Architecture and Channels.

1. GSM

2. What is GSM ?
Global System for Mobile (GSM) is a
second generation cellular standard
developed to cater voice services and data
delivery using digital modulation .

3. GSM: HISTORY
o Developed by Group Spéciale Mobile (founded 1982) which was
an initiative of CEPT ( Conference of European Post and
Telecommunication )
o Under ETSI, GSM is named as “ Global System for Mobile
communication “ in 1989
o Full set of specifications phase-I became available in 1990
o Phase 2 of the GSM specifications occurs in 1995. Coverage is
extended to rural areas

4. GSM SYSTEM ARCHITECTURE

5. GSM SYSTEM ARCHITECTURE-I
o Mobile Station (MS)
o Mobile Equipment (ME)
o Subscriber Identity Module (SIM)
o Base Station Subsystem (BSS)
o Base Transceiver Station (BTS)
o Base Station Controller (BSC)
o Network Switching Subsystem(NSS)
o Mobile Switching Center (MSC)
o Home Location Register (HLR)
o Visitor Location Register (VLR)
o Authentication Center (AUC)
o Equipment Identity Register (EIR)

6. SYSTEM ARCHITECTURE
MOBILE STATION (MS)
The Mobile Station is made up of two entities:
1. Mobile Equipment (ME)
2. Subscriber Identity Module (SIM)

7. MOBILE EQUIPMENT
o Portable, vehicle mounted, hand held device
o Uniquely identified by an IMEI (International Mobile
Equipment Identity)
o Voice and data transmission
o Monitoring power and signal quality of surrounding cells
for optimum handover
o Power level : 0.8W – 20 W
o 160 character long SMS.

8. SUBSCRIBER IDENTITY MODULE(SIM)
o Smart card contains the International Mobile Subscriber
Identity (IMSI)
o Allows user to send and receive calls and receive other
subscribed services
o Protected by a password or PIN
o Can be moved from phone to phone – contains key information
to activate the phone

9. SYSTEM ARCHITECTURE
BASE STATION SUBSYSTEM (BSS)
Base Station Subsystem is composed of two parts that
communicate across the standardized Abis interface allowing
operation between components made by different suppliers
1. Base Transceiver Station (BTS)
2. Base Station Controller (BSC)

10. BASE TRANSCEIVER STATION (BTS):
o Encodes, encrypts, multiplexes, modulates and feeds
the RF signals to the antenna.
o Communicates with Mobile station and BSC
o Consists of Transceivers (TRX) units

11. BASE STATION CONTROLLER (BSC)
o Manages Radio resources for BTS
o Assigns Frequency and time slots for all MS’s in its area
o Handles call set up
o Handover for each MS
o It communicates with MSC and BTS

12. SYSTEM ARCHITECTURE
NETWORK SWITCHING
SUBSYSTEM(NSS)
The system contains the following functional units
Mobile Switching Center (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Authentication Center (AUC)
Equipment Identity Register (EIR)

13. MOBILE SWITCHING CENTER (MSC)
o Heart of the network
o Manages communication between GSM and other networks
o Billing information and collection
o Mobility management
- Registration
- Location Updating
- Inter BSS and inter MSC call handoff

14. HOME LOCATION REGISTERS (HLR)
o Stores information about each subscriber that belongs to it MSC in
permanent and temporary fashion.
o As soon as mobile subscriber leaves its current local area, the
information in the HLR is updated.
o database contains IMSI, MSISDN, prepaid/ postpaid, roaming
restrictions, supplementary services.

15. VISITOR LOCATION REGISTERS (VLR)
o Temporary database which updates whenever new MS enters its
area, by HLR database
o Assigns a TMSI to each MS entering the VLR area which keeps on
changing.
o Controls those mobiles roaming in its area
o Database contains IMSI, MSISDN, Location Area, authentication
key

16. AUTHENTICATION CENTER (AUC)
o Contains the algorithms for authentication as well as the keys for
encryption.
o Protects network operators from fraud.
o Situated in special protected part of the HLR.

17. EQUIPMENT IDENTITY REGISTER (EIR)
o Stores all devices identifications registered for this network.
o Database that is used to track handsets using the IMEI
(International Mobile Equipment Identity)
o Prevents calls from stolen, unauthorised or defective mobile
devices

18. OPERATION AND MAINTENANCE CENTRE
(OMC)
o The centralized operation of the various units in the system and
functions needed to maintain the subsystems.
o Dynamic monitoring and controlling of the network.
o Functions :
o - configuration management
o - fault report and alarm handling
o - performance supervision/management
o - storage of system software and data

19. CHANNELS

21. TRAFFIC CHANNELS (TCH)
o One physical channel (1 timeslot) can support:
• 1 TCH/F or 2 TCH/H
• TCH/F: 13 kb/s voice or 9.6 kb/s data
• TCH/H: 6.5 kb/s voice or 4.8 kb/s data
o Uplink / Downlink Synchronization
The MS transmit burst is delayed by 3 timeslots after the
BTS burst.. This delay allows enables: Use of the same
UL and DL timeslot number in TDMA frame

22. BROADCAST CHANNELS (BCH)
o BCH channels are all downlink and are allocated to timeslot
zero.
o BCH channels include:
. FCCH:
Frequency control channel sends the MS a burst of all ‘0’ bits
which acts as a beacon and allows MS to fine tune to the
downlink frequency and time synchronize.
. SCH:
Synchronization channel enables TDMA-Frame number
synchronization by sending the absolute value of the frame
number (FN), together with the BTS’s BSIC.
. BCCH:
Broadcast Control Channel sends network-specific
information such as radio resource management and control
messages, Location Area Code etc.

23. COMMON CONTROL CHANNELS (CCCH)
o CCCH contains all point to multi-point downlink
channels (BTS to several MSs) and the uplink Random
Access Channel:
. RACH: Random Access Channel is sent by the MS to
request a resource from the network e.g. an SDCCH
channel for call setup.
• AGCH: Access Grant Channel is used to allocate a
dedicated channel (SDCCH) to the mobile.
• PCH: Paging Channel sends paging signal to inform
mobile of a call.

24. . CBCH: Cell Broadcast Channel is an optional GSM
Phase II implementations for SMS broadcast
messages, for example road traffic reports or
network engineering messages.
• NCH: Used for GSM Phase II voice services
such as Voice Broadcast Service (VBS) or Voice
Group Calling Service (VGCS).

25. DEDICATED CONTROL CHANNELS (DCCH)
o DCCH comprise the following bi-directional (uplink /
downlink) point to point control channels:
• SDCCH: Standalone Dedicated CHannel is used for call
set up, location updating and SMS.
• SACCH: Slow Associated Control CHannel is used for
link measurements and signalling during a call.
• FACCH: Fast Associated Control CHannel is used (when
needed) for signalling during a call, mainly for delivering
handover messages and for acknowledgement when a
TCH is assigned.

26. THANK YOU.