Frame Hierarchy: |
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One of the problems with TDMA systems is the difficulty of
identifying on which timeslot and on which carrier a logical channel is present at any
given time. To resolve this problem, GSM introduces the frame hierarchy. The lowest level
of the frame hierarchy is one TDMA-frame that consists of 8 timeslots. The duration of the
TDMA-frame is 8 x 577 µs = 4.615 ms. The figure underneath illustrates the frame hierarchy that is used on all timeslots and all carriers of the GSM air interface. Figure 1: The Frame Hierarchy Certain restrictions apply to the different types of multiframes: The 26-multiframe consists of 26 consecutive TDMA-frames and is only applicable for circuit-switched traffic channels. The configuration of the 26-multiframe is highlighted below (FN = Frame Number). Figure 2: The 26-Multiframe The 52-multiframe consists of 52 consecutive TDMA-frames and is only applicable for packet data channels (PDCH). The 52-multiframe can be regarded as two consecutive 26-multiframes. Please move your mouse over the different radio blocks to view the allowed channel allocations.
Figure 3: The 52-Multiframe
The 51-multiframe consists of 26
consecutive TDMA-frames and can only be used for circuit-switched signaling channels like
the SDCCH or the CCCHs. Therefore, there may never be a
timeslot simultaneously providing circuit-switched control and traffic channels Figure 4: Example of 51-Multiframe Configuration The next level of the frame hierarchy is the superframe with a duration of 6.12 s. One superframe combines 26 x 51-multiframes, 51 x 26-multiframes or 25.5 x 52-multiframes. Finally, there is the hyperframe that consists of 2048 superframes. Note that GSM introduces the hyperframe level only to achieve a longer repetition rate of the frame number because of the ciphering procedure Each mobile station determines the current frame number via the SCH that is broadcast on timeslot 0 of the BCCH-carrier. [2GTS 05.01 / 2GTS 05.02] |