GSM: GSM stands for Global System for Mobile Communication. It is the most successful second generation digital mobile radio system. GSM became popular very quickly because it provided improved speech quality and through a uniform international standard, made it possible to use a single telephone number and mobile unit around the world. The European Telecommunications Standardization Institute (ETSI) adopted the GSM standard in 1991, and GSM is now used in 135 countries.

The benefits of GSM include:

• Support for international roaming
• Distinction between user and device identification
• Excellent speech quality
• Wide range of services
• Interworking i.e. with ISDN,DECT etc.
• Extensive security features

GSM also stands out from other technologies with its wide range of services i.e.

• Telephony, asynchronous and synchronous data services (2.4/ 4.8/ 9.6 kbit/s), access to packet data network (X.25), telematic services (SMS, fax, videotext etc.), many value added features (call forwarding, caller ID, voice mailbox) and, e-mail and internet connections.

GSM architecture:

The best way to create a manageable communications system is to divide into various subgroups that are interconnected using standardized interfaces. A GSM network can be divided into three groups as shown in below mentioned figure i.e.,

• The mobile station (MS)
• The base station subsystem (BSS)
• The network subsystem (NSS)

GSM network architecture

This architecture divided by in three interfaces i.e. Air interface (Um), A interface and Abis interface as shown in below mentioned figure, where

• Air interface (Um) lies between MS (mobile station) and BTS(Base trans-receiver station) station
• Abis interface lies between BTS and BSC station and
• A interface lies between BSC and MSC (mobile switching centre) station.

GSM interfaces

GSM CHANNELS:

GSM network consists of two types of channels i.e. logical and physical channels.  Several logical channels are mapped onto the physical channels. The organisation of logical channels depends on the application and the direction of information flow (uplink/downlink or bidirectional). A logical channel can be either a traffic channel (TCH), which carries user data, or a signalling channels shown below:

TRAFFIC CHANNELS:

Traffic channels are divided into two parts i.e. full rate and half rate. GSM supports both full rate and half rate channels. Every time slots carries 144 bits of data. The full rate channel has a transmission rate of 22.8 kbps. The GSM speech coder outputs 260 bits every frame time. These are error protected to produce 456 bits that are transmitted over 4 frames (8 blocks of 57 bits, 2 blocks per frame (114 bits)).        ``

SIGNALLING CHANNEL: This channel carries control information to enable the system to operate correctly.  The signalling channels on the air interface are used for call establishment, paging, call maintenance, synchronisation etc.The main categories of signalling channels in GSM which are further divided in several categories:

1.BROADCAST CHANNELS (BCH): This channel carry only downlink information and are responsible mainly for synchronization and frequency correction. This is the only channel type enabling point to multi-point communications in which short messages are simultaneously transmitted to the several mobiles. The BCHs include the following channels:

Broadcast Control Channel (BCCH): This channel handle General information, cell specific, e.g. local area code (LAC), network operator, access parameters, list of neighbouring cells, etc. The mobile station (MS) receives signals via the BCCH from many BTSs within the same network and/ or different networks.

Frequency Correction Channel (FCCH): This channel carry only downlink information and handle the correction of MS frequencies, transmission of frequency standard to MS, it is also used for synchronization of an acquisition by providing the boundaries between timeslots and the position of the first timeslot of a TDMA frame..

Synchronization Channel (SCH): This channel carry only downlink information and handle

The frame synchronization and identification of base station. The valid reception of one SCH burst will provide the MS with all the information needed to synchronize with a BTS.

2. COMMON CONTROL CHANNELS (CCCH):

This channel carry a group of uplink and downlink channels between the MS card and the BTS. These channels are used to convey information from the network to MSs and provide access to the network. The CCCHs include the following channels:

Paging Channel (PCH): This channel carry only downlink information. MS is informed by the BTS for incoming calls via the PCH.

Random Access Channel (RACH): This channel carry only uplink information. It allows the MS to request an SDCCH in response to a page or due to a call. The MS chooses a random time to send on this channel. This creates a possibility of collisions with transmissions from other MS. The PCH and AGCH are transmitted in one channel called the paging and access grant channel (PAGCH). They are separated by time.

Access Grant Channel (AGCH): This channel carry only downlink information. BTS allocates a TCH or SDCCH to the MS, thus allowing the access to the network.

3. DEDICATED CONTROL CHANNELS (DCCH):

These channels are responsible for roaming, handovers, encryption etc. These channels include the following channels:

Standalone Dedicated Control Channel (SDCCH): This channel carry both uplink and downlink information. In response of RACCH, network allocates SDCCH over AGCH for further communication between BTS and MS.This channel is used for the location update, voice call setup and SMS.

Fast Associated Control Channel (FACCH): This channel carry both uplink and downlink information. This channel is used to convey handover information. There is no TS and frame allocation dedicated to this channel. This channel can be associated with SDCCH or TCH and works on the principle of stealing. The burst of TCH is replaced by FACCH signalling when required.

Slow Associated Control Channel (SACCH): This channel carry both uplink and downlink information. This channel is always associated with TCH or SDDCH used for control and supervision of signals associated with the traffic channels. This channel is used to convey the periodic carrier signal strength measurements to the network, transmit power control and timing advance.