THE GSM CHANNELS:

• 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. There are six different types of TCHs provided for in GSM, and an even larger number of CCHs

THE GSM TRAFFIC CHANNELS:

• GSM traffic channels may be either full-rate or half-rate and may carryeither digitized speech or user data.
• When transmitted as full-rate, user data iscontained within one TS per frame.
• When transmitted as half-rate, user data is mapped onto the same time slot, but is sent in alternate frames.
• That is, two half-rate channel users would share the same time slot, but would alternatelytransmit during every other frame.
• Each group of twenty-six consecutive TDMA frames is called amultifranie (or speech multiframe, to distinguish it from the control channel maltiframe ).
• For every twenty-six frames, the thirteenth and twenty-sixth frames consist of Slow Associated Control Channel (SACCH) data, or the idle frame, respectively.
• The twenty-sixth frame contains idle bits for thecase when full-rate TCHs are used, and contains SACCH data when half-rateTCHs are used.

THE FULL RATE TRAFFIC CONTROL CHANNEL:

• 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 Channel for 9600 bps (TCH/F9.6) .
• The full-rate traffic data channel carries raw user data which is sent at 9600 bps.
• With additional forward error correction coding applied by the GSM standard, the 9600 bps data is sent at 22.8 kbps. Full-Rate Data Channel for 4800 bps (TCH/F4.8)
• The full-rate traffic data channel carries raw user data which is sent at 4800 bps.
• With additional forward error correction coding applied by the GSM standard, the 4800 bps is sent at 22.8 kbps.
• Full-Rate Data Channel for 2400 bps (TCH/F2.4) .
• The full-rate traffic data channel carries raw user data which is sent at 2400 bps. With additional forward error correction coding applied by the GSM standard, the 2400 bps is sent at 22.8 kbps.

HALF RATE TRAFFIC CONTROL CHANNEL :

• 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.
• GSIVI anticipates the availability of speech coders which can digitize speech at about 6.5 kbps.
• With GSM channel coding added to the digitized speech, the half-rate speech channel will carry 11.4 kbps.Half-Rate Data Channel for 4800 bps (TCH/H4.8)
• The half-rate trafficdata channel carries raw user data which is sent at 4800 bps.
• With additional forward error correction coding applied by the GSM standard, the4800 bps data is sent at 11.4 kbps.
• Half-Rate Data Channel for 2400 bps (TCHIH2.4) — The half-rate traffic data channel carries raw user data which is sent at 2400 bps.
• With additional forward error correction coding applied by the GSM standard, the2400 bps data is sent at 11.4 kbps.

THE GSM CONTROL CHANNEL:

• There are three main control channels in the GSM system.
• These are the broadcast channel (BCH), the common control channel (CCCH), and the dedicated control channel (DCCH). Each control channel consists of several logical channels which are distributed in time to provide the necessary GSM control functions.
• The BCH and CCCH forward control channels in GSM are implemented only on certain ARFCN channels and are allocated timeslots in a very specific manner.
• Specifically, the BCH and CCCH forward control channels are allocated only TS 0 and are broadcast only during certain frames within a repetitive fifty one frame sequence (called the control channel multiframe) on those ARFCNs which are designated as broadcast channels. TS1 through TS7 carry regular TCH traffic, so that ARFCNs which are designated as control channels are still able to carry full-rate users on seven of the eight time slots.
• The GSM specification defines thirty-four ARFCNs as standard broadcast channels.
• For each broadcast channel, frame 51 does not contain any BCH/ CCCH forward channel data and is considered to be an idle frame.
• However, the reverse channel CCCH is able to receive subscriber transmissions during TS 0 of any frame (even the idle frame).
• On the other hand, DCCH data may be sent during any time slot and any frame, and entire frames are specifically dedicated to certain DCCH transmissions.

THE BROADCAST CHANNEL:

• Broadcast Channels (BCE) The broadcast channel operates on the forward link of a specific ARFCN within each cell, and transmits data only in the first time slot (TS 0) of certain GSM frames.
• Unlike TCHs which are duplex, BCHs only use the forward link. Just as the forward control channel (FCC) in AJYIPS is used as a beacon for all nearby mobiles to camp on to, the BCH serves as a TDMA beacon channel for any nearby mobile to identify and lock on to.
• The BCH provides synchronization for all mobiles within the cell and is occasionally monitored by mobiles in neighboring cells so that received power and MAHO decisions may be made by out-of-cell users. Although BCH data is transmitted in TSO, the other seven timeslots in a GSM frame for that same ARFCN are available for TCH data, DCCH data, or are filled with dummy bursts.
• Furthermore, all eight timeslots on all other ARFCNs within the cell are available for TCH or DCCH data.
• 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 BCCII 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.

THE FREQUENCY CONTROL CHANNEL:

• Frequency Correction Channel (FCCH) The FCCH is a special data burst which occupies PS 0 for the very first GSM frame (frame 0) and is repeated every ten frames within a control channel multi frame.
• The FCCH allows each subscriber unit to synchronize its internal frequency standard (local oscillator) to the exact frequency of the base station.
• Synchronization Channel (SCH) SCH is broadcast in TS 0 of the frame immediately following the FCCH frame and is used to identilS' the serving base station while allowing each mobile to frame synchronize with the base station.
• The frame number (FN), which ranges from 0 to 2,715,647, is sent with the base station identity code (BSIC) during the SCH burst.
• The BSIC is uniquely assigned to each BST in a GSM system.
• Since a mobile may be as far as 30 km away from a serving base station, 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 COMMON CONTROL CHANNEL:

• Common Control CHannels (CCCH) On the broadcast (13CM) 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: the paging channel (PCH), which is a forward link channel, the random access channel (RACH) which is a reverse link channel, and the access grant channel (AGCH), which is a forward link channel.
• CCCHs are the most commonly used control channels and are used to page specific subscribers, assign signaling channels to specific users, and receive mobile requests for service. These channels are described below.
• Paging CHannel (PCH) The PCH provides paging signals from the base station to all mobiles in the cell, and notifies a specific mobile of an incoming call which originates from the PSTN.
• The PCH transmits the IMSI of the target subscriber, along with a request for acknowledgment from the mobile unit on the RACH.
• Alternatively, the PCH may be used to provide cell broadcast ASCII text messages to all subscribers, as part of the SMS feature of GSM.
• Random Access Channel (R.ACH) — The RACH is a reverse link channel used by a subscriber unit to acknowledge a page from the PCH, and is also used by mobiles to originate a call. The RACH uses a slotted ALOHA access scheme.