Structure of Multi-Frame

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There are two different kind of Multi-Frames in GSM. One is called 'Control Multiframe' and the other one is called 'Traffic Multi Frame'. (Do you remember where the multiframe is located in overall GSM frame structure ? Let's refresh our memory. Slot --> Frame --> MultiFrame (Control Multiframe, Traffic MultiFrame) --> Super Frame --> HyperFrame. Do this questions and answer by yourself whenever you have chance until your brain automatically pops up a frame structure diagram as soon as you see some key words related to GSM frame).

Following is one example showing a control multiframe. As I said, even a single multi frame has 51 x 8 slots, so it is very hard to visualize the full details on a page. You will see a better way to visualize this structure at later section, for now just try to get some 'sense(feeling)' of how a multiframe looks like. One thing worth noticing would be the first slot in each frame seems to be used as some control channel. In this diagram, the second slot in each frame is being used as a control channel but it is not always the case.

Following is the common example of a Traffic Multiframe. This illustration shows 26-multiframe structure for TCH/FR and TCH/ER. This example shows 2 users using full rate voice traffic channels. (One user uses the slot 2 at every frame and the other user use the slot 4 at every frame).
At the center of the 26-frame traffic channel multiframe (i.e, Frame 12) is the Slow Associated Control Channel (SACCH) which carries link control information to and from the MS–BTS. At the last frame is 1 idle frame. All the remaining frame are allocated for Traffic. There is no dedicated FACCH frame or slots. FACCH steals TCH whenever it needs.




Following is an example of control channel multiframe structure for Beacon Channel (Base Control Channel) which is made up of FCCH, SCH, BCCH, CCCH. The illustration below is for Downlink multiframe structure. In Uplink base control channel case, every frame is for single channel - RACH. Some key facts about DL Base Control Channel are
  • There are five FCCH equally spaced within the 51 multiframe.
  • Each FCCH is followed by a SCH, meaning that there are five SCH as well.
  • Four frames (Frame 2~5) are allocated for BCCH.
  • The last frame (Frame 50) is allocated for Idle.
  • All the remaining Frames are allocated for CCCH(e.g, PCH or AGCH).





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