GSM Logical & Physical Channels

- a tutorial, description, overview of GSM channels including transport and logical channels, SACCH, SDCCH, FACCH, etc.

GSM primer includes:
GSM introduction     Network architecture     Network interfaces     RF interface / slot & burst     GSM frames     Power classes & control     Channels     Audio codecs / vocoders     Handover    

GSM uses a variety of 'channels' in which the data is carried. In GSM, these channels are separated into physical channels and logical channels. The Physical channels are determined by the timeslot, whereas the logical channels are determined by the information carried within the physical channel. It can be further summarised by saying that several recurring timeslots on a carrier constitute a physical channel. These are then used by different logical channels to transfer information. These channels may either be used for user data (payload) or signalling to enable the system to operate correctly.

Common and dedicated channels

The channels may also be divided into common and dedicated channels. The forward common channels are used for paging to inform a mobile of an incoming call, responding to channel requests, and broadcasting bulletin board information. The return common channel is a random access channel used by the mobile to request channel resources before timing information is conveyed by the BSS.

The dedicated channels are of two main types: those used for signalling, and those used for traffic. The signalling channels are used for maintenance of the call and for enabling call set up, providing facilities such as handover when the call is in progress, and finally terminating the call. The traffic channels handle the actual payload.

The following logical channels are defined in GSM:

  • TCHf   Full rate traffic channel.
  • TCH h   Half rate traffic channel.
  • BCCH   Broadcast Network information, e.g. for describing the current control channel structure. The BCCH is a point-to-multipoint channel (BSS-to-MS).
  • SCH   Synchronisation of the MSs.
  • FCHMS   frequency correction.
  • AGCH   Acknowledge channel requests from MS and allocate a SDCCH.
  • PCHMS   terminating call announcement.
  • RACHMS   access requests, response to call announcement, location update, etc.
  • FACCHt   For time critical signalling over the TCH (e.g. for handover signalling). Traffic burst is stolen for a full signalling burst.
  • SACCHt   TCH in-band signalling, e.g. for link monitoring.
  • SDCCH   For signalling exchanges, e.g. during call setup, registration / location updates.
  • FACCHs   FACCH for the SDCCH. The SDCCH burst is stolen for a full signalling burst. Function not clear in the present version of GSM (could be used for e.g. handover of an eight-rate channel, i.e. using a "SDCCH-like" channel for other purposes than signalling).
  • SACCHs   SDCCH in-band signalling, e.g. for link monitoring.
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