2. Global System for Mobile Communications
Originally GSM stood for Groupe Speciale Mobile
GSM to meet the following business objectives
1. Support for international roaming
2. Good speech quality
3. Ability to support handheld terminals
4. Low terminal and service cost
5. Spectral efficiency
6. Support for a range of new services and facilities
7. ISDN compatibility
4. GSM System Hierarchy
Consists at the minimum one administrative region
assigned to one MSC (Mobile Switching Centre)
Administrative region is commonly known as PLMN
(Public Land Mobile Network)
Each administrative region is subdivided into one or
many Location Area (LA)
One LA consists of many cell groups and each cell group
is assigned to one BSC (Base Station Controller)
For each LA, there will be at least one BSC while cells in
one BSC can belong to different LAs
7. Mobile Station
Mobile Station (MS) consists of two main elements:
mobile equipment or mobile device (that is the phone
without the SIM card) and Subscriber Identity Module (SIM)
Terminals distinguished principally by their power and
application
SIM is installed in every GSM phone and identifies the
terminal
SIM cards used in GSM phones are smart processor
cards with a processor and a small memory
SIM card contains the International Mobile Subscriber
Identity (IMSI) used to identify the subscriber to the system,
a secret key for authentication, and other security
information
8. Radio subsystem
• The Radio Subsystem (RSS) comprises the cellular mobile network up
to the switching centers
• Components
– Base Station Subsystem (BSS):
• Base Transceiver Station (BTS): radio components including sender, receiver,
antenna - if directed antennas are used one BTS can cover several cells
• Base Station Controller (BSC): switching between BTSs, controlling BTSs,
managing of network resources, mapping of radio channels (Um) onto
terrestrial channels (A interface)
• BSS = BSC + sum(BTS) + interconnection
– Mobile Stations (MS)
9. System architecture: radio subsystem
•Interfaces
– Um : radio interface
– Abis : standardized, open interface with
16 kbit/s user channels
– A: standardized, open interface with
64 kbit/s user channels
Um
Abis
A
BSS
radio
subsystem
network and switching
subsystem
MS MS
BTS
BSC MSC
BTS
BTS
BSC
BTS
MSC
10. Network and Switching Subsystem
Central component of the Network Subsystem is the
Mobile Switching Center (MSC)
Signaling between functional entities in the Network
Subsystem uses Signaling System Number 7 (SS7)
MSC together with Home Location Register (HLR) and
Visitor Location Register (VLR) databases, provide the
call-routing and roaming capabilities of GSM
MSC does the following functions:
1. It acts like a normal switching node for mobile
subscribers of the same network (connection between
mobile phone to mobile phone within the same network)
11. Network and switching subsystem
• NSS is the main component of the public mobile network GSM
– switching, mobility management, interconnection to other networks,
system control
• Components
– Mobile Services Switching Center (MSC)
controls all connections via a separated network to/from a mobile
terminal within the domain of the MSC - several BSC can belong to a MSC
– Databases (important: scalability, high capacity, low delay)
• Home Location Register (HLR)
central master database containing user data, permanent and semi-
permanent data of all subscribers assigned to the HLR (one provider can have
several HLRs)
• Visitor Location Register (VLR)
local database for a subset of user data, including data about all user
currently in the domain of the VLR
12. Mobile Services Switching Center
• The MSC (mobile services switching center) plays a central role in GSM
– switching functions
– additional functions for mobility support
– management of network resources
– interworking functions via Gateway MSC (GMSC)
– integration of several databases
• Functions of a MSC
– specific functions for paging and call forwarding
– termination of SS7 (signaling system no. 7)
– mobility specific signaling
– location registration and forwarding of location information
– provision of new services (fax, data calls)
– support of short message service (SMS)
– generation and forwarding of accounting and billing information
13. Operation subsystem
• The OSS (Operation Subsystem) enables centralized operation,
management, and maintenance of all GSM subsystems
• Components
– Authentication Center (AUC)
• generates user specific authentication parameters on request of a VLR
• authentication parameters used for authentication of mobile terminals and
encryption of user data on the air interface within the GSM system
– Equipment Identity Register (EIR)
• registers GSM mobile stations and user rights
• stolen or malfunctioning mobile stations can be locked and sometimes even
localized
– Operation and Maintenance Center (OMC)
• different control capabilities for the radio subsystem and the network
subsystem
15. GSM Frequency Allocation
Each way the bandwidth for the GSM system is 25 MHz which
provides 125 carriers uplink/downlink each having a bandwidth of
200 kHz.
ARFCN (Absolute Radio Frequency Channel Numbers) denote
a forward and reverse channel pair which is separated in
frequency by 45 MHz.
Practically, a guard band of 100 kHz is provided at the upper
and lower end of the GSM 900 MHz spectrum and only 124
(duplex) channels are implemented.
16. GSM Frequency Allocation
GSM uses TDMA and FDMA
One or more carrier frequencies are assigned to each base
station and each of these carrier frequencies is then divided in
time using a TDMA scheme where fundamental unit is called a
burst period lasting approximately 0.577 ms.
Eight burst periods are grouped into a TDMA frame of
approximately 4.615 ms which forms the basic unit for the
definition of logical channels.
One physical channel is one burst period per TDMA frame
while, normally, channels are defined by the number and position
of their corresponding burst periods.
