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GSM tutorial

The full form of GSM is Global System for Mobile Communications. It is also known as 2G or Second Generation technology. GSM is globally accepted for mobile communication to support voice, text messaging and data services through its networks. GSM ensures interoperability and seamless roaming across different countries. There are various GSM standards such as GSM900, EGSM900, GSM1800 and GSM 1900 based on RF carrier frequency bands and bandwidths. Due to many compelling advantages of GSM architecture, it has gained popularity and became the dominant mobile communication standard worldwide.

Architecture of gsm

GSM network is consists of Mobile station, Base station subsystem and Network and operation subsystem. This is shown in the block diagram below.

gsm network

Mobile Station- This Mobile station is GSM mobile phone equipment which houses DSP,RF chip and SIM(subscriber Identity Module). This SIM is enough to carry to avail the service of GSM network. SIM contains subscriber related all the information, network with which subscriber is subscribed with and encryption related information.

Base station Subsystem- Base station subsystem houses Base Transceiver station-BTS and Base station controller-BSC. This subsystem take care of radio control related functions and provides GSM air interface for GSM mobile phones to connect with GSM network. To provide GSM service, region/city on earth is divided into various cells. The cell size is usually about 100m to about 35 km. BTS coverage is limited to this cell. Like this many BTSs cover entire region. All this BTSs are interfaced with one BSC in various ways mesh, star etc. This BSC takes care of radio frequency assignments to the mobile phones, takes care of handoff within BSS i.e. between one BTS and the other BTS.

Network Subsystem (NSS) - This subsystem provides interface between cellular system and circuit switched telephone network i.e. PSTN. It performs switching and operation & maintenance related functions. NSS takes care of call processing functions such as call setup, switching, tear down and also hand over between BSCs. NSS takes care of security and authentication related functions. There are various network elements in this subsystem as mentioned in GSM architecture above. They are explained below. These are basically database elements.
HLR-Home Location Register, it stores permanent and temporary subscriber related information.
VLR- Visitor Location Register, it stores visitor subscriber related information about its facilities, the network it is subscribed to, and its home location and so on.
AUC- Authentication center, used to authenticate activities in the system. It holds encryption (A5 key) and authentication keys (A3 key)in both HLR and VLR.
EIR- Equipment Identification Register, it helps in security as it keeps track of equipment type available in Mobile Station or Terminal.

GSM network interfaces

Air interface between Mobile station and BTS
Abis interface between BTS and BSC
A interface between BSC and MSC
SS7 interface between MSC and PSTN
Refer GSM network interfaces >>

GSM system Specifications

Access Method- TDMA/FDMA
Uplink frequency band- 890 to 915 MHz
Downlink frequency band- 935 to 960 MHz
System Bandwidth- 200 KHz
No. of frequency channels or ARFCN (Absolute Radio Frequency Channel Number)-124
Users per channel-8
Frame duration-4.615ms
Spectral efficiency-1.35 b/s/Hz
Data rate per user- 33.6 kbps (270.833 kbps Gross data rate for 8 users/8users)

GSM Frame Structure

gsm frame structure

The GSM frame structure is designated as hyperframe, superframe, multiframe and frame. The minimum unit being frame (or TDMA frame) is made of 8 time slots. One GSM hyperframe composed of 2048 superframes. Each GSM superframe composed of multiframes (either 26 or 51 as described below). Each GSM multiframe composed of frames (either 51 or 26 based on multiframe type). Each frame composed of 8 time slots. Hence there will be total of 2715648 TDMA frames available in GSM and the same cycle continues.

As shown in the gsm hyperframe diagram, there are two varients to GSM multiframe structure.
1. 26 frame multiframe - Called traffic multiframe,composed of 26 bursts in a duration of 120ms, out of these 24 are used for traffic, one for SACCH and one is not used.
2. 51 frame multiframe- Called control multiframe,composed of 51 bursts in a duration of 235.4 ms. This type of multiframe is divided into logical channels. These logical channels are time sheduled by BTS. Always occur at beacon frequency in time slot 0, it may also take up other time slots if required by system for example 2,4,6.

Following are possible channel combinations in GSM system which network(BTS) will adopt based on need of traffic channels versus signalling(control) gsm channels. They are called as combined and non-combined type, refer following links for more.

Noncombined 51-frame multiframe configuration

Following are the two gsm time slot hierarchy used in combined and noncombined 51 frame multiframe configurations.

GSM noncombined channel configuration TS0 GSM noncombined channel configuration TS1

Follow link below for the complete chart of this configurations for TS0 and TS1. In noncombined configuration, dedicated signaling channels are not combined with BCCH/CCCH and thus require separate time slot(TS1). FCCH,SCH,BCCH and CCCH channels are mapped on TS0.
Refer GSM Noncombined channel Configuration >>

Combined 51 frame multiframe configuration

GSM combined channel configuration

Follow link below for the complete chart of this configurations for TS0. In combined configuration, FCCH,SCH,BCCH,CCCH channels are present along with SDCCH on ime slot TS0. Hence dedicated signaling channels SDCCH are combined with BCCH/CCCH on the same time slot TS0. SDCCH also can be mapped on TS1 in addition to TS0; even SDCCH can be mapped on to anyother time slots also.
GSM Combined channel configuration >>
In the following section of GSM tutorial we will explore GSM burst types, frame structure, channel types, physical layer and protocol stack.

GSM Channels

gsm channels

There are two main types of GSM channels viz. physical channel and logical channel. Physical channel is specified by specific time slot/carrier frequency. Logical channel run over physical channel i.e. logical channels are time multiplexed on physical channels; each physical channel(time slot at one particular ARFCN) will have either 26 Frame MF(Multi-frame) or 51 Frame MF structure describe here. logical channels are classified into traffic channel and control channel. Traffic channel carry user data. Control channels are interspersed with traffic channels in well specified ways.

There are various control channels such as BCCH (Broadcast control channel), SCH (synchronous channel), FCCH ( Frequency control channel), DCCH(Dedicated control channel). Traffic channel is designated as TCH.
Refer links below under heading "GSM Channel Types" for more information.

GSM Burst Types

GSM burst types includes normal burst, frequency correction burst, Synchronization burst, dummy burst and access burst. Each of these bursts are used for specific purposes. Normal burst is used for all common applications which include exchange of signaling traffic and user traffic. Access burst is used by mobile to make first attempt to communicate with the network (i.e. BTS). Before normal burst and access bursts are used, BTS in gsm uses two special bursts known as frequency correction burst and synchronization burst in order to allow mobile subscriber (MS) to synchronize in frequency and time.

GSM Protocol Stack

gsm protocol stack

The figure depicts gsm protocol stack layers at various GSM network elements viz. MS, BTS, BSC and MSC. Refer gsm protocol stack layers >> for more information.

GSM Physical Layer

gsm physical layer transmitter

GSM physical layer i.e. layer-1 which sits below GSM layer-2(LAPDm). As shown it consists of various modules viz. cyclic encoder, convolution encoding, interleaving, ciphering, burst formation, differential encoding, GMSK modulation and RF upconversion. Refer GSM PHYSICAL LAYER modules >> for more information.

Conclusion: GSM technology has laid foundation for subsequent generations of mobile communication technologies such as 3G, 4G LTE, 5G NR and beyond while continuing to serve as a reliable mobile communication platform in various regions worldwide. GSM architecture has evolved further to include standards such as GSM-R (GSM Railway), EGPRS (Enhanced GPRS), GERAN (GSM/EDGE Radio Access Network), TACS, NMT etc.

GSM Channel Types

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