GSM Burst

The Below diagram illustrates a GSM burst. It consists of several different elements.

GSM Burst and TDMA Frame

These elements are as below:

Info

This is the area in which the speech, data or control information is held.

Guard Period

The BTS and MS can only receive the burst and decode it, if it is received within the timeslot designated for it. The timing, therefore, must be extremely accurate, but the structure does allow for a small margin of error by incorporating a ‘guard period’ as shown in the diagram. To be precise, the timeslot is 0.577 ms long, whereas the burst is only 0.546 ms long, therefore there is a time difference of 0.031 ms to enable the burst to hit the timeslot.

Stealing Flags

These two bits are set when a traffic channel burst has been ‘‘stolen” by a FACCH (the Fast Associated Control Channel). One bit set indicates that half of the block has been stolen.

Training Sequence

This is used by the receiver’s equalizer as it estimates the transfer characteristic of the physical path between the BTS and the MS. The training sequence is 26 bits long.

Tail Bits

These are used to indicate the beginning and end of the burst.

Burst Types

The diagram below shows the five types of burst employed in the GSM air interface. All bursts, of whatever type, have to be timed so that they are received within the appropriate timeslot of the TDMA frame.

GSM Burst Types

The burst is the sequence of bits transmitted by the BTS or MS, the timeslot is the discrete period of real time within which it must arrive in order to be correctly decoded by the receiver:

Normal Burst

The normal burst carries traffic channels and all types of control channels.

Frequency Correction Burst

This burst carries FCCH downlink to correct the frequency of the MS’s local oscillator, effectively locking it to that of the BTS.

Synchronization Burst

So called because its function is to carry SCH downlink, synchronizing the timing of the MS to that of the BTS.

Dummy Burst

Used when there is no information to be carried on the unused timeslots of the BCCH Carrier (downlink only).

Access Burst

This burst is of much shorter duration than the other types. The increased guard period is necessary because the timing of its transmission is unknown. When this burst is transmitted, the BTS does not know the location of the MS and therefore the timing of the message from the MS can not be accurately accounted for. (The Access Burst is uplink only.)