By Erick O

1
GSM GPRS Primer

• By Erick OConnor
• February 2005

2
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

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A
3
History of Cellular Communications

• 1960s to the Present Day

4
the early years

• 1960 1970s
• Idea of a cell-based mobile radio system
  developed by ATTs 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.

5
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 Europes 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

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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 industrys 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

7
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
8
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.

9
The market today key statistics
10
GSM design

• Radio Network subsystems, Signalling
  Transmission

11
Basic GSM network elements
A
AUC Authentication Centre BSC Basestation
Controller BTS Basestation Transceiver EIR Equipme
nt Identity Register GMSC Gateway Mobile
Switching Centre HLR Home Location
Register ISC International Switching
Centre ISDN Integrated Services Digital
Network MSC Mobile Switching Centre PDN Packet
Data Network (X25) PSTN Public Switched Telephony
Network SIWF Shared Interworking
Function VLR Visitor Location Register XCDR Transc
oder (16 / 64kbps coding)
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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)

13
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 _at_ 2 Grade of Service
• c.2,000 users per BTS _at_ 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 _at_ 900 MHz

Frequency reuse cluster formation
14
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)

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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

16
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 Subscribers 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 subscribers 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

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GSM call setup Signalling
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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
  _at_ 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

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Signalling Mobile Application Part interfaces
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
20
ITU-T Common Channel Signalling System Number 7
A
Application Parts Actually carry the specific
messages for Mobile (MAP), Intelligent Network
(INAP) or Operations Maintenance (OMAP)
GSM interfaces B, C, D, E G carried as Mobile
Application Part
Standard Telephone User Part (TUP) Most basic
CSS7 signalling
Transaction Control Application Part component
responsible for carrying higher level
Application Parts to their correct destinations
ISO Layers 1 thro 7
Signalling Connection Control Part Functionally
equivalent to TCP layer, carries Connectionless
messages between Network elements
ISDN User Part Add functionality to permit ISDN
signalling (i.e. fully digital) between networks

• Signalling 101
• Line signalling tell the other end you want to
  make call
• Register signalling tell the other end the
  destination of the call

Message Transfer Part Lowest level, permits
interconnection with underlying physical
transmission medium
21
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
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140 Mbps
Plesiochronous Digital Hierarchy (PDH)
34 Mbps
Voice / Data Timeslot
2 Mbps
Abis - Voice GSM Codec 4 x 13 kbps Timeslots
STM-16
Synchronous Digital Hierarchy (SDH) (SONET - USA)
STM-4
STM-1
22
Circuit-switched network architecture(Transmissio
n Signalling planes)
A
BSS Basestation Subsystem CCS7 Common Channel
Signalling 7 CO Central Office HLR Home Location
Register MSC Mobile Switching Centre SDH Synchrono
us Digital Hierarchy SSP Service Switching
Point STP Signalling Transfer Point
23
Call setup
A

• 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

24
GPRS Design
25
GPRS network elements
A
BG Border Gateway BSC Basestation
Controller BTS Basestation Transceiver GGSN Gatewa
y GPRS Support Node HLR Home Location
Register 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 VLR Visitor
Location Register
26
How GSM GPRS co-exist
A
OSS
BSC Basestation Controller BTS Basestation
Transceiver CCS7 Common Channel Signalling
7 CG Charging Gateway DHCP Dynamic Host
Configuration Protocol DNS Domain Name
Server GSN GPRS Serving Node (Serving /
Gateway) HLR Home Location Register IWF Interworki
ng Function (Circuit / Packet) LIAN Legal
Intercept Attendance Node MAP Mobile Application
Part (CCS7) MSC Mobile Switching Centre (Serving
/ Gateway) OSS Operational Support System
PCU Packet Control Unit PSTN Public Switched
Telephony Network VLR Visitor Location
Register XCDR Transcoder (16 / 64kbps coding)
Voice or Data link
Signalling Name of Interface
27
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 todays 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,

Downlink
Uplink
28
Protocol layers in GPRS
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 Subnet
work Dependent Convergence Protocol
29
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

30
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
• 101 Activity factor
• 101 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

31
Evolution towards UMTS All IP core
GSM GPRS
Packet Gateway
Circuit Gateway
All IP Packet Network
Call Control Server
BTS
RNC Server
BTS
BSC
UMTS Node B
BTS
3rd Generation UMTS
32
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