Wednesday, May 22, 2019

What is means by GSM BURST and its types


As we know the GSM network uses the time slot to transmit information and Burst is a type of information that contains in particular time slots of GSM network. Bursts are designed to fit in these time slots to transmit information. GSM consist of five bursts which are
  1. Normal Burst 
  2. Synchronization Burst 
  3. Frequency Correction Burst 
  4. Access Burst
  5. Dummy Burst

1. Normal Burst

Gsm Normal Burst

Typical burst contains 114 bit of valuable information and divided into two groups 57 bits each. These bits contain signaling and user data.
Stealing flag helps to indicate special events like Handovers etc. It also helps to verify that whether the block contains data or is stolen.
Tail bits found at the start and end of data. It is used as a guard time. Tail bit are three (3) in number and set to zero and placed at the beginning and at the end of a burst.
The Guard Period (GP) helps to avoid overlapping with other bursts that may carry some additional information. The length of the Guard Period is 8.25 bits.
The training sequence has a length of 26 bits and is used to synchronizes the receivers, hence this effects multipath propagation. The Training sequence helps in adapting the receiver parameters to the current path propagation.

2. Synchronization Burst:

GSM Synchronization Burst

As its name suggests, this burst is used to synchronize Mobile Station
and BTS using a synchronization channel (SCH). The function of both Tail and Guard bit the same as to mention above in Normal Burst.
Coded Data tells about TDMA frame number.
The training sequence tells about the base station number.
Mobile Station at first demodulates synchronization burst. That's why the length is 64 bits. It helps to avoid Larger multipath delay spreads.

3. Frequency Correction Burst

Frequency Correction Burst

The most simple format of all the bursts is used for the frequency correction burst, which is transmitted only in the frequency correction channel (FCCH).
All 148 bits are coded with zeros and frequency, which is a pure sine wave (GMSK modulator Produce sine wave) is transmitted that enables mobile to synchronize its frequency with the master frequency.
Frequency correction burst appears on frequency correction channel.

4. Random Access Burst:

Random Access Burst
Random Access Burst has a unique format as compared to others because of its special task. When first switched on, A mobile station (MS) uses the access burst for initial access to a BTS. The first job is to synchronize itself with BTS in frequency and time using synchronization burst.
Burst has a length of 88 bits plus 68.25 guard bits. The access burst length is set large because to make it possible, that access burst appears at the BTS. It can be increased more by adding a guard band of 68.25.

5. Dummy Burst:

GSM Dummy Burst
Dummy burst contains no user data information. Dummy is used to fill out idle time slots on the channel when a user is not sending any data. Dummy burst sent by the base station on the base channel. When a mobile station is turned on, it easily discovers the base channel. The pseudo-random bit sequence is used to avoid accidental confusion with frequency correction burst.

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