Improved spectrum efficiency.
Low-cost mobile sets and base stations (BSs).
Compatibility with Integrated Services Digital Network
(ISDN) and other telephone company services.
• Support for new services.
Bearer or Data Services
• Telecommunication services that enable voice communication
via mobile phones
• Offered services
• Include various data services for information transfer
between GSM and other networks like PSTN, ISDN etc
at rates from 300 to 9600 bps
• Short Message Service (SMS)
–up to 160 character alphanumeric data transmission
to/from the mobile terminal
• Unified Messaging Services(UMS)
• Group 3 fax
• Voice mailbox
• Electronic mail
• Call related services : Call Waiting- Notification of an incoming
call while on the handset.
Call Hold- Put a caller on hold to take another call
Call Barring- All calls, outgoing calls, or incoming calls
Call Forwarding- Calls can be sent to various numbers defined by
• Multi Party Call Conferencing - Link multiple calls together
• CLIP – Caller line identification presentation
• CLIR – Caller line identification restriction
• CUG – Closed user group
• A GSM network consists of several functional entities, whose
functions and interfaces are defined. The GSM network can be
divided into following broad parts.
• The Mobile Station (MS)
• The Base Station Subsystem (BSS)
• The Network Switching Subsystem (NSS)
• The Operation Support Subsystem (OSS)
• Mobile Station (MS)
Mobile Equipment (ME)
Subscriber Identity Module (SIM)
• Base Station Subsystem (BSS)
Base Transceiver Station (BTS)
Base Station Controller (BSC)
• Network Switching Subsystem(NSS)
Mobile Switching Center (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Authentication Center (AUC)
Equipment Identity Register (EIR)
Uniquely identified by an IMEI (International Mobile Equipment
Identity).The Mobile Station is made up of two entities:
• Mobile Equipment (ME):There are different types of
terminals (MN) distinguished principally by their power and
application: Portable , vehicle mounted, hand held device with
Power level : 0.8W – 20 W
Subscriber Identity Module(SIM): It is protected by a
four-digit Personal Identification Number (PIN). Allows
user to send and receive calls and receive other subscribed
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):
Encodes,encrypts,multiplexes,modulates and feeds the RF
signals to the antenna.
Communicates with Mobile station and BSC
Consists of Transceivers (TRX) units
2. Base Station Controller (BSC)
• Manages Radio resources for BTS
• Assigns Frequency and time slots for all MS’s in its
• Handles call set up
• Transcoding and rate adaptation functionality
• Handover for each MS
• Radio Power control
• It communicates with MSC and BTS
Mobile Switching Center (MSC)
• Heart of the network
• Manages communication between GSM and other networks
• Call setup function and basic switching
• Call routing
• Billing information and collection
• Mobility management
- Location Updating
- Inter BSS and inter MSC call handoff
• MSC does gateway function while its customer roams to other
network by using HLR/VLR
• Home Location Registers (HLR)
- permanent database about mobile subscribers in a large
service area(generally one per GSM network operator)
- database contains IMSI,MSISDN,prepaid/postpaid,roaming
• Visitor Location Registers (VLR)
- Temporary database which updates whenever new MS enters
its area, by HLR database
- Controls those mobiles roaming in its area
- Reduces number of queries to HLR
- Database contains IMSI,TMSI,MSISDN,MSRN,Location
• Authentication Center (AUC)
- Protects against intruders in air interface
- Maintains authentication keys and algorithms and
provides security triplets ( RAND,SRES,Kc)
- Generally associated with HLR
• Equipment Identity Register (EIR)
- Database that is used to track handsets using the IMEI
(International Mobile Equipment Identity)
- Made up of three sub-classes: The White List, The
Black List and the Gray List
- Only one EIR per PLMN
• In a GSM network, the following areas are defined:
Cell: Cell is the basic service area: one BTS covers one cell.
Each cell is given a Cell Global Identity (CGI), a number that
uniquely identifies the cell.
Location Area: A group of cells form a Location Area. This is
the area that is paged when a subscriber gets an incoming call.
Each Location Area is assigned a Location Area Identity
(LAI). Each Location Area is served by one or more BSCs.
MSC/VLR Service Area: The area covered by one MSC is
called the MSC/VLR service area.
PLMN: The area covered by one network operator is called
PLMN. A PLMN can contain one or more MSCs.
• GSM uses TDMA and FDMA to let everybody talk.
• FDMA: 25MHz freq. is divided into 124 carrier frequencies.
Each base station gets few of those.
• TDMA: Each carrier frequency is divided into bursts [0.577
ms]. 8 bursts are a frame.
• RF Spectrum
Mobile to BTS (uplink): 890-915 Mhz
BTS to Mobile(downlink):935-960 Mhz
Bandwidth : 2* 25 Mhz
Mobile to BTS (uplink): 1710-1785 Mhz
BTS to Mobile(downlink) 1805-1880 Mhz
Bandwidth : 2* 75 Mhz
When a mobile subscriber makes a call to a PSTN telephone
subscriber, the following sequence of events takes place:
• The MSC/VLR receives the message of a call request.
• The MSC/VLR checks if the mobile station is authorized to
access the network. If so, the mobile station is activated. If the
mobile station is not authorized, service will be denied.
• MSC/VLR analyzes the number and initiates a call setup with
• MSC/VLR asks the corresponding BSC to allocate a traffic
channel (a radio channel and a time slot).
• The BSC allocates the traffic channel and passes the
information to the mobile station.
• The called party answers the call and the conversation takes
• The mobile station keeps on taking measurements of the radio
channels in the present cell and neighbouring cells and passes
the information to the BSC. The BSC decides if handover is
required, if so, a new traffic channel is allocated to the mobile
station and the handover is performed. If handover is not
required, the mobile station continues to transmit in the same
When a PSTN subscriber calls a mobile station, the sequence of
events is as follows:
• The Gateway MSC receives the call and queries the HLR for
the information needed to route the call to the serving
• The GMSC routes the call to the MSC/VLR.
• The MSC checks the VLR for the location area of the MS.
• The MSC contacts the MS via the BSC through a broadcast
message, that is, through a paging request.
• The MS responds to the page request.
• The BSC allocates a traffic channel and sends a message to the
MS to tune to the channel. The MS generates a ringing signal
and, after the subscriber answers, the speech connection is
1. MS sends dialed number to BSS
2. BSS sends dialed number to MSC
3,4 MSC checks VLR if MS is
allowed the requested service. If so,
MSC asks BSS to allocate resources
5 MSC routes the call to GMSC
6 GMSC routes the call to local
exchange of called user
7, 8,9,10 Answer back(ring back) tone
is routed from called user to MS via
Calling a GSM subscribers
Forwarding call to GSMC
Signal Setup to HLR
5. Request MSRN from VLR
Forward responsible MSC to
7. Forward Call to current MSC
8. 9. Get current status of MS
10.11. Paging of MS
12.13. MS answers
14.15. Security checks
16.17. Set up connection
• the use of signals for controlling communications
• the information exchange concerning the establishment and
control of a telecommunication circuit and the management of
the network, in contrast to user information transfer
• the sending of a signal from the transmitting end of
a telecommunication circuit to inform a user at the receiving
end that a message is to be sent.
1.In-band versus out-of-band signaling
2. Line versus register
3 .Channel-associated versus common-channel signaling
4 .Compelled signaling
5 .Subscriber versus trunk signaling
• In the public switched telephone network (PSTN), in-band
signaling is the exchange of call control information within the
same channel that the telephone call itself is using. An
example is dual-tone multi-frequency signaling (DTMF),
which is used on most telephone lines to customer premises.
• Out-of-band signaling is telecommunication signaling on a
dedicated channel separate from that used for the telephone
call. Out-of-band signaling is used in Signalling System No.
7 (SS7), the standard for signaling among exchanges that have
controlled most of the world's calls for over twenty years
• Line signaling is concerned with conveying information on
the state of the line or channel, such as on-hook, off-hook ,
ringing current (alerting), and recall.
• Register signaling is concerned with conveying addressing
information, such as the calling and/or called telephone
• Channel-associated signaling (CAS) employs a signaling
channel which is dedicated to a specific bearer channel.
• Common-channel signaling (CCS) employs a signaling
channel which conveys signaling information relating to
multiple bearer channels. These bearer channels therefore have
their signaling channel in common.
• Compelled signaling refers to signaling where receipt of each
signal from an originating register needs to be explicitly
acknowledged before the next signal is able to be sent.
• The term is only relevant in the case of signaling
systems that use discrete signals , as opposed to
signaling systems which are message-oriented where
each message is able to convey multiple items of
• Subscriber signaling refers to signaling between the
telephone and the telephone exchange. Trunk signaling
is signaling between exchanges