390 Chapter 14
Real Time TDMA Formats
Enhanced Observed Time Difference (E-OTD)
26 Frame Multiframe
The most basic of GSM multiframes is the traffic channel multiframe. This is made up of 26 frames and has
a time interval of 120 msec. Each frame of the 26-frame multiframe has an eight time slot TDMA frame
(4.62 msec). The first 12 frames are used to transmit traffic data. Then one frame is used by the Slow
Associated Control Channel (SACCH) which provides channel maintenance. Then another 12 frames of
traffic data followed by an IDLE frame where not data is sent. This IDLE frame allows the mobile station to
perform other tasks such as measuring the signal strength of other neighboring cells. Refer to Figure 14-2 on
page 391.
51 Frame Multiframe
The 51-frame multiframe is comprised of 51 TDMA frames and has a duration of 235.4 ms. This multiframe
is used for control channels. Control channels always occur on the master frequency in the downlink
direction from the BTS to the mobile station. Different formats exist for the makeup of the 51-frame
multiframe but for E-OTD, type IV and V are used. The common features of these channels are the
frequency correction burst (FCCH), the synchronization burst (SCH), and the broadcast control channel
(BCCH). These different channel types allow the mobile station to synchronize to the base station in the
frequency domain and the time domain and provide information about the current broadcast channel. Refer
to Figure 14-2 on page 391.
Superframe
A superframe is composed of 51 26-frame multiframe or 26 51-frame multiframes which is equal to 1326
frames which is the lowest common denominator and is the least number of frames that can absorb the
contents of all the 26 or 51 multiframes and finish with no spare timeslots. Refer to Figure 14-2 on page 391.
Hyperframe
The time interval of the hyperframe is over three hours long which is much longer than the typical telephone
connection. The encryption process used is not likely to repeat itself during a call of reasonable length and
the privacy is not compromised. The hyperframe is made up of 2048 superframes. Refer to Figure 14-2 on
page 391.
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