1.Each user transmits a burst of data during the time slot assigned to it. These data bursts may have one of five specific formats. Normal bursts are used for TCH and DCCH transmissions on both the forward and reverse link. FCCH and SCH bursts are used in TS0 of specific frames to broadcast the frequency and time synchronization control messages on the forward link. The RACH burst is used by all mobiles to access service from any base station, and the dummy burst is used as filler information for unused time slots on the forward link.
2.A frame is one where no time slot is repeated. A frame contains eight time slots TS0 to TS7.
One time slot duration= 576.92 μsec
Number of bits transmitted during 1 time slot= 156.25 bits
Thus duration of one frame = 576.92 μsec ×8=4.6153 msec
Each bit duration= $\frac{576.92 μsec}{156.25bits}$=3.6922 μsec
Transmission rate= $\frac{56.25bits}{576.92 μsec}$ =270.833kbps
One user’s transmission rate = 270.833 kbps / 8=33.854 kbps
3.One single time slot in frame has eight fields as follows:
Tale bits: They are present at the start and end of every time slot to distinguish one time slot from another. It identifies beginning and end of the burst.
Coded data: This is the actual information to be transmitted. Out of 156.25 bits 114 are information bearing bits that are transmitted as two 57 bits sequences close to beginning and end of the time slot.
Stealing flag: This bit helps base station to distinguish whether the coded data is control or actual information. At the time of urgency control its are also sent using voice channel at that time stealing bit becomes ‘0’ indicating coded data carries control information.
Midamble: This consists of 26 bits training sequence. It helps the adaptive equalizer in mobile or base station receiver to analyze radio channel characteristics before coding. It gives the amount of fading the channel provides helping to decide which equalizer to be used.
Guard period: A guard time of 8.25 bits is provided at the end of every time slot to prevent overlapping with next time slot preventing actual information from getting tampered.
4.During a frame a GSM subscriber uses on one time slot to transmit and one time slot to receive and may use spare five time slots to measure signal strength on adjacent five base station as well as its own base station.
5.Each normal speech frames are grouped into larger structures called Multiframes which in turn are grouped into superframes and hyperframes.
6.One Multiframe contains 26 TDMA frames, one superframe contains 51 multiframes or 1326 TDMA frames and one hyperframe contains 2048 superframes or 2715648 TDMA frames. One hyperframe is transmitted after every 3.4815 hours.
7.Hyperframe is important in GSM because the encryption algorithms rely on particular frame number and sufficient security can only be obtained by using a large number of frames as provided by hyperframe.
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