GSM tutorial logical and physical channels
an overview or tutorial of the channels and their structure for the GSM
system
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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