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Explain different traffic channels and control channels in GSM.

Subject : Mobile Communication

Topic : GSM

Difficulty : Medium

1 Answer
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A. Traffic Channel (TCH): TCH cab either be Full-rate or Half-rate and carry either digitalized speech or users data. When transmitted as full –rate. User data is contained within one TS per frame. When transmitted as half0rate, user datais mapped onto the same time slot , but is sent in alternate frames.

  • Full-Rate Speech Channel (TCH/FS): the full-rate speech channel carries user speech which is digitalized at raw rate of 13 kbps, with GSM channel coding added to the digitalized speech, the full rate speech channel carries 22.8 kbps.

  • Full-Rate Data Channel for 9600 bps (TCH/F 9.6): The full-rate traffic data channel carries raw user date which is sent at 9600 bps. With additional foeward 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/F 4.8): The full-rate traffic data channel carries raw user date which is sent at 4800 bps. With additional foeward error correction coding applied by the GSM standard, the 4800 bps data is sent at 22.8 kbps.

  • Full-Rate Data Channel for 2400 bps (TCH/F 2.4): The full-rate traffic data channel carries raw user date which is sent at 2400 bps. With additional foeward error correction coding applied by the GSM standard, the 2400 bps data is sent at 22.8 kbps.

  • Half-Rate Speech Channel (TCH/HS): the half-rate speech channel carries user speech which is digitalized and sampled at the rate half that of the full arte channel i.e.6.5 kbps, with GSM channel coding added to the digitalized speech, the full rate speech channel carries 11.4 kbps.

  • Half-Rate Data Channel for 4800 bps (TCH/H 4.8): The half-rate traffic data channel carries raw user date which is sent at 4800 bps. With additional foeward error correction coding applied by the GSM standard, the 4800 bps data is sent at 11.4 kbps.

  • Half-Rate Data Channel for 2400 bps (TCH/H 2.4): The half-rate traffic data channel carries raw user date which is sent at 2400 bps. With additional foeward error correction coding applied by the GSM standard, the 2400 bps data is sent at 11.4 kbps.

B. Control channel:

  • There are three control channels in GSM system. These are Broadcas Channel (BCH), Common Control Channel (CCCH) and Dedicated Control Channel (DCCH).
  • Each control channel consists of several logical channel which are distributed in time to provide the necessary GSM control functions.
  • GSM specification defines 34 standard broadcast channels. There are three DCCH in GSM and they are bidirectional and have same format and function on both forward and reverse links.
  • DCCH can exist in any time slot and on any ARFCN except TS0 of BCH ARFCN.
  • DCCH is of three types Stand-alone Dedicated Control Channel (SDCCH) used for providing signaling services required by users.
  • Slow-Associated control Channel (FACCH) and Slow-Associated Control Channel (SACCH) used for supervisory data transmission between mobile station and the base station during a call.
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