1. Cellular Mobile Communication Systems Lecture 7
Engr. Shahryar Saleem
Assistant Professor
Department of Telecom Engineering
University of Engineering and Technology
Taxila TI TI

2. GSM Architecture GSM is a PLMN (Public Land Mobile Network)
Several providers can setup mobile networks following the GSM standard within each country
Major components
MS (mobile station)
BTS (base transceiver station) or BS or cell site
BSC (base station controller)
MSC (mobile switching centre)
LR (location registers): VLR, HLR
AUC (Authentication database), EIR (Equipment Identity Register) TI

3. GSM Architecture Subsystems
RSS (radio subsystem): covers all radio aspects
NSS (network and switching subsystem): call forwarding, handoff, switching, location tracking, etc.
OSS (operation support subsystem): management of the network
Standardized interfaces
Allows provider to mix and match vendor equipment TI

4. GSM System ArchitectureTI

5. GSM Functional Architecture

6. GSM System ArchitectureTI

7. Mobile Station
An MS consists of the physical equipment used by a user to access a PLMN
A mobile station (MS) comprises several functional groups
MT (Mobile Terminal)
End-point of the radio interface (Um)
TA (Terminal Adaptor)
Terminal adaptation, hides radio specific characteristics
TE (Terminal Equipment)
Peripheral device of the MS, offers services to a user
SIM (Subscriber Identity Module)
Stores user parameters such as subscriber number, authentication Key, PIN etc
An Ms has a number of identities
IMEI, IMSI, TMSI etc TI

8. Radio Station Subsystem
Components
MS (Mobile Station)
BSS (Base Station Subsystem)
Physical equipment to provide radio coverage to cells
BSS consists of two functional units BTS and BSC
BTS (Base Transceiver Station): Antenna + digital radio equipment
BSC (Base Station Controller): Controlling several transceivers, map radio channels (Um) onto terrestrial channels A TI

9. RSS Interfaces Interfaces Um : radio interface (From MS to BTS)
Abis : standardized, open interface with 16 kbit/s user channels (from BTS to BSC)
A: standardized, open interface with 64 kbit/s user channels as in wired telephone network (from BSC to MSC) TI

10. System Architecture Network and Switching Subsystems
Components
MSC (Mobile Services Switching Center):
IWF (Interworking Functions)
ISDN (Integrated Services Digital Network)
PSTN (Public Switched Telephone Network)
PSPDN (Packet Switched Public Data Net.)
CSPDN (Circuit Switched Public Data Net.)
Databases
HLR (Home Location Register)
VLR (Visitor Location Register)
EIR (Equipment Identity Register) TI

11. NSS NSS includes the main switching function of GSM
Databases for users
Mobility management
Most important role is to manage the communication between GSM and other network users
Components
Mobile Switching Center (MSC)
Performs the necessary switching functions
Monitors the mobility of its users
Manages Handoff functions
Involved in inter-working functions to communicate with other networks such as PSTN and ISDN etc.
The inter-working functions depends on the types of networks and the types of services to be performed TI

12. NSS Components Home Locator Register (HLR) HLR contains
Functional unit for management of mobile subscriber
HLR stores two types of information: Subscriber information and part of mobile information (to allow incoming calls to be routed to the MSC for a particular MS)
HLR contains
Subscriber ID (IMSI and MSISDN)
Current Subscriber VLR (Current Location)
Supplementary Services
Authentication KEY and AUC functionality
MSRN TI

13. NSS Components Visitor Locator Register (VLR)
The VLR is linked to one or more MSCs
Functional unit which dynamically stores subscriber information when the subscriber is located in the area covered by the VLR TI

14. VLR
When a roaming MS enters an MSC area, the MSC infoems the associated VLR about the MS
The MS goes through a registration process which includes
The VLR recognizes that the MS is from another PLMN
If roaming is allowed, the VLR finds the MS’s HLR in home PLMN
VLR constructs a Global Title (GT) from the IMSI to allow signalling form the VLR to the MS’s HLR via PSTN/ISDN networks
VLR generates a Mobile Subscriber Roaming Number (MSRN) that is used to route incoming calls to the MS
The MSRN is sent to the MS’s HLR
The information in the VLR includes MSRN, IMSI, TMSI, HLR address or GT, the location area where the MS has registered, local MS identity TI

15. Subscriber Identities
IMSI:
This number identifies the MS subscriber. It is only transmitted over the air during initialization.
TMSI:
VLR controls the allocation of Temporary Mobile Subscriber Identity
TMSI is updated frequently
Makes it very difficult for the call to be traced and therefore provides a high degree of security for the subscriber.
TMSI may be updated in any of the following situations
Call Setup
On entry to a new LAI
On entry to a new VLR TI

16. Operation Subsystem Authentication Center (AUC)
OSS (Operation Subsystem) enables centralized operation, management, and maintenance
Components
Authentication Center (AUC)
Generates user specific authentication parameters on the request of VLR
Normally co-located with the HLR as it is required to continuously access and update subscriber records
Authentication parameters used for
Mobile authentication
User data encryption TI

17. Operation Subsystem Equipment Identity Register (EIR)
Contains a centralized database for validating the IMEI
The database is concerned solely with MS equipment and not with the IMSI.
The EIR database consists of lists of IMEIs organized as follows
White List
Contains those IMEIs which are known to have been assigned to valid MS equipment.
Black List
Contains IMEIs of MS which have been reported stolen or which are to be denied service for some other reason.
Grey List
Contains IMEIs of MS which have problems (for example, faulty software). These are not, however, sufficiently significant to warrant a ‘‘black listing”. TI

18. EIR
Call Processing Functions TI

19. Operation Subsystem
Operation and Maintenance Center (OMC)
The Operations and Maintenance Centre (OMC) is a centralized facility that supports the day to day management of a cellular network as well as providing a database for long term network engineering and planning tools. An OMC manages a certain area of the PLMN thus giving regionalized network management. TI

20. GSM Interfaces The Um Radio Interface (MS to BTS)
Uses Physical FDMA/TDMA/FDD physical
In 900 MHz band: MHz Uplink band, MHz Downlink
Radio carrier is a 200kHz channel => 125 pairs of radio channels
Called Absolute Radio Frequency Channel Number (ARFCN)
ARFCN numbers given by f(n) = n MHz for Uplink band n = 0,…124
Corresponding downlink is f(n) + 45 MHz
Channels and ARFCN slightly different in other frequency bands
A TDMA frame is defined on the radio carrier (8 users per carrier)
Channel rate is kbps
Two types of logical channels map onto physical channels
Control Channels (call setup, power adjustment, etc..)
Traffic Channels (voice or data) = 22.8kbps = 1 slot in a TDMA frame TI

21. GSM TDMA / FDMA MS Transmission Band : 890 – 915 MHZ
BS Transmission Band : 935 – 960 MHZ
Year Introduced
1990
Access method
TDMA
Channel Bandwidth
200 kHz
Number of duplex channels
125
Users per channel 8
Speech coding bit rate
13 kbps
Data coding bit rate
12 kbps
Frame size
4.6 ms
45 MHz TI

22. GSM TDMA / FDMA TI

23. GSM Interfaces (cont) Abis Interface (BTS to BSC)
The inter-connection between BTS and BSC is through a standard Abis
Primary functions are traffic channel transmission, terrestrial channel management and radio channel management
Supports two types of communication links
Traffic channels at 64 Kbps carrying speech or data
Signalling channels at 16Kbps carrying information for BSC-BTS and BTS-BSC signalling
A Interface (BSC to MSC)
Inter-connection between BSc and MSC
Physical layer is a 2 Mbps standard TI

24. GSM Protocol Stack Three Layers specified in the protocol
Network layer has three sub layers
1. Call Management
Establishment, maintenance, and termination of circuit-switched calls
2. Mobility Management
Registration, authentication, and location tracking
3. Radio Resource Management
Establishment, maintenance, and termination of radio channel connections
Link Layer
Uses variation of ISDN Link Access Protocol on D channel protocol – termed LAPDm
Physical layer (already discussed)
Time slot on a 200 KHz carrier – absolute radio frequency channel number (ARFCN) TI

25. GSM Protocol Stack TI

26. GSM Channels Physical Channel Traffic Channels Control Channels
125 radio carriers, 8 slots per carrier =>
Traffic Channels
Full rate (TCH/F) at 22.8 kb/s or half rate (TCH/H) at 11.4 kb/s
Physical channel = full rate traffic channel (1 timeslot) or 2 half rate traffic channels (1 timeslot in alternating frames)
Full rate channel may carry 13 kb/s speech or data at 12, 6, or 3.6 kb/s
Half rate channel may carry 6.5 kb/s speech or data at 6 or 3.6 kb/s
Control Channels
1. BCH (broadcast channels): point-to-multipoint downlink only
2. CCCH (common control channel): for paging and access
3. DCCH (dedicated control channel): bi-directional point-to-point signalling TI

27. Control Channels
Broadcast Channels (BCH): Point-to-multipoint unidirectional channels
Frequency Correction Channel (FCCH): correcting mobile frequency in downlink.
This channel is required for the correct operation of the radio sub-system and allow an MS accurately turn to a BS
Synchronization Channel (SCH): Frame synchronization
The SCH has a 64-bit binary sequence known to the MS
MS achieves the exact timing synchronization with respect to the GSM frame by correlating the bits with the internally stored 64-bits.
Broadcast Control Channel (BCCH): control channel structure TI

28. Control Channels (cont.)
Common Control Channel (CCCH): Point-to-multipoint bidirectional channels
Paging Control Channel (PCH): Downlink
Used to page MS
Random Access Channel (RACH): Uplink
Used to request assignment of DCCH
Access Grant Channel (AGCH): Downlink
Used to assign an MS to a specific DCCH
Dedicated Control Channel (DCCH): Used for signalling and control after call establishment
Stand Alone dedicated control channel (SDCCH):
Used for authentication of MS, location updates, assignments to TCH TI

29. Control Channels (cont.)
Slow Associated Control Channel (SACCH):
Used to carry general control information
Fast Associated Control Channel (FACCH):
Used to transmit handoff orders. TI

30. GSM Channels TI

31. Framing Scheme in GSM (Traffic Channels)TI

32. END TI