OR

With neat labeled diagram describe GSM frame architecture.

Subject: Mobile Computing

Difficulty: Medium

Marks: 4 Marks

Frame structure in GSM:

• The length of GSM frame in a frequency channel is 4.615 ms.

• The frame is divided into 8 bursts of length of 0.577ms

• The timeslots in the uplink are derived from downlink by a time delay of 3 time slots

• This arrangement prevents an MS from transmitting and receiving at the same time

• However, due to propagation delay (when MS is far away from BTS) the 3 TS delay cannot be maintained accurately.

GSM Burst structure:

• Each burst contains 148 bits (0.546ms) followed by 0.031ms guard time (8.25bits)

• The burst begins with 3 head bits and 3 tail bits (logical Zeroes)

• Two groups are data bits are separated by an equalizer Training sequence of 26 bits

• Each data group consists of 57 bit information bits and 1 flag that indicate whether the information bits are for user speech/ data or signaling.