GSM traffic channel (TCH)

GSM TCH may be wither full rate or half rate and may carry either digitized speech or user data.

(a) Full rate TCH:

(I) Full rate speech channel (TCH/FS): This channel carries user speech which is digitized at a raw data rate of 13kbps. With GSM channel coding added to the digitized speech, this channel carries 22.8kbps.

(II) Full rate data channel for 9600 bps(TCH/F9.6): This channel carries raw user data which is sent at 9600bps. With additional forward error correction coding applied by GSM standard the 9600bps data is sent at 22.8kbps.

(III) Full rate data channel for 4800bps(TCH/F4.8) This channel carries raw user data which is sent at 4800bps. With additional forward error correction coding applied by GSM standard the 4800bps data is sent at 22.8kbps.

(IV) Full rate data channel for 2400bps (TCH/F2.4) This channel carries raw user data which is sent at 2400bps. With additional forward error correction coding applied by GSM standard the 2400bps data is sent at 22.8kbps.

(b) Half rate TCH:

(I) Half rate speech channel (TCH/HS): This channel has been designed to carry digitized speech which is sampled at half rate of 6.5kbps. with GSM channel coding added to the digitized speech the half rate speech channel will carry 11.4kbps.

(II) Half rate data channel for 4800bps (TCH/H4.8): This channel carries raw user data which is sent at 4800bps. With additional forward error correction coding applied by GSM, this channel will carry data at 11.4kbps.

(III) Half rate data channel for 2400bps (TCH/H2.4): This channel carries raw user data which is sent at 2400bps. With additional forward error correction coding applied by GSM, this channel will carry data at 11.4kbps

GSM control channel (CCH):

1) Broadcast channel (BCH):

Broadcast control channel (BCCH):

The BCCH is a forward control channel that is used to broadcast information such as cell and network identity, operating characteristics of the cell (current control channel structure, channel availability and congestion).

The BCCH also broadcast a list of channels that are currently in use within the cell.

(a) Frequency correction channel (FCCH): The FCCH allows each subscriber unit to synchronize its internal frequency standard (local oscillator) to the exact frequency of the base station.

(b) Synchronization channel (SCH):SCH is used to identify the serving BS while allowing each mobile to frame synchronizes with the BS. The frame number (FN) is sent with the base station identity code (BSIC) during the SCH burst.

2) Common control channel (CCCH):

(a) Paging channel (PCH):The PCH provides paging signals from the BS to all mobiles in the cell, and notifies a specific mobile of an incoming call which originates from PSTN.PCH may be used to provide cell broadcast ASCII text messages to all subscribers.

(b) Random Access Channel (RACH):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.

(c) Access grant channel (AGCH):The AGCH is used by the BS to provide forward link communication to the mobile, and carries data which instructs the mobile to operate in a particular physical channel.

3) Dedicated control channel (DCCH):

(a) Stand-alone Dedicated control channel (SDCCH):The SDCCH carries signaling data following the connection if the mobile with the BS, and just before TCH assignment issued by the BS. The SDCCH ensures that the mobile station and base station remain connected while the BS and MSC verifies subscriber unit.

(b) Slow Associated Control Channel (SACCH):On the forward link the SACCH is used to send slow but regularly changing control information to the mobile such a transmit power level instruction. On the reverse link the SACCH carries information about the received signal strength.

(c) Fast Associated Control Channel (FACCH):FACCH carries urgent messages and contains essentially the same type of information as SDCCH.