MULTIFRAME

MULTIFRAMES

Multiframes are used to denote a pattern of timeslots that repeat on a cyclic basis for example, a traffic channel multiframes sequence repeats every 26 timeslots whereas a control channel multiframe sequence repeats every 51 timeslots. As each of these timeslots represent a physical channel or single timeslot in a TDMA frame it can also be said that these sequence repeat every 26 and 51 TDMA frames respectively.

                                          Multiframes allow one timeslot allocation(physical channel) in aTDMA frame to be used for a variety of purpose(logical channel). This is achieved by multiplexing several logical channels onto a single physical timeslot

Traffic Channel Multiframes 

A traffic channel multiframes repeats cyclically every 26 TDMA frames . Its structure is shown in the illustration below

• The TCH multiframes consists of 26 timeslots.
• This multiframes maps the following logical channels *TCH    *SAACH    *FACCH
• TCH multiframe structure

     T=TCH, S=SACCH, I=IDLE
FACCH is not allocated slots in the multiframe. It steals TCH slots when required -indicated by stealing flags in the normal burst.

Control Channel Multiframes

Control channels are used by the MS to establish a communication with the network in the idle mode and also in initiating calls to enter the dedicated mode. Timeslot 0 is grouped into structure of 51 frames referred to as  control channel multiframes. The control channel are grouped as Broadcast control channels(BCCH) common control channels (CCCH) and Dedicated control channels(DCCH).

• The control channel multiframe is formed of 51 timeslot.
• CCH multiframe maps the following logical channels:-

    Downlink:- FCCH ,SCH, BCCH, CCCH(combination of PCH and AGCH)

    Uplink :- RACH

BCH Channel Configuration

Each cell has one carrier designated as a BCH carrier . The BCH carrier has all 8 time slots continuously on , either with traffic or dummy bursts.Timeslot 0 of the BCH carrier contains logical control channel.

        The BCCH occurs once in the 51-frame cycle, and contains information that is packed in a block of 4-frames . The information on the BCCH is known as system information and include network identities, call parameters, cell channels and option configuration. A GSM mobile reads the BCCH when it first camps on a cell and at intervals thereafter to detect any change in parameter settings.

             On the BCH carrier there are 3 or 9 blocks of the common control channel. Each block comprises 4-TDMA frames and contains one signalling message. The common control channel blocks are further subdivided into the Access Grant channel(AGCH) and paging channel(PCH).

• Each cell has a designated BCH carrier.
• All BCH timeslots transmit continuously on full power.
• TS0 contains logical control channel.
• TS1-7 optionally carry traffic
• BCCH block occurs once each 51-frame multiframe.
• Each block comprises 4 frames carrying 1 message.

•    On the downlink , CCCH consists of paging (PCH) and AcCess Grant channel(AGCH) messages.
• A combined multiframe has only 3 CCCH blocks to allow for SDCCH and SACCH.
• A non-combined multiframe has 9  CCCH blocks on timeslot 0.

         

To save battery power , a mobile does not monitor all the paging channels in a multiframe : it only monitors the paging channel belonging to its paging group.Each paging channel in a multiframe has a different group number.