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Explain Traffic & Control GSM channels along with its sub types and characteristics.

Subject: Mobile Computing

Difficulty: Medium

Marks: 6 Marks


1 Answer
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There are mainly two types of GSM logical channels:

(i) Traffic channels (TCHs). (ii) Control channels (CCHs).

Traffic channels carry digitally encoded user voice or user data and have identical formats of both forward link and reverse link. Control channels carry signal and synchronization commands between the base station and mobile station. Other control channels are used only for forward and reverse link.

There are six types of GSM traffic channels (TCHs). GSM traffic channel carry digital voice and user data either at half rate or at full rate. When signal is transmitted at full rate, user data is contained within one TS per frame. When signal is transmitted at half rate, user data is mapped on to the same time slots, but it is sent in alternative frames.

Full Rate Traffic Channels (TCH):

(i) Full-rate speech channel (TCH/FS): This channel carries user speech in digitized form at a raw data rate of 13 kbps. GSM channel coding is added to digitized speech then the full rate speech channel carries 22.8 kbps.

(ii) Full-rate data channel for 9600 bps (TCH/F9.6): This channel carries raw user data which is transfers at 9600 bps with additional forward error correction applied by GSM, the 9600 bps data sent at 22.8 kbps.

(iii) Full-rate data channels for 4800 bps (TCH/F4.8): This channel carries raw user data which is transferred at 4800 bps with additional forward error correction applied by GSM, the 4800 bps is sent at 22.8 kbps.

(iv) Full rate data channel for 2400 bps (TCH/F2.4): This channel carries raw user data which is transferred at 2400 bps with additional forward error correction coding by GSM, the 2400 bps is sent at 22.8 bps.

Half-rate Traffic Channels:

(i) Half-rate speech channel (TCH/HS): This channel carries digitized speech which is sampled at a half rate then the full rate GSM channel coding added to digitized speech and half rate speech channel carry 11.4 kbps.

(ii) Half-rate data channels for 4800 bps (TCH/H4.8): This channel carries raw user data which is to be transferred at 4800 bps. With additional forward error correction applied by GSM, the 4800 bps data sent at 11.4 kbps.

(iii) Half-rate data channels for 2400 bps (TCH/H2.4): This channel carries raw user data which is to be transferred at 2400 bps with additional forward error correction by the GSM, the 2400 bp data sent at 11.4 kbps.

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GSM Control Channels (CCH):

There are three control channels in GSM:

  1. Broadcast control channels.
  2. Common control channels.
  3. Dedicated control channels.

1. Broadcast control channels (BCH) : The BTS uses this channel to give information to all MSs within a cell. Information uses by this channel is cell and network identity, current control channel structure, channel availability and congestion. The broadcast control channel also sends the list of channels that are currently used within cell.

(a) Frequency Correction Channel (FCCH): The BTS sends information for frequency correction via the Frequency Correction Channel (FCCH). The FCCH is special data burst, which occupies first frame (i.e. frame 0) and repeated after every ten frames in control channel multiframe.

(b) Synchronization Channel (SCH): BTS broadcast information about time synchronization to all MSS via synchronization channel (SCH). If the mobile station is 30 km away from serving base station, it is often necessary to adjust the timing of particular mobile user. The SCH is transmitted once after every ten frames within the control channel multiframe.

2. Common Control Channels (CCCH):

All the information regarding setting up a connection between MS and BS is exchanged via the CCCH. The common control channel occupies TSO (framo) of GSM frame and that is not used by BCH and ideal channels.

(a) Paging Channel (PCH): The PCH gives paging signal from the base station to all mobile stations within cell. It also notify particular mobile for an incoming call from PSTN. Alternatively, the PCH is used to provide cell broadcast ASCII text message to all subscriber, as a GSM SMS features.

(b) Random Access Channel (RACH): If MS wants to setup a call, it uses Random Access Channel (RACH) to send data to BTS. All mobile must request access or respond to a PCH with TSO of GSM frame. At BTS, every frame will accept RACH transmission from mobile during TSO. (c) Access Grant Channel (AGCH): The AGCH channel is used by base station to provide forward link communication to mobile station and carries instructional data which tells mobile to operate in particular physical channel with particular control channel. The AGCH is the final common control channel message sent by the base station before subscriber is roaming or moving off the control channel.

3. Dedicated Control Channels (DCCH):

There are mainly three types of dedicated control channels in GSM, same as traffic channel, they are bidirectional. They have same format and function on both forward and reverse links.

(a) Stand-alone Dedicated Control Channels (SDCCH): SDCCH carries signaling data which follows the connection of mobile with base station. The SDCCH ensures that the mobile and base station connection remains constant while the base station and MSC verify the subscriber unit and resource allocation to mobile. The SDCCH is also used to send authentication and alert messages but not speech.

(b) Slow Associated Control Channel (SACCH): The SACCH is always associate with traffic channel or SDCCH, the SACCH carries general information between the MS and BTS. On the forward link, the SACCH is used to send slow but regularly changing control information to the mobile, such as power level instruction, and specific timing advance instruction for each user. The reverse SACCH carries information about the received signal strength and quality of traffic channel as well as BCH measurement result from neighbouring cell.

(c) Fast Associated Control Channels (FACCH): FACCH carries urgent messages, and contain the same type of information as SDCCH. A FACCH is assigned to a particular user when SDCCH has not been dedicated to particular user. The FACCH access the time slots by taking frame from traffic channel, this is done by using two special bits, called stealing bits, in TCH channel.

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