GSM & GPRS Primer

GSM & GPRS Primer
Erick O‟Connor
February 2005

Topics

Background General Packet Radio System (GPRS)
• The history of cellular communications • Protocol layers
• Key statistics • Key information
– Worldwide subscribers • Dimensioning a Network
– Top 20 global mobile operators • Mobility Management

Global System for Mobile (GSM) Third-Generation Systems (UMTS)
• The Radio environment • Evolution paths
• Basestation & Network subsystems • Core components
• Subscriber data & addressing
• Circuit-switched network architecture
• Overview of PDH transmission
• Common Channel Signalling & GSM MAP

©2001 - 2005 Erick O’Connor 2

History of Cellular Communications
1960s 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

And, by technology.…

www.gsmworld.com

….Subscribers


…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 design
Radio & Network subsystems, Signalling & Transmission

Basic GSM network elements
PSTN
Network Subsystem ISDN
PDN
ISC
BTS
GMSC SIWF
BSC XCDR

MSC User Data &
Authentication
EIR
BTS BSC AUC
HLR
VLR
AUC Authentication Centre
BTS BSC Basestation Controller
BTS Basestation Transceiver
EIR Equipment Identity Register
MS + SIM GMSC Gateway Mobile Switching Centre
HLR Home Location Register
ISC International Switching Centre
Radio Subsystem ISDN
MSC
Integrated Services Digital Network
Mobile Switching Centre
PDN Packet Data Network (X25)
PSTN Public Switched Telephony Network
SIWF Shared Interworking Function
VLR Visitor Location Register
©2001 - 2005 Erick O’Connor XCDR Transcoder (16 / 64kbps coding) 11

GSM air interface design

• Access Techniques
– Time Division Multiple Access
– Frequency Division Multiple Access
– Space Division Multiple Access Multiple cells
-400 kHz f0 +400 kHz

• Radio characteristics 8 timeslots GMSK Spectrum
– Gaussian Minimum Shift Keying (GMSK) f3
– Slow Frequency Hopping
f2

Frequency
• Logical structure
f1
– 8 Timeslots per Carrier
f0
– 1 Downlink Timeslot reserved for signalling
– 3 timeslot difference between uplink & downlink FDMA & TDMA
Time

• Frame structure used for synchronisation
– 51-frame Multiframe (235.4 ms)
0 1 2 3 4 5 6 7 Downlink
– 51 or 26 Multiframe Superframe (6.12 sec)
– 2048 Superframe Hyperframe (3 hr 28 mins) Uplink 0 1 2 3 4 5 6 7
Delay


Radio subsystem (i)

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

BSC XCDR
• 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 BTS BSC
– 3 x 7 Timeslots x 3 = 63 Timeslots total
– c.52 Erlangs @ 2% Grade of Service BTS

MS + SIM
– c.2,000 users per BTS @ 25 mErl / User (90 seconds) 3
2
1
• Frequency reuse depends on terrain, Frequency reuse &
3 f3
2 f2
frequencies available etc. cluster formation 1 f1
K=3
• Paired spectrum shared by Operators
7
– 900 / 1800 MHz in Europe / Asia (25 & 75 MHz) 6 2 7
1 6 2
– 1900 MHz in N.America 5 3 1
4 5 3
f7 4
• 200 kHz channel separation f6 f2
f1
• 125 Channels @ 900 MHz f5 f3
f4 K=7


Radio subsystem (ii)
BTS

BSC XCDR
• Basestation Controller (BSC) controls a number of BTS
– Acts as a small switch
BTS
– Assists in handover between cells and between BTS BSC

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

MS + SIM

• 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)


Network subsystem (i) PSTN
ISDN
PDN
ISC
• Core component is Mobile Switching Centre (MSC)
GMSC SIWF

– Performs all switching functions of a fixed-network switch
MSC
– 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


Network subsystem (ii)

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

– Stores all permanent subscriber data &
relevant temporary data including: • Authentication Centre (AUC) &
Equipment Identity Register (EIR)
• MS-ISDN (Mobile Subscriber‟s telephone no.)
• MSRN (Mobile Station Roaming no.) – GSM is inherently secure using
encryption over the air-interface and for
• Current Mobile Location Area
authentication / registration
– Actively involved in incoming call set-up &
– AUC holds each subscriber‟s secret key
supplementary services
(Ki) & calculates “triplet” for challenge /
respond authentication with mobile
• Visitor Location Register (VLR)
– SIM is sent data and must calculate
associated with individual MSCs
appropriate response
– VLR stores temporary subscriber information
– EIR is used to store mobile terminals
obtained from HLR of mobiles currently
serial numbers
registered in serving area of MSC
– Involved in registration of mobiles
– Assists in delivery of supplementary service
features such as Call Waiting / Call Hold


Signalling – Air interface

Air Interface Signalling • Uplink signalling (from MS) more
complicated
• Downlink signalling (to Mobile Station) – Random Access Channel (RACH) –
– Relies on Bearer Control Channel competitive multi-access mode using
(BCCH) set at fixed frequency per cell slotted ALOHA to request dedicated
signalling channel (SDCCH)
• Mobile Stations use this to lock-on to
network
• Bidirectional channels include
• Mobile Stations periodically scan
– Traffic Channels (TCH) – Carrying full
environment and report back other
rate voice @ 13 kbps / half-rate voice
BCCH power levels to BSC to assist
in handover – Standalone Dedicated Control Channel
(SDCCH) – used for updating location
– Access Grant Channel (AGCH) – used
information or parts of connection set-up
to assign a Control or Traffic Channel to
the mobile – Slow Associated Control Channel
(SACCH) – used to report radio conditions
– Paging Channel (PCH) – paging to find
& measurement reports
specific mobiles
– Fast Associated Control Channel
(FACCH) – uses “stolen” traffic channel
capacity to add extra signalling capacity


Signalling – Mobile Application Part interfaces

Network Signalling GSM Specific Signalling Interfaces
(Mobile Application Part)
Um Air interface signalling
MSC
Abis Radio management
A BSS management, connection EIR
control & mobility management MSC

B Subscriber data, location F E
information, supplementary
service settings
BSC
C
C Routing information requests
A
HLR
D Exchange of location-dependent B D
subscriber data & subscriber BTS
Abis
management VLR

E Inter-MSC handover signalling
G
F Subscriber & equipment identity BTS
VLR
check
G Inter-MSC handover, transfer of Um
subscriber data MS + SIM


ITU-T Common Channel Signalling System Number 7 A

Application Parts
Actually carry the specific GSM interfaces B, C, D, E & G
messages for Mobile (MAP), Standard Telephone
carried as Mobile Application Part
Intelligent Network (INAP) or User Part (TUP)
Operations & Maintenance
Most basic CSS7 signalling
(OMAP)

MAP INAP OMAP
Transaction Control
Application Part –
component responsible for TCAP

ISO Layers 1 thro 7
“carrying” higher level ISUP
Application Parts to their TUP
correct destinations
SCCP
Signalling Connection ISDN User Part
Control Part
Add functionality to
Functionally equivalent to TCP permit ISDN signalling
layer, carries “Connectionless” (i.e. fully digital)
messages between Network
elements MTP Layers 1/2/3 between networks

Signalling 101
• Line signalling – “tell the other end you want to make call” Message Transfer Part
Lowest level, permits
• Register signalling – “tell the other end the destination of the call” interconnection with
underlying physical
transmission medium

PDH transmission …composition of 32 channel E1 bearer

TS 0 Synchronisation
Header
TS16 Signalling

ITU-T G.703 E1 link 2048 kbps
32 x 64 kbps Timeslots

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Plesiochronous Digital Hierarchy (PDH)
140 Mbps

34 Mbps
Voice / Data Timeslot
2 Mbps

Abis - Voice GSM Codec
4 x 13 kbps Timeslots Synchronous Digital Hierarchy (SDH)
STM-16
(SONET - USA)
STM-4

STM-1


Circuit-switched network architecture
(Transmission & Signalling planes)
BSS Basestation Subsystem
CCS7 Common Channel Signalling #7
CO Central Office
CSS7 Signalling Plane
HLR Home Location Register SSP
MSC Mobile Switching Centre HLR
SDH Synchronous Digital Hierarchy CCS7 Links
SSP Service Switching Point
STP Signalling Transfer Point CO Switch
STP
MSC

SDH Fibre
Optic Network

Synchronisation

Drop & Insert Other Networks
Multiplexers

BSS Transmission Plane


Call setup

Data held in HLR: Call is placed to a mobile subscriber by dialling the mobile
1 number (MS-ISDN).
• Subscriber & Subscription Data
– International Mobile Subscriber
Identity (IMSI)
– Mobile Station ISDN (MS-ISDN)
– Bearer & teleservice subscriptions MS-ISDN 1
– Service restrictions PSTN

– Parameters for additional services
– Information on subscriber BTS
equipment GMSC
BSC XCDR
– Authentication data
MSC
• Tracking & Routing Information
– Mobile Station Roaming Number
(MSRN) BTS
BSC
HLR
– Temporary Mobile Subscriber VLR
Identity (TMSI)
BTS
– Current VLR address
MS + SIM
– Current MSC address
– Local Mobile Subscriber Identity Principle of routing call to mobile subscribers


Call setup

Data held in HLR: Using the MS-ISDN the MSC interrogates the HLR to find status
2 and location of mobile subscriber.
Identity (IMSI)

equipment GMSC
BSC XCDR
– Authentication data 2 MS-ISDN
MSC
(MSRN) BTS
BSC
HLR
Identity (TMSI)
BTS
MS + SIM


Call setup

Data held in HLR: The HLR returns the MSRN – a “virtual” number telling the
3 GMSC how to route the call to the serving MSC.
Identity (IMSI)

equipment GMSC
BSC XCDR
– Authentication data 3 MSRN 2 MS-ISDN
MSC
(MSRN) BTS
BSC
HLR
Identity (TMSI)
BTS
MS + SIM


Call setup

Data held in HLR: Using the MSRN the GMSC routes the call to the serving MSC.
4
Identity (IMSI)
– Service restrictions MSRN PSTN
4
equipment GMSC
BSC XCDR
MSC
(MSRN) BTS
BSC
HLR
Identity (TMSI)
BTS
MS + SIM


Call setup

Data held in HLR: When
Using the MS-ISDN the MSC interrogates the queries findVLR to
MSC receives the incoming call it HLR to its status
5 obtain the TMSI for the subscriber.
and location of mobile subscriber.
Identity (IMSI)
4
equipment GMSC
BSC XCDR
MSC
(MSRN) BTS
BSC
5 MSRN
HLR
Identity (TMSI)
BTS
MS + SIM


Call setup

Data held in HLR: The TMSI is assigned at registration and is another “virtual”
6 number used for security purposes. Together with cell ID
location information stored in the VLR the MSC now has
Identity (IMSI) sufficient information to be able to route the call.
4
equipment GMSC
BSC XCDR
MSC
(MSRN) BTS
BSC
5 MSRN
HLR
Identity (TMSI) 6
BTS TMSI
MS + SIM


Call setup

Data held in HLR: The MSC directs the BSC to page the subscriber and inform the
7 handset of an incoming call.
Identity (IMSI)
4
equipment GMSC
BSC XCDR 7
MSC
• Tracking & Routing Information 7

(MSRN) BTS
BSC
5 MSRN
HLR
– Temporary Mobile Subscriber 7 TMSI VLR
Identity (TMSI) 6
BTS TMSI
MS + SIM


Call setup

Data held in HLR: The handset acknowledges the incoming call and the call is
8 established between the two parties. The handset may also
signal the BSC / MSC during the call to set up supplementary
Identity (IMSI) services such as Call Hold, 3-way calling etc.
4
equipment GMSC
BSC XCDR 7
MSC
• Tracking & Routing Information 7

(MSRN) BTS
BSC
5 MSRN
HLR
– Temporary Mobile Subscriber 7 TMSI VLR
Identity (TMSI) 6
BTS TMSI
MS + SIM 8
– Current MSC address TMSI


GPRS network elements

Other GPRS
SM-SC PLMN

BTS
GGSN
BSC PCU
BG

SGSN

PDN
GGSN

BTS BSC
BG Border Gateway
BSC Basestation Controller
VLR HLR
BTS
GGSN Gateway GPRS Support Node
HLR Home Location Register
GPRS MS + SIM
PCU Packet Control Unit
PDN Packet Data Network (X25)
PLMN Public Land Mobile Network
SM-SC Short Message Service Centre
SGSN Serving GPRS Support Node


How GSM & GPRS co-exist
DHCP Radius
X.25 / IP / PDN
PSTN Internet
Firewall DNS
De facto interfaces
G.703 E1 64kbps Gi (IP)
SMSC
OSS CG
GMSC IWF GGSN
MAP C LIAN
MAP E Gn (IP) DNS
HLR
VLR
MAP D MAP Gr MAP Ga
SMSC SGSN
GSM XCDR GPRS
A (G.703 E1 16kbps)
BSC Basestation Controller
BSC PCU CCS7 Common Channel Signalling #7
Gb (Frame Relay) CG Charging Gateway
DHCP Dynamic Host Configuration Protocol
DNS Domain Name Server
BTS GSN GPRS Serving Node (Serving / Gateway)
Abis (G.703 E1) HLR Home Location Register
Cells IWF Interworking Function (Circuit / Packet)
LIAN Legal Intercept Attendance Node
MAP Mobile Application Part (CCS7)
Voice or Data link MSC Mobile Switching Centre (Serving / Gateway)
OSS Operational Support System
Signalling & Name of Interface PCU Packet Control Unit
PSTN Public Switched Telephony Network
©2001 - 2005 Erick O’Connor XCDR Transcoder (16 / 64kbps coding) 33

GPRS key information

• Four Coding Schemes defined • GSM offsets uplink timeslots (Ts) from
– CS1 9.05 kbit / second per timeslot downlink by 3 to save on radio transmit /
receive hardware
– CS2 13.40
– Therefore today‟s handsets are typically:
– CS3 15.60
• 1 Ts downlink
– CS4 21.40
• 2 to 3 Ts uplink
– Higher speed = Trade off of Forward Error
Correction & hence quality • Class B
• CS1 & CS2 capable
• Three Handset Types defined • Equals 3 x 13.40 = 40.20 kbit/s maximum
– Class A – simultaneous voice & data
– Class B – voice or data only at one time – Handsets can exceed this limit
– Class C – data only • But cost more…
• Use more power etc,
GPRS

0 1 2 3 4 5 6 7 8 Downlink

Signalling

Uplink

GPRS


Protocol layers in GPRS

Application Protocol (http / ftp)

TCP Transmission Control Protocol (TCP) TCP TCP

IP IP IP IP

SNDCP SNDCP GTP GTP

LLC LLC UDP / UDP /
TCP TCP
RLC RLC BSSGP BSSGP
IP IP

Network Network
MAC MAC L2 L2
Service Service

GSM
GSM RF L1 bis L1 Bis L1 L1
RF

Laptop GPRS MS BSS SGSN GGSN
/ PDA

BSSGP Basestation System GPRS Protocol
GSM RF Radio Frequency
GTP Gateway Tunnelling Protocol
LLC Logical Link Control
MAC Medium Access Control
RLC Radio Link Control
SNDCP Subnetwork Dependent Convergence Protocol

Mobility management

• Mobility management • Packet Data Protocol (PDP) Contexts
– Attach – Every mobile must have an address for each
• Know who is the MS PDP Context in use
• Know what the user is allowed to do – Addresses are statically or dynamically assigned
– Detach – Context information includes:
• Leave the system • PDP Type
– Location updates • PDP address (optional)
• Know location of MS • Quality of Service (5 classes – Service
Precedence / Reliability / Delay /
• Route mobile terminated (MT) packets to MS Throughput Maximum & Mean)
– SGSN has main control of QoS
• GPRS Service Descriptions
– Point-to-Point
• Connection-orientated (X25)
• Connection-less (IPv4 / IPv6)
– Point-to-Multipoint (Release 2)
• Multicast
• Groupcast
– Short Message Service (SMS)


GPRS dimensioning

• 900MHz UK Network • Dimensioning
– 7 Timeslots per Carrier – 8 million subscribers
– 1 to 6 RF carriers / cell – 10% GPRS handset penetration
– 1 to 3 cells / BTS – 800,000 users
– 5,000 BTS – 10:1 Activity factor
– 250 BSC – 10:1 x 800,000 = 80,000 simultaneous users
– 50 MSC – 8 SGSN / 2 GGSN
– 10 GMSC
• Exact dimensioning depends on:
• GPRS – Number of users
– SGSN c.10,000 simultaneous users – Geography
– GGSN c.45,000 simultaneous users – Population density
– 10 to 1 contention ratio – Data profile & activity
– GPRS growth


Evolution towards UMTS – All IP core
Internet Packet Data

GSM & GPRS
PSTN
Packet
Gateway

HLR CAMEL
Circuit
All IP Packet
Gateway
Network
Call Control
Server

BTS

RNC Server

BTS
BSC

UMTS
BTS Node B
3rd Generation UMTS


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


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