GSM needs to transmit a large amount of signalling information apart from the payload data. These different types of data are transmitted via several logical channels. The name stems from the fact that each of the data types is transmitted on specific timeslots that are parts of physical channels. Logical channel means just a part of unique information meant for a unique user or a group of users. All these logical pieces of information require a carrier frequency to be transmitted. This carrier frequency is the physical channel of 200 KHz. So effectively, every logical channel needs a physical channel but viceversa is not true. There are fixed time slots (predecided) on a physical channel on which this logical channels will be transmitted. This is called as mapping of logical channel on a physical channel.

The categories of logical channels is given below:

There are two types of GSM logical channels, called Traffic CHannels (TCH) and Control CHannels (CCH). Traffic channels carry digitally encoded user speech or user data and have identical functions and formats on both the forward and reverse link. Control channels carry signaling and synchronizing commands between the Base station and the mobile station. Certain types of control channels are defined for just the forward or reverse link.

1. GSM traffic channels: GSM traffic channels may be either full-rate or half-rate and may carry either digitized speech or user data. Full rate channels are those in which a user gets a time slot per frame. Half rate channels are those in which the user gets a time slot per alternate frame. More number of users i.e. 16 users can get the same channel simultaneously at a compromise of data rate. Whether a physical channel will be used as full rate or half rate totally depends upon the service provider who will decide according to the congestion in the network.

a) Full-Rate TCH

The following full rate speech and data channels are supported:The notations of these channels are given in the bracket.

• Full-Rate Speech Channel (TCH/FS) :The full-rate speech channel carries user speech which is digitized at a raw data rate of 13 Kbps. With GSM channel coding added to the digitized speech, the full-rate speech channel carries 22.8 Kbps.Full Rate data channels are used in three configurations and are termed as TCH/F9.6, TCH/F4.8 and TCH/F2.4.

-Full-Rate Data channels: These channels carry raw user data at rates of 9.6 Kbps, 4.8 Kbps and 2.4 Kbps respectively. With additional forward error correction coding applied by the GSM standard, the data is sent at 22.8 Kbps in all three configurations.

b) Half-Rate TCH

The following half-rate speech and data channels are supported:

• Half-Rate Speech Channel (TCH/HS): The half-rate speech channel has been designed to carry digitized speech which is sampled at a rate half that of the full-rate channel. With GSM channel coding added to the digitized speech, the half-rate speech channel will carry 11.4 Kbps. Half rate channels are of two types : TCH/H4.8 and TCH/H2.4

• Half-Rate Data channels: These channels carry user data at 4.8 and 2.4 Kbps respectively. With additional forward error correction coding applied by the GSM standard, the data is sent at 11.4 Kbps for both the channels.

2. GSM Control Channels (CCH): There are three main control channels in the GSM system. These are the

i) Broadcast CHannel (BCH), ii) The Common Control CHannel (CCCH), and the iii) Dedicated Control CHannel (DCCH)

Each control channel consists of several logical pieces of information which is discussed in detail:

i) Broadcast Channels (BCH): 34 ARFCNs out of 124 are defined as Broadcast control channels. BCH is sent only on the TS0 ( Time slot 0 ) of each frame. The other 7 time slots of each frame can be used for sending user traffic. Unlike TCHs which are duplex, BCHs only use the forward link. BCH carries information About frequency, time synchronization, occasionally monitored by neighbouring mobiles for MAHO decisions.

Three types of broadcast channels exist which are:

a) Broadcast Control CHannel (BCCH) : The BCCH is a forward control channel that is used to broadcast information such as cell and network identity, and operating characteristics of the cell (current control channel structure, channel availability, and congestion). The BCCH also broadcasts a list of channels that are currently in use within the cell. Frame 2 through frame 5 in a control multiframe (4 out of every 51 frames) contain BCCH data.

b) Frequency Correction CHannel (FCCH): The FCCH is a special data burst which occupies TS 0 for the very first GSM frame (frame 0) and is repeated every ten frames within a control channel multiframe. The FCCH allows each subscriber unit to synchronize its internal frequency standard (local oscillator) to the exact frequency of the Base Station.

c) Synchronization CHannel (SCH): SCH is broadcasted in TS 0 of the frame immediately following the FCCH frame and is used to time synchronize the serving Base Station while allowing each mobile to frame synchronize with the BS. The Frame Number (FN), which ranges from 0 to 2,715,647 ( 26 frames $\times$ 51 multiframes $\times$ 2048superframe), is sent with the base station identity code (BSIC) during the SCH burst. The BSIC is uniquely assigned to each BTS in a GSM system. Since a mobile may be as far as 30 km away from a serving BS, it is often necessary to adjust the timing of a particular mobile user such that the received signal at the base station is synchronized with the Base Station clock. The BS issues coarse timing advancement commands to the mobile stations over the SCH,as well. The SCH is transmitted once every ten frames within the control channel multiframe, as shown in Figure 2(a).

ii) Common Control CHannels (CCCH): On the broadcast (BCH) ARFCN,the common control channels occupy TS 0 of every GSM frame that is not otherwise used by the BCH or the Idle frame. CCCH consists of three different channels:

a) Paging CHannels (PCH):

• Notifies a mobile about an incoming call.
• Transmits the IMSI of the target subscriber.
• Provides the cell broadcast ASCII text msgs.

b) Random Access CHannels (RACH):

• It is reverse link channel
• To acknowledge the page message from the PCH or request for a call setup.

c) Access Grant CHannel( AGCH):

• It is forward link channel.
• It is a response to the RACH message from mobile.
• It conveys the information about time slot and ARFCN allotted to a mobile station for authentication process or for starting the communication.

iii) Dedicated Control CHannels (DCCH): There are three types of dedicated control channels in GSM, and, like traffic channels they are bidirectional and have the same format and function on both the forward and reverse links. Like TCHs, DCCHs may exist in any time slot and on any ARFCN except TS0 of the BCH ARFCN.

a) Stand alone Dedicated Control CHannel (SDCCH)

• Carries data following connection of mobile.
• Ensures mobile and BS connectivity during authentication and alert messages.

b) Slow Associated Control CHannel (SACCH)

• To acknowledge a page message from PCH
• Transmitted during the 13th frame and 26th frame
• SACCH is used to send slow but regularly changing data like transmit power level instructions.

c) Fast Associated Control CHannel (FACCH)

• Carries urgent messages (HO requests)
• Gains access to a time slot by stealing frames from the traffic channel
• The FACCH gains access to a time slot by "stealing" frames from the traffic channel to which it is assigned. This is done by setting two special bits, called stealing bits, in a TCH forward channel burst. If the stealing bits are set, the time slot is known to contain FACCH data, not a TCH, for that frame.

For proper understanding of the reader, a control channel multiframe in the forward and reverse channel is as shown in Figure 2. Unlike traffic multiframe, a control channel mutiframe has 51 TDMA frames and is of 235 ms duration. This is done intentionally to ensure that any GSM subscriber (whether in the serving or adjacent cell) will be certain to receive the SCH and FCCH transmissions from the BCH, no matter what particular frame or time slot they are using.