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Explain traffic channel, control channel, broadcast channel and common control channel in GSM.

Mumbai university > Comp > SEM 8 > Mobile Communication

Marks: 10M

Year: Revised 2012

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.

C. Common Control Channel (CCCH): CCCh consists of three different channels-Paging Channel (PCH) which is forward link channel, Random Access Channel (RACH)which is reverse link channel and Access Grant Channel (AGCH) which is forward link channel. CCCH are most commonly used control channels and are used to page specific subscribers, assign signaling channels to specific users and receive mobile requests for service.

  • 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 acknowledgement from the mobile unit on the RACH. PCH may be used to provide cell broadcast ASII text message to all subscribers as part of the SMS feature of GSM.

  • Random Access Channel (RACH): It is a reverse link channel used by subscriber to acknowledge the page from PCH, and also usedby mobiles to originate a call. The RACH uses ALOHA access scheme. At the BTS every frame will accept RACH transmission from mobiles during TS0. In establishing service the GSM base station must respond to the RACH transmission by allocating a channel and assigning a stand alone dedicated control channel for signaling during a call. This connection is confirmed by the base station over AGCH.

  • Access Grant Channel (AGCH): The AGCH is used by the base station to provide foeward link communication to the mobile and carries data which instructs the mobile to operate in aparticular physical channel with particular dedicated control channel. It is the final CCCH message sent by the base station before a subscriber is moved off the control channel. It is used by the base station to respond to a RACH sent by a mobile station in a previous CCCH frame.

D. Broadcast Channel (BCH): The broadcast channel operates on forward link of a specific ARFCN within each cell and transmits data only in the first time slot of a certain GSM frames. The BCH provides synchronization for all mobiles in neighbouring cells so that received power and MAHO decisions may be made by out of cell user. BCH is defined by three separate channels which are given access TS0 during various trains of the 51 frame sequence.

  • Broadcaste 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. 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 contain BCCH data.

  • Frequency Correction Chennel (FCCH): The FCCH is special data burst which occupies TS0 for the very first GSM frame and is repeated every 10 frames within a control channel multiframe. The FCCH allows each subscriber unit to synchrinize its internal frequency standard to the exact frequency of the base station. When mobile is switched on FCCH helps mobile to get tuned to the appropriate network i.e MSC.

  • Synchroization Channel (SCH): SCH is broadcast in TS0 of the frame immediately following the FCCH frame and is used to identify the serving base station while allowing each mobile to frame synchronize with the base station. The frame number is sent with the base station identity code (BSIC) during the SCH burst. The base station issues time advancement commands to the mobile stations over SCH. SCH is transmitted once every ten frames within the control channel multiframe.

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