GSM & GPRS - Aalto University · Antti Siitonen, 2003 5 Wireless environment › Path loss ›...

1Antti Siitonen, 2003

GSM & GPRS

Antti Siitonen

Head of Department, MSc (EE)

[email protected]

› T-110.300 Telecommunications architectures

› Lectures on 12.11.2003

› Introduction to GSM & GPRS


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


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


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, ..


Wireless environment

› Path loss

› Radio signals decrease exponentially with distance

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

› Shadow Fading and Multipath Fading

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

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

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

› Traffic loading

› Mobile speed and users density are elusive in nature


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 Multiple Access(GSM, D-AMPS, PDC)

› Same frequency, different timeslots

› Timing advance!

Picture: Ericsson


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


Handover

1. Call in progress cell 1

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

cell 2 cell 1

› BSC


Soft handover

› 1. Call in progress cell 1

2. Cell 2 has good signal strength

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

cell 2 cell 1


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 Fixed Wireless Access

(Available)

100 000 FUTURE

SYSTEMS

( >2003)

BLUETOOTH

SCOPE


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)


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 12 countries)

› 1989 GSM specification responsibility is passed to ETSI

› 1990 GSM First Phase specification

› 1991 First commercial GSM service (Global System for Mobile Communications)

› 1992 More operators signed the MoU

› 1995 GSM Second Phase specification

› check http://www.gsmworld.com/


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


› GSM1900


› 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


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


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)

Standard

SectorizedUmbrella


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)


MS BTS BSC MSCVLR

HLR

GSM Protocols

LAPDm LAPD MTP

SCCP

RIL3-CC

RIL3-MM

RIL3-RR

RSMBSSMAP

TCAP

MAP/DCM

MM

RR

MTP

SCCP

RIL3 = Radio Interface Layer 3 MAP = Mobility/Management Application Part ISUP = ISDN User PartCM = Call Management TCAP = Transaction Capabilities Application Part GMSC = Gateway MSCCC = Call Control SCCP = Signalling Connection Control Part DTAP = Direct Transfer Application Part for GSMMM = Mobility Management´ LAPD = Link Access Protocol for D-channelsRR = Radio Resource MTP = Message Transfer Part

ISDN (PSTN)

ISUP

RIL3-protocols• Radio managementMAP-protocols• Mobilty management for switchingSS7-protocols• Call management

GMSC

DTAP

ISUP


Simplfied connection establishment example1. The subscriber in the fixed network dials the B-subscriber's mobile number. The PSTN identifies the number and sets up a connection to the called

network

› Default signalling protocol is the signalling protocol between the GMSC and the PSTN is ISUP, but TUP(Telephone User Part) or a channel-associated protocol could also be used. ISUP and TUP use MTP as a signalling bearer.

2. The GMSC does not know through which MSC the mobile can be reached; nor does it know whether the mobile is free, busy, turned on or turned off. To be able to continue, the GMSC must therefore request a routing number from the HLR

› The GMSC uses the MAP protocol for this request. MAP uses the transaction capabilities application part (TCAP), which in turn uses the SCCP as a bearer.

3. The registration function continuously updates the HLR on the location of the mobile (MSC service area). Provided the mobile is turned on and free, the HLR will call that service area's VLR to request a free routing number. The mobile's subscriber data is sent along with this request.

› The MAP protocol is used for this communication

4. The GMSC receives the routing number and uses it to select a route in the mobile network. GMSC forwards the call to MSC

› ISUP is used for signalling between the GMSC and the MSC

5. MSC consults its VLR to find out in which group of cells (location area) the mobile is at the moment. (Keeping the VLR informed of the mobile's location is also part of the registration function.) MSC then orders BSC to find the mobile.

› Communication at this stage is BSSMAP. These signals are carried between the MSC and the BSC by the connectionless SCCP service.

6. BSC sends a paging call to all cells in its service area that can be visited by the mobile at the moment.

› The paging procedure is described in the GSM standard. The paging call is carried by LAPD between the BSC and the base station, and on control channel across the air interface.

7. Next, the call is answered by the mobile.

› The answering procedure is described in the GSM standard. The answer is carried by control channel across the air interface and then sent on to the BSC on LAPD.

8. BSC allocates the mobile a control channel for signalling with MSC.

› This information is carried by LAPD between the BSC and the base station, and by control channel across the air interface.

› A traffic channel over radio access and between MSC and BSC is also reserved for the call.

9. Now the mobile communicates directly with the MSC

› This communication is in accordance with the DTAP protocol. The signals are carried by SCCP/MTP between the MSC and the BSC, relayed through the BSC, and carried by LAPD between the BSC and the base station

› The DTAP signalling concludes with the set-up of a traffic channel through the switches in BSC and MSC. An acknowledgement of this through-connection is sent backwards to the PSTN


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


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 = code for VLR and HLR), MSISDN, Access control classes

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

› PLMN data: Network identifier and home network frequencies


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 ciphering algorithms, 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. HLR forwards triplets to VLR. VLR negotiates authentication and ciphering with MS

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


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- Key Kc- RAND- A3(Ki&RAND) = SRES


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


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)


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 module or alternatively a standalone SMS Center. It is not possible to make the Short Message Service available without an SMS Center since all short messages pass through the SMS Center.

› SMS Mobile terminate

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

› SMS Mobile originate

› True two-way SMS capability.


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 implement applications and menus executed on the SIM card

› Wireless Application Protocol WAP

› Attemp to standardize mobile applications

› Optimized for wireless networks (not only for GSM)


Operator service offering

› Operators

› Traditionally 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, MMS ..

› Mobile number portability

› Possibility to change service operator and keep the old mobile number


Data in GSM-network

Internet

ISP(accessserver)

GSM-network

PSTN

BSMSC

Connection time5-30 s

Data rate9.6 kbit/s

Poor user interface fordata applications

Time basedbilling

Circuitswitchedconnection

- features and restrictions


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


New service requirements

› Location Related Services

› Can be implemented to the current architecture

› 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


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


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


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


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

BSS=Base Station Subsystem, GGSN=Gateway GPRS Support Node, HLR=Home Location Register, MS=Mobile Station,MSC=Mobile Switching Centre, SGSN=Serving GPRS Support Node, PDN=Packet Data Network, VLR=Visitor Location Register


GPRS Network architecture

BTS

SGSN GGSN

BSC MSC/VLR HLR

IPNetwork

X.25NetworkBackbone

NetworkIP

MS


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


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)


GGSN = 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


RL service network

WAP GW

Internet

RL ISP ExternalWAP GWor ISP

GPRS- connections

GPRS

GSM

HLR (GSM&GPRS)

GGSN

APN= internet

APN= wap

APN= yritys.fi

SGSN

soittosarja

Corporatenetwork

WAP GWAPN = Access Point Name(Logical Name)


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

BSSGP=BSS GPRS Protocol, GTP=GPRS Tunneling Protocol, LLC=Logical Link Protocol, MAC=Medium Access Control, GSMRF=GSM Radio Physical Layer, SNDCP=Subnetwork Dependent Convergence, UDP=User Datagram Protocol,TCP=Transmission Control Protocol


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 timeslot and multiple timeslots per user

› Support for Class B and C Mobiles

› GPRS Charging (packet based billing)

› IP and X.25 packet data networks

› Static and dynamic IP address allocation

› Authentication and Ciphering

›Phase 2

› Enhanced QoS support

› Access to ISP and Intranets

› GPRS prepaid

› Group call

› Point to multipoint services


GPRS roaming

› International Roaming Experts Group (IREG) Proposal

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

› GPRS Roaming Exchange (GRX) at least in every continent; some cases 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


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


Three categories of terminals›Class A

› simultaneous circuit switched and packet connection

›Class B

› both circuit and packed switched connections possible but not at the same time

›Class C

› only packet switched or circuit switched connection


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


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

Card phone+Notebook

PDAWAP

Smart phone


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/s up to 384 kbit/s

› Needs new radio interface & terminals

› Transport network needs to be also upgraded

› Currently EDGE deployments are mainly going on in USA

› During 2003 several European operators have announced EDGE deployments


GSMcapable systems

IMT-2000capable systems

Functionality& bitrate

Time

Speech

Circuit data

HSCSD

GPRS

WCDMAEDGE

Summary and conclusion


GSM-based network evolution

GSMGSM

HSCSDHSCSD

GPRSGPRS

EDGEEDGE UMTSUMTS

1997 1998 1999 2000 2001 2002 2003

Non-UMTS


For those who are interested to know more

Literature beyond course material:

› Yi-Bing Lin, Imric Chlamtac: ”Wireless and Mobile Network Architectures”

› Christoffer Andersson: ”GPRS and 3G Wireless Applications: Professional Developer’s Guide”

› Timo Halonen, Javier Romero, Juan Melero: ” GSM, GPRS and EDGE Performance: Evolution Toward 3G/UMTS”

Internet:

http://www.3gpp.org/

http://www.cs.hut.fi/~hhk/GPRS/gprs_index.html

http://www.google.com/ ☺

GSM & GPRS - Aalto University · Antti Siitonen, 2003 5 Wireless environment › Path loss ›...

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