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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
History of Cellular Communications
1960s to the Present Day
4 ©2001 - 2005 Erick O’Connor
…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….
5 ©2001 - 2005 Erick O’Connor
… 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…
6 ©2001 - 2005 Erick O’Connor
…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
7 ©2001 - 2005 Erick O’Connor
…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
….Subscribers
And, by technology.…
www.gsmworld.com
8 ©2001 - 2005 Erick O’Connor
…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 ©2001 - 2005 Erick O’Connor
The market today – key statistics
GSM design
Radio & Network subsystems, Signalling & Transmission
11 ©2001 - 2005 Erick O’Connor
Basic GSM network elements
AUC Authentication Centre BSC Basestation Controller BTS Basestation Transceiver EIR Equipment 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 Transcoder (16 / 64kbps coding)
BTS
BSC
BSC
BTS
BTS
MS + SIM
XCDR
Radio Subsystem
GMSC
ISC
PSTN
ISDN
PDN
MSC
SIWF
Network Subsystem
VLR
EIR AUC
HLR
User Data &
Authentication
12 ©2001 - 2005 Erick O’Connor
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
quency
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
13 ©2001 - 2005 Erick O’Connor
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
1 5
4 3
2 7
6
K=7
f1 f5
f4
f3
f2
f7 f6
1 5
4 3
2 7
6
1
3 2
f1
f3 f2
1
3 2
K=3
Frequency reuse &
cluster formation
14 ©2001 - 2005 Erick O’Connor
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
15 ©2001 - 2005 Erick O’Connor
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
16 ©2001 - 2005 Erick O’Connor
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
EIR AUC
HLR
GSM call setup & Signalling
18 ©2001 - 2005 Erick O’Connor
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
19 ©2001 - 2005 Erick O’Connor
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
20 ©2001 - 2005 Erick O’Connor
ITU-T Common Channel Signalling System Number 7
MTP Layers 1/2/3
TCAP
SCCP
MAP INAP OMAP
ISUP TUP
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
21 ©2001 - 2005 Erick O’Connor
PDH transmission …composition of 32 channel E1 bearer
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
TS 0 Synchronisation
Header TS16 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-16 Synchronous Digital Hierarchy (SDH)
(SONET - USA)
22 ©2001 - 2005 Erick O’Connor
SDH Fibre
Optic Network
Transmission Plane
Synchronisation
Other Networks Drop & Insert
Multiplexers
BSS
Circuit-switched network architecture (Transmission & Signalling planes)
BSS Basestation Subsystem
CCS7 Common Channel Signalling #7
CO Central Office
HLR Home Location Register
MSC Mobile Switching Centre
SDH Synchronous Digital Hierarchy
SSP Service Switching Point
STP Signalling Transfer Point
STP
HLR
CSS7 Signalling Plane
MSC
CCS7 Links
SSP
CO Switch
23 ©2001 - 2005 Erick O’Connor
BTS
BSC
BSC
BTS
BTS
MS + SIM
XCDR GMSC
PSTN
MSC
VLR HLR
Principle of routing call to mobile subscribers
1 MS-ISDN
Call is placed to a mobile subscriber by dialling the mobile
number (MS-ISDN). 1
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
24 ©2001 - 2005 Erick O’Connor
2 Using the MS-ISDN the MSC interrogates the HLR to find status
and location of mobile subscriber.
BTS
BSC
BSC
BTS
BTS
MS + SIM
XCDR GMSC
PSTN
MSC
VLR HLR
Principle of routing call to mobile subscribers
1 MS-ISDN
2 MS-ISDN
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
25 ©2001 - 2005 Erick O’Connor
3 The HLR returns the MSRN – a “virtual” number telling the
GMSC how to route the call to the serving MSC.
BTS
BSC
BSC
BTS
BTS
MS + SIM
XCDR GMSC
PSTN
MSC
VLR HLR
Principle of routing call to mobile subscribers
1 MS-ISDN
2 MS-ISDN 3 MSRN
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
26 ©2001 - 2005 Erick O’Connor
4 Using the MSRN the GMSC routes the call to the serving MSC.
BTS
BSC
BSC
BTS
BTS
MS + SIM
XCDR GMSC
PSTN
MSC
VLR HLR
Principle of routing call to mobile subscribers
1 MS-ISDN
2 MS-ISDN 3 MSRN
4 MSRN
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
27 ©2001 - 2005 Erick O’Connor
Using the MS-ISDN the MSC interrogates the HLR to find status
and location of mobile subscriber. 5 When the MSC receives the incoming call it queries its VLR to
obtain the TMSI for the subscriber.
BTS
BSC
BSC
BTS
BTS
MS + SIM
XCDR GMSC
PSTN
MSC
VLR HLR
Principle of routing call to mobile subscribers
1 MS-ISDN
2 MS-ISDN 3 MSRN
4 MSRN
5 MSRN
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
28 ©2001 - 2005 Erick O’Connor
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
BTS
BSC
BSC
BTS
BTS
MS + SIM
XCDR GMSC
PSTN
MSC
VLR HLR
Principle of routing call to mobile subscribers
1 MS-ISDN
2 MS-ISDN 3 MSRN
4 MSRN
5 MSRN
6 TMSI
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
29 ©2001 - 2005 Erick O’Connor
The MSC directs the BSC to page the subscriber and inform the
handset of an incoming call. 7
BTS
BSC
BSC
BTS
BTS
MS + SIM
XCDR GMSC
PSTN
MSC
VLR HLR
Principle of routing call to mobile subscribers
1 MS-ISDN
2 MS-ISDN 3 MSRN
4 MSRN
5 MSRN
6 TMSI
7
7
7
TMSI
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
30 ©2001 - 2005 Erick O’Connor
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
BTS
BSC
BSC
BTS
BTS
MS + SIM
XCDR GMSC
PSTN
MSC
VLR HLR
Principle of routing call to mobile subscribers
1 MS-ISDN
2 MS-ISDN 3 MSRN
4 MSRN
5 MSRN
6 TMSI
7
7
7
TMSI
8 TMSI
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
GPRS Design
32 ©2001 - 2005 Erick O’Connor
GPRS network elements
BG Border Gateway
BSC Basestation Controller
BTS Basestation Transceiver
GGSN Gateway 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
HLR VLR
BTS
BSC
BSC
BTS
BTS
GPRS MS + SIM
PCU
Other GPRS
PLMN
GGSN
SGSN
PDN GGSN
SM-SC
BG
33 ©2001 - 2005 Erick O’Connor
Cells
BTS
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
Internet X.25 / IP / PDN
PSTN
G.703 E1 64kbps
DHCP
Firewall DNS
Radius
Gi (IP)
GSM
PCU
SGSN
Gb (Frame Relay)
GPRS
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 Interworking 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)
34 ©2001 - 2005 Erick O’Connor
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
35 ©2001 - 2005 Erick O’Connor
Protocol layers in GPRS
Laptop / PDA
GPRS MS BSS SGSN GGSN
Application Protocol (http / ftp)
Transmission Control Protocol (TCP)
GSM RF
IP
MAC
RLC
LLC
SNDCP
GSM
RF
MAC
RLC
L1 bis
Network
Service
BSSGP
L1 Bis
Network
Service
BSSGP
LLC
SNDCP
L1
L2
IP
UDP /
TCP
GTP
L1
L2
IP
UDP /
TCP
GTP
IP
TCP
IP
TCP
IP
TCP
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
36 ©2001 - 2005 Erick O’Connor
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
37 ©2001 - 2005 Erick O’Connor
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
38 ©2001 - 2005 Erick O’Connor
Evolution towards UMTS – All IP core
BTS
BSC BTS
UMTS
Node B
RNC Server
All IP Packet
Network
Packet
Gateway
Circuit
Gateway
BTS
Call Control
Server
PSTN
CAMEL HLR
GSM & GPRS
3rd Generation UMTS
Internet Packet Data
39 ©2001 - 2005 Erick O’Connor
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