 kbit/s Fax

› Data

› 9,6 kbit/s transparent data (no error correction)

› 9,6 kbit/s non-transparent data (with error correction)

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GSM Services (cont.)› Voice mail

› Answering machine type of service. Colocated with MSC

› SMS

› The initial SMS Center may be simply a voice mail platform moduleor alternatively a standalone SMS Center. It is not possible to makethe Short Message Service available without an SMS Center sinceall short messages pass through the SMS Center.

› SMS Mobile terminate

› Often SMS Mobile Terminate Services are offered along with voicemail notifications.

› SMS Mobile originate

› True two-way SMS capability.

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Value added services (examples)› Mainly SMS based services

› Edited content for SMS

› Ringtones

› Logos and picture messages

› CHAT

› Payment methods

› Menu driven SMS applications

› Sim Application Toolkit (STK) - set of tools possible to implementapplications and menus executed on the SIM card

› Wireless Application Protocol WAP

› Attemp to standardize mobile applications

› Optimized for wireless networks (not only for GSM)

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Operator service offering› Operators

› Typically Network and Service operators are same corporation

› Pure service operators have entered during last years

› Subscriptions for different needs

› Business use, Private use, Pure data

› Basic services without extra subscription

› Call waiting, hold, group call, SMS, Calling line identity, Roaming,data

› Premium services

› Voice mail, SMS-service packets, Billing limits, Mobile E-mail, ..

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Data in GSM-network

Internet

ISP(accessserver)

GSM-network

PSTN

BSMSC

Connection time5-30 s

Data rate 9.6 kbit/s

Poor user interface fordata applications

Time basedbilling

Circuitswitchedconnection

- features and restrictions

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GSM data development› Basic GSM data:

› Connections to any modem service

› 9,6 Kbit/s data rate

› Enhancements:

› ISDN type connection - faster connection time

› V.42bis compression - 4:1 basic text compression (limited)

› HSCSD - High Speed Circuit Switched Data» Non transparent: 1-4 time slot per MS - up to 38,4 kbit/s - dynamic

allocation of channels» Transparent: Up to 8 time slots per MS - static allocation of channels

› 14,4 kbit/s channel coding» Less bits for error correction if signal is good enough» Can be combined to HSCSD and/or V.42bis

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New service requirements› Location Related Services

› Can be implemented to the current architecture» Still no established way to implement (Oct. 2001)

› Different methods give different accuracy

› Need for packet based mobile data?

› Asymmetric traffic» WAP-services» Internet services: Web browsing

› Multiple Services with Variable Bit Rates

› Multi-Session and “Always On” Features

› Variable QoS Requirements

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time

traffic

Traffic in mobile network

08:00 17:00

› GSMnetwork is planned for

circuit switched voice services

› ”Rush hour” is the capacity

planning key

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WWW browsing traffic

Rate (kbit/s) uplink downlink14.4 4.8% 19.5%28.8 3.9% 20%64 3.0% 11.7%

Rate (kbit/s) uplink downlink14.4 4.8% 19.5%28.8 3.9% 20%64 3.0% 11.7%

- Measured usage rate on a circuit switched* data connection**

* GSM and HSCSD are circuit switched* measurements are made during six days on may 1999 from the Kolumbus ISP’s access server

aika

Typical downlink traffic

Home page... WWW download...Reading...Reading...

GPRS uses channel only when there is traffic » Resources areavailbe for otherusers when there isno traffic

Data traffic

No traffic

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GPRS Data rates - Link layer

9,05 kbps

13,4 kbps

15,6 kbps

21,4 kbps

CS-1

CS-2

CS-3

CS-4

1-8channel

171,2 kbpsin theory

Practical data rate is 10 - 40 kbit/s

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GPRS Reference network architecture

Gf

Gi

Gn

GbGc

D

Gp

Gs

Signalling and Data Transfer InterfaceSignalling Interface

MSC/VLR

MS BSS TEPDN

Um

Gr

HLR

Other GPRS Nw

SGSN

GGSN

GGSN

EIR

A

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GPRS Network architecture

BTS

SGSN GGSN

BSC MSC/VLR HLR

IPNetwork

X.25NetworkBackbone

NetworkIP

MS

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GPRS updates to GSM network

Element Software HardwareMS Upgrade required Upgrade requiredBTS Upgrade required No changeBSC Upgrade required PCU Interface

TRAU No change No changeMSC/VLR Upgrade required No change

HLR Upgrade required No changeSGSN New NewGGSN New New

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SGSN = Serving GPRS Support Node› Authentication, Authorization

› GTP tunneling to GGSN

› Ciphering and compression

› Mobility management

› Session management

› Interaction with HLR, MSC/VLR

› Collects charging and statistics information about data networkusage

› Interfaces towards OSS (Operation and Management)

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Gateway GPRS Support Node› Interfaces external data networks

› Internet, Intranets, Operator ISP network

› Encapsulates end user data in GTP packets

› Routes mobile originated packets to right destination

› Filters end user traffic

› Collects charging and statistics information about data networkusage

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RL service network

WAP GW

Internet

RL ISP ExternalWAP GWor ISP

GPRS- connections

GPRS

GSM

HLR (GSM&GPRS)

GGSN

APN= www.radiolinja.gprs.fi

APN= www.radiolinja.wap.fi

APN= www.yritys.wap.fi

SGSN

soittosarja

Corporatenetwork

WAP GWAPN = Access Point Name(Logical Name)

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Connection protocol stack

Relay

NetworkService

GTP

Application

IP / X.25

SNDCP

LLC

RLC

MAC

GSM RF

SNDCP

LLC

BSSGP

L1bis

RLC

MAC

GSM RF

BSSGP

L1bis

Relay

L2

L1

IP

L2

L1

IP

GTP

IP / X.25

Um Gb Gn GiMS BSS SGSN GGSN

NetworkService

UDP /TCP

UDP /TCP

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GPRS phases›Phase 1

› Point-to-Point packet service

› CS1 and CS2 channel coding

› Internal interfaces

› Flexible radio resource allocation,I.e. Multiple users per timeslotand multiple timeslots per user

› Support for Class B and CMobiles

› GPRS Charging (packet basedbilling)

› IP and X.25 packet data networks

› Static and dynamic IP addressallocation

› Authentication and Ciphering

›Phase 2

› Enhanced QoS support

› Access to ISP and Intranets

› GPRS prepaid

› Group call

› Point to multipoint services

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GPRS roaming› International Roaming Experts Group (IREG) Proposal

› Roaming traffic will be carried over central managed hierarchicalRoaming Network where commonly agreed policies are followed

› GPRS Roaming Exchange (GRX) at least in every continent; somecases thre could be many GRX’s in one country

› Every Roaming Operator will have connection to GRX using some ofthe following methods

» Layer 1 connection (Leased Line, fibre, etc.)» Layer 2 logical connection (ATM/Frame Relay, etc)» Layer 3 IP VPN connection over public IP Network (IPsec tunnel)

› Connection will carry BGP routing (Border Gateway)

› Public addressing in GPRS backbone networks

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Roaming network

BTS

FW

BSC

Home networkHome network

Visited networkVisited network

FW

VisitedNetwork

BB

BG

BTS

SGSN DNS

GGSN

Inter-operatorBB

HomeBB GGSN

DNS

SGSN

Internet

BSC• Border Gateway

• Inter-operator Backbone

BG

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Three categories of terminals›Class A

› simultaneous circuitswitched and packetconnection

›Class B

› both circuit and packedswitched connectionspossible but not at the sametime

›Class C

› only packet switched orcircuit switched connection

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Different capabilities

›Terminals are typically asymmetric

› Receiving is more simple than sending

› Full duplex radio

› Tx Rx

› 1 + 1 Ch

› 1 + 2 Ch

› 1 + 4 Ch

› 2 + 2 Ch

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GPRS-terminal types and service usage GPRS-terminal types and service usage

Card phone+Notebook

PDAWAP

Smart phone

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EDGE› Enhanced Data rates for Global (GSM) Evolution

› Modulation update: from GMSK to 8-PSK

› In theory modulation efficiency will be tripled

› In practice the maximum bit rate increases from GPRS’s 171,2 kbit/sup to 384 kbit/s

› Needs new radio interface & terminals

› Transport network needs to be also upgraded

› At the moment (11/2001) only AT&T and Verizon have announcedto implement EDGE in their network

› Some vendors have announced to implement EDGE only to 1900 MHz

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GSMcapable systems

IMT-2000capable systems

Functionality& bitrate

Time

Speech

Circuit data

HSCSD

GPRS

WCDMAEDGE

Summary and conclusion

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GSM-based network evolution

GSMGSM

HSCSDHSCSD

GPRSGPRS

EDGEEDGE UMTSUMTS

1997 1998 1999 2000 2001 2002 2003

Non-UMTS