by erick o’connor february 2005 gsm & gprs primer

By Erick O’Connor

February 2005

GSM & GPRS Primer

2©2001 - 2005 Erick O’Connor

Topics

Background• The history of cellular communications

• Key statistics– Worldwide subscribers

– Top 20 global mobile operators

Global System for Mobile (GSM)• The Radio environment

• Basestation & Network subsystems

• Subscriber data & addressing

• Circuit-switched network architecture

• Overview of PDH transmission

• Common Channel Signalling & GSM MAP

General Packet Radio System (GPRS) • Protocol layers

• Key information

• Dimensioning a Network

• Mobility Management

Third-Generation Systems (UMTS)• Evolution paths

• Core components

AThe following symbol indicates that the slide contains animations. Press the space bar to advance the animation

History of Cellular Communications1960s to the Present Day


…the early years

1960 – 1970s

• Idea of a cell-based mobile radio system developed by AT&T’s Bell Labs in late 1960s• First commercial analogue mobile cellular systems deployed 1978

1980s (1st Generation Analogue Systems)

• Usage in N.America grows rapidly– Advanced Mobile Phone System (AMPS) becoming the de facto standard

• Europe, run by the PTTs, characterised by multiple incompatible analogue standards– Nordic Mobile Telecommunications (NMT-450)– Total Access Communications (TAC) – United Kingdom– C-Netz – West Germany– Radiocom 2000 – France– RTM / RTMS – Italy etc. etc.

• Capacity limitations already becoming apparent by end of decade….


… going digital

Late 1980s to early 1990s (2nd Generation Digital Systems)

• N.America relies on de facto “let the best technology win” standardisation

• By contrast Europe decides to rely on standardisation & co-operation

– Huge pent-up demand for mobility can not be met by upgrading existing purely analogue systems. Parallel advances in digital techniques and Very Large Scale Integration (VLSI) chipset manufacture suggest a new way forward

– However European domestic markets individually too small to achieve the economies of scale necessary for vendors to take the risk of developing such a risky new solution

– Enter the European Commission with a political agenda – demonstrate Europe’s “technology leadership” and ensure European manufacturers can compete globally

• New spectrum auctions in USA in early 1990 (PCS 1900) lead to plethora of standards

– D-AMPS IS-54 – Motorola sponsored, TDMA IS-136, CDMA IS-95 – Qualcomm sponsored

– Plus, limited GSM

• Meanwhile in Europe…


…GSM is born

Late 1980s to early 1990s (2nd Generation Digital Systems)

• Guided by European Commission & European Telecommunications Standard Institute

• 26 European telecommunication administrations establish the Groupe Spéciale Mobile (GSM) in 1982 with aim to develop a new specification for a fully digital pan-European mobile communications network

• The Group notes that the “new industry’s economic future will rely on unprecedented levels of pan-European co-operation”

• Political decision to force member countries to:

– allocate frequencies at 900 MHz in every EC country (later 1800 MHz)

– specify the exact technology to be used and;

– deploy systems by 1991

• First commercial GSM networks deployed in 1992 – Denmark / Finland / France / Germany / Italy / Portugal / Sweden / United Kingdom


…beginning of the GSM success story

By End of 1993

• One million subscribers using GSM

• GSM Association has 70 members, 48 countries

• First non-European operator, Telstra of Australia

North America - 20%

Caribbean, Central &Latin America - 8%

Western Europe - 32%

Central & EasternEurope - 3%

Central Asia - 11%

Asia-Pacific - 22%

Middle East & Af rica - 4%

2000 (470 million)

North America - 20%




Central Asia - 11%

Asia-Pacific - 22%

Middle East & Af rica - 4%

2000 (470 million)

North America - 17%




Central Asia - 13%

Asia-Pacific - 19%

Middle East &Af rica - 6%

2004 (1,192 million)

North America - 17%




Central Asia - 13%

Asia-Pacific - 19%

Middle East &Af rica - 6%

2004 (1,192 million)

….Subscribers

And, by technology.…

www.gsmworld.com


…the turn of the century & 3rd generation services

• Multiple operators per country & worldwide (800+) – intense price based competition

– Huge growth in subscribers thanks to pre-paid but falling ARPU & high churn (c.25%)

– Market close to saturation – slowing subscriber penetration growth rates (c.85%)

• The challenge – what to do in future?

• Europe keen to replicate commercial success of GSM but, Americans & Japanese had different views and needs

– Japan had run out of spectrum for voice

– Americans unhappy at being “dictated to” by a European standard

– European vision of always on data & rich value added content services

• America & Japan jointly force Europe to open up standardisation process so as not to once again “lock-out” other trading blocs’ vendors

– Creation of 3rd Gen Partnership Programme (3GPP) body

– Heated standardisation on Wideband CDMA (Qualcomm vs Ericsson)

– Final agreement on Universal Mobile Telecommunications Standard (UMTS) in 1998….


The market today – key statistics

GSM designRadio & Network subsystems, Signalling & Transmission


Basic GSM network elements

AUC Authentication CentreBSC Basestation ControllerBTS Basestation TransceiverEIR Equipment Identity RegisterGMSC Gateway Mobile Switching CentreHLR Home Location RegisterISC International Switching CentreISDN Integrated Services Digital NetworkMSC Mobile Switching Centre PDN Packet Data Network (X25)PSTN Public Switched Telephony NetworkSIWF Shared Interworking FunctionVLR Visitor Location RegisterXCDR Transcoder (16 / 64kbps coding)

BTS

BSC

BSC

BTS

BTS

MS + SIM

XCDR

Radio Subsystem

GMSC

ISC

PSTN

ISDN

PDN

MSC

SIWF

Network Subsystem

VLR

EIRAUC

HLR

User Data & Authentication

A


GSM air interface design

• Access Techniques

– Time Division Multiple Access

– Frequency Division Multiple Access

– Space Division Multiple Access

• Radio characteristics

– Gaussian Minimum Shift Keying (GMSK)

– Slow Frequency Hopping

• Logical structure

– 8 Timeslots per Carrier

– 1 Downlink Timeslot reserved for signalling

– 3 timeslot difference between uplink & downlink

• Frame structure used for synchronisation

– 51-frame Multiframe (235.4 ms)

– 51 or 26 Multiframe Superframe (6.12 sec)

– 2048 Superframe Hyperframe (3 hr 28 mins)

Multiple cells

Time

Fre

qu

en

cy

8 timeslots

f3

f2

f1

f0

FDMA & TDMA

f0

GMSK Spectrum

+400 kHz-400 kHz

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

Downlink

Uplink

Delay


BTS

BSC

BSC

BTS

BTS

MS + SIM

XCDR

Radio subsystem (i)

• Basestation Transceiver (BTS) provides radio channels for signalling & user data

• A BTS has 1 to 6 RF carriers per sector and 1(omni) to 6 sectors

– e.g. 3/3/3 = 3 sector with 3 carriers per sector

– 3 x 7 Timeslots x 3 = 63 Timeslots total

– c.52 Erlangs @ 2% Grade of Service

– c.2,000 users per BTS @ 25 mErl / User (90 seconds)

• Frequency reuse depends on terrain, frequencies available etc.

• Paired spectrum shared by Operators

– 900 / 1800 MHz in Europe / Asia (25 & 75 MHz)

– 1900 MHz in N.America

• 200 kHz channel separation

• 125 Channels @ 900 MHz

15

43

27

6

K=7

f1f5

f4

f3

f2

f7f6

15

43

27

6

1

32

f1

f3f2

1

32

K=3

Frequency reuse & cluster formation


Radio subsystem (ii)

• Basestation Controller (BSC) controls a number of BTS

– Acts as a small switch

– Assists in handover between cells and between BTS

– Manages the Radio Resource, allocating channels on the air interface

• Transcoding (XCDR) function is logically associated with BTS

– But, typically located at BSC to save on transmission costs

– XCDR provides 13 kbps Coding / Decoding between GSM Codec & standard 64 kbps A-law encoded voice

• Interfaces

– “Abis” – BTS to BSC interface (never fully standardised so vendor-specific variants exist)

– “A” – BSC to MSC interface carrying voice, BSC signalling and Radio

– Traffic Channels are mapped one-to-one between BTS and Transcoder

– BTS can be connected in “Star” or “Daisy-chain” arrangement to BSC (max. 15)

BTS

BSC

BSC

BTS

BTS

MS + SIM

XCDR


Network subsystem (i)

• Core component is Mobile Switching Centre (MSC)

– Performs all switching functions of a fixed-network switch

– Allocates and administers radio resources & controls mobility of users

– Multiple BSC hosted by one MSC

• Gateway MSC (GMSC) provide interworking with other fixed & mobile networks

– Crucial role in delivering in-coming call to mobile user in association with Home Location Register (HLR) interrogation

• Shared Interworking Function (SIWF)

– Bearer Services are defined in GSM including 3.1 kHz Voice, ISDN, 9.6 kbps Data & 14.4 kbps

– IWF provides “modem” capabilities to convert between digital bearer & PSTN, ISDN & PDN

• International Switching Centre (ISC)

– Provides switching of calls internationally. Switch may be provided by another carrier

GMSC

ISC

PSTN

ISDN

PDN

MSC

SIWF


Network subsystem (ii)

• Home Location Register (HLR) holds master database of all subscribers

– Stores all permanent subscriber data & relevant temporary data including:

• MS-ISDN (Mobile Subscriber’s telephone no.)

• MSRN (Mobile Station Roaming no.)

• Current Mobile Location Area

– Actively involved in incoming call set-up & supplementary services

• Visitor Location Register (VLR) associated with individual MSCs

– VLR stores temporary subscriber information obtained from HLR of mobiles currently registered in serving area of MSC

– Involved in registration of mobiles

– Assists in delivery of supplementary service features such as Call Waiting / Call Hold

• Authentication Centre (AUC) & Equipment Identity Register (EIR)

– GSM is inherently secure using encryption over the air-interface and for authentication / registration

– AUC holds each subscriber’s secret key (Ki) & calculates “triplet” for challenge / respond authentication with mobile

– SIM is sent data and must calculate appropriate response

– EIR is used to store mobile terminals serial numbers

VLR

EIRAUC

HLR

GSM call setup & Signalling


Signalling – Air interface

Air Interface Signalling

• Downlink signalling (to Mobile Station)

– Relies on Bearer Control Channel (BCCH) set at fixed frequency per cell

• Mobile Stations use this to lock-on to network

• Mobile Stations periodically scan environment and report back other BCCH power levels to BSC to assist in handover

– Access Grant Channel (AGCH) – used to assign a Control or Traffic Channel to the mobile

– Paging Channel (PCH) – paging to find specific mobiles

• Uplink signalling (from MS) more complicated

– Random Access Channel (RACH) – competitive multi-access mode using slotted ALOHA to request dedicated signalling channel (SDCCH)

• Bidirectional channels include

– Traffic Channels (TCH) – Carrying full rate voice @ 13 kbps / half-rate voice

– Standalone Dedicated Control Channel (SDCCH) – used for updating location information or parts of connection set-up

– Slow Associated Control Channel (SACCH) – used to report radio conditions & measurement reports

– Fast Associated Control Channel (FACCH) – uses “stolen” traffic channel capacity to add extra signalling capacity


Signalling – Mobile Application Part interfaces

BTS

BSC

BTS

MS + SIM

VLR

EIR

HLR

MSC

MSC

VLR

A

F E

Abis

C

B

G

D

Um

GSM Specific Signalling Interfaces

(Mobile Application Part)

Network Signalling

Um Air interface signalling

Abis Radio management

A BSS management, connection control & mobility management

B Subscriber data, location information, supplementary service settings

C Routing information requests

D Exchange of location-dependent subscriber data & subscriber management

E Inter-MSC handover signalling

F Subscriber & equipment identity check

G Inter-MSC handover, transfer of subscriber data


ITU-T Common Channel Signalling System Number 7

MTP Layers 1/2/3

TCAP

SCCP

MAP INAP OMAP

ISUPTUP

Standard Telephone User

Part (TUP)

Most basic CSS7 signalling

ISDN User Part

Add functionality to permit ISDN signalling

(i.e. fully digital) between networks

Message Transfer Part

Lowest level, permits interconnection with underlying physical

transmission medium

Signalling Connection Control

Part

Functionally equivalent to TCP layer, carries

“Connectionless” messages between Network elements

Application Parts

Actually carry the specific messages for Mobile (MAP), Intelligent Network (INAP)

or Operations & Maintenance (OMAP)

Transaction Control Application

Part – component responsible for “carrying” higher level Application

Parts to their correct destinations

ISO

Layers

1 t

hro

7

Signalling 101

• Line signalling – “tell the other end you want to make call”

• Register signalling – “tell the other end the destination of the call”

GSM interfaces B, C, D, E & G carried as Mobile Application Part

A


PDH transmission …composition of 32 channel E1 bearer

0 1 2 3 4 5 6 7 8 9 10 111213141516171819202122232425262728293031

TS 0 Synchronisation

HeaderTS16 Signalling

ITU-T G.703 E1 link 2048 kbps

32 x 64 kbps Timeslots

Voice / Data Timeslot

Abis - Voice GSM Codec

4 x 13 kbps Timeslots

2 Mbps

34 Mbps

140 Mbps

Plesiochronous Digital Hierarchy (PDH)

STM-1

STM-4

STM-16Synchronous Digital Hierarchy (SDH)

(SONET - USA)


SDH Fibre Optic

Network

Transmission Plane

Synchronisation

Other NetworksDrop & Insert

MultiplexersBSS

Circuit-switched network architecture(Transmission & Signalling planes)

BSS Basestation SubsystemCCS7Common Channel Signalling #7CO Central OfficeHLR Home Location RegisterMSC Mobile Switching CentreSDH Synchronous Digital HierarchySSP Service Switching PointSTP Signalling Transfer Point

STP

HLR

CSS7 Signalling Plane

MSC

CCS7 Links

SSP

CO Switch

A


BTS

BSC

BSC

BTS

BTS

MS + SIM

XCDRGMSC

PSTN

MSC

VLRHLR

Principle of routing call to mobile subscribers

1MS-ISDN

Call is placed to a mobile subscriber by dialling the mobile number (MS-ISDN).

1

2 MS-ISDN

Using the MS-ISDN the MSC interrogates the HLR to find status and location of mobile subscriber.2

3 MSRN

The HLR returns the MSRN – a “virtual” number telling the GMSC how to route the call to the serving MSC.3

4MSRN

Using the MSRN the GMSC routes the call to the serving MSC.4

5 MSRN

When the MSC receives the incoming call it queries its VLR to obtain the TMSI for the subscriber. 5

6TMSI

The TMSI is assigned at registration and is another “virtual” number used for security purposes. Together with cell ID location information stored in the VLR the MSC now has sufficient information to be able to route the call.

6

7

7

7

TMSI

The MSC directs the BSC to page the subscriber and inform the handset of an incoming call.7

8TMSI

The handset acknowledges the incoming call and the call is established between the two parties. The handset may also signal the BSC / MSC during the call to set up supplementary services such as Call Hold, 3-way calling etc.

8

Call setup

Data held in HLR:

• Subscriber & Subscription Data– International Mobile Subscriber

Identity (IMSI)

– Mobile Station ISDN (MS-ISDN)

– Bearer & teleservice subscriptions

– Service restrictions

– Parameters for additional services

– Information on subscriber equipment

– Authentication data

• Tracking & Routing Information– Mobile Station Roaming Number

(MSRN)

– Temporary Mobile Subscriber Identity (TMSI)

– Current VLR address

– Current MSC address

– Local Mobile Subscriber Identity

A

GPRS Design


GPRS network elements

BG Border GatewayBSC Basestation ControllerBTS Basestation TransceiverGGSN Gateway GPRS Support NodeHLR Home Location RegisterPCU Packet Control UnitPDN Packet Data Network (X25)PLMN Public Land Mobile NetworkSM-SC Short Message Service CentreSGSN Serving GPRS Support NodeVLR Visitor Location Register

HLRVLR

BTS

BSC

BSC

BTS

BTS

GPRS MS + SIM

PCU

Other GPRS PLMN

GGSN

SGSN

PDNGGSN

SM-SC

BG

A


CellsBTS

How GSM & GPRS co-exist

OSS

CG

LIAN

DNS

Abis (G.703 E1)

BSC

SMSC

VLR

XCDR

A (G.703 E1 16kbps)

GGSN

Gn (IP)

MAP Gr MAP Ga

GMSC

MAP E

Signalling & Name of Interface

Voice or Data link

HLR

MAP D

MAP C

SMSC

De facto interfaces

IWF

InternetX.25 / IP / PDNPSTN

G.703 E1 64kbps

DHCP

Firewall

DNS

Radius

Gi (IP)

GSM

PCU

SGSN

Gb (Frame Relay)

GPRS

BSC Basestation ControllerBTS Basestation TransceiverCCS7 Common Channel Signalling #7CG Charging GatewayDHCP Dynamic Host Configuration ProtocolDNS Domain Name ServerGSN GPRS Serving Node (Serving / Gateway)HLR Home Location RegisterIWF Interworking Function (Circuit / Packet)LIAN Legal Intercept Attendance NodeMAP Mobile Application Part (CCS7)MSC Mobile Switching Centre (Serving / Gateway)OSS Operational Support System PCU Packet Control UnitPSTN Public Switched Telephony NetworkVLR Visitor Location RegisterXCDR Transcoder (16 / 64kbps coding)

A


GPRS key information

• Four Coding Schemes defined– CS1 9.05 kbit / second per

timeslot

– CS2 13.40

– CS3 15.60

– CS4 21.40

– Higher speed = Trade off of Forward Error Correction & hence quality

• Three Handset Types defined– Class A – simultaneous voice & data

– Class B – voice or data only at one time

– Class C – data only

• GSM offsets uplink timeslots (Ts) from downlink by 3 to save on radio transmit / receive hardware

– Therefore today’s handsets are typically:

• 1 Ts downlink

• 2 to 3 Ts uplink

• Class B

• CS1 & CS2 capable

• Equals 3 x 13.40 = 40.20 kbit/s maximum

– Handsets can exceed this limit

• But cost more…

• Use more power etc,

1 2 3 4 5 6 7 8

GPRS

GPRS

0

Signalling

Downlink

Uplink


Protocol layers in GPRS

Laptop/ PDA

GPRS MS BSS SGSN GGSN

Application Protocol (http / ftp)

Transmission Control Protocol (TCP)

GSM RF

GSM RF

IPIP

MACMAC

RLCRLC

LLCLLC

SNDCP

SNDCP

GSM RF

GSM RF

MACMAC

RLCRLC

L1 bisL1 bis

Network

Service

Network

Service

BSSGPBSSGP

L1 BisL1 Bis

Network Service

Network Service

BSSGPBSSGP

LLCLLC

SNDCP

SNDCP

L1L1

L2L2

IPIP

UDP / TCP

UDP / TCP

GTPGTP

L1L1

L2L2

IPIP

UDP / TCP

UDP / TCP

GTPGTP

IPIP

TCPTCP

IPIP

TCPTCP

IPIP

TCPTCP

BSSGP Basestation System GPRS ProtocolGSM RF Radio FrequencyGTP Gateway Tunnelling ProtocolLLC Logical Link ControlMAC Medium Access ControlRLC Radio Link ControlSNDCP Subnetwork Dependent Convergence Protocol


Mobility management

• Mobility management– Attach

• Know who is the MS

• Know what the user is allowed to do

– Detach

• Leave the system

– Location updates

• Know location of MS

• Route mobile terminated (MT) packets to MS

• GPRS Service Descriptions– Point-to-Point

• Connection-orientated (X25)

• Connection-less (IPv4 / IPv6)

– Point-to-Multipoint (Release 2)

• Multicast

• Groupcast

– Short Message Service (SMS)

• Packet Data Protocol (PDP) Contexts– Every mobile must have an address for each

PDP Context in use

– Addresses are statically or dynamically assigned

– Context information includes:

• PDP Type

• PDP address (optional)

• Quality of Service (5 classes – Service Precedence / Reliability / Delay / Throughput Maximum & Mean)

– SGSN has main control of QoS


GPRS dimensioning

• 900MHz UK Network– 7 Timeslots per Carrier

– 1 to 6 RF carriers / cell

– 1 to 3 cells / BTS

– 5,000 BTS

– 250 BSC

– 50 MSC

– 10 GMSC

• GPRS– SGSN c.10,000 simultaneous users

– GGSN c.45,000 simultaneous users

– 10 to 1 contention ratio

• Dimensioning– 8 million subscribers

– 10% GPRS handset penetration

– 800,000 users

– 10:1 Activity factor

– 10:1 x 800,000 = 80,000 simultaneous users

– 8 SGSN / 2 GGSN

• Exact dimensioning depends on:– Number of users

– Geography

– Population density

– Data profile & activity

– GPRS growth


Evolution towards UMTS – All IP core

BTS

BSCBTS

UMTS

Node B

RNC Server

All IP Packet Network

Packet Gateway

Circuit Gatewa

y

BTS

Call Control Server

PSTN

CAMELHLR

GSM & GPRS

3rd Generation UMTS

Internet Packet Data


Further Reading

• ‘GSM Switching, Services and Protocols’ – Jörg Eberspöcher & Hans-Jörg Vögel, John Wiley & Sons, 2000

• ‘GPRS General Packet Radio Service’ – Regis J. “Bud” Bates, McGraw-Hill Telecom Professional, 2002

• ‘GPRS Networks’ – Geoff Sanders, Lionel Thorens, Manfred Reisky, Oliver Rulik, Stefan Deylitz, John Wiley & Sons, 2003

by erick o’connor february 2005 gsm & gprs primer

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