GSM Channels

This page on GSM tutorial covers channel types logical GSM channels,physical GSM channels and mapping between them. It also covers GSM noncombined channel configuration(51 frame multiframe) and GSM combined channel configuration(51 frame multiframe).

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.

Logical vs physical gsm channels

This tutorial on GSM covers following:

For example, every 26 TDMA frames a logical channel gets bandwidth in a physical channel. Traffic channel are mainly of two types half rate and full rate traffic channels. There are various control channels such as BCCH (Broadcast control channel), SCH (synchronous channel), FCCH ( Frequency control channel), DCCH(Dedicated control channel).

All these gsm channels help maintain GSM network and also helps GSM mobile phone connect to GSM network and maintain the connection and help tear down the connection. Figure below mention all the channels used in GSM.

gsm channels

Fig.5 GSM Channels

GSM Channel Links

• GSM BCCH(Broadcast Control Channel)
• GSM FCCH(Frequency Correction Channel)
• GSM SCH(Synchronization Channel)
• GSM RACH(Random Access Channel)
• GSM PCH(Paging Channel)
• GSM CCCH(Common Control Channel)
• GSM FACCH(Fast Associated Control Channel)
• GSM SACCH(Slow Associated Control Channel)
• GSM SDCCH(Standalone Dedicated Control Channel)
• GSM TCH(Traffic Channel)
• GSM AGCH(Access Grant Channel)
• GSM MS Power Class
• GSM BTS Transceiver Power Class

GSM Possible Channel combinations(Noncombined and combined)

There are certain rules by which different channel types are used in different time slots. These rules are used to map logical channels to physical channels. The most important slot is the time slot TS0 where in BCCH is mapped and is very useful as SI messages(system information messages) are transmitted over this channel.

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

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.
GSM Noncombined channel Configuration➤➤

Combined 51 frame multiframe 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➤➤

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