GSM frame structure uses slots, frames, multi-frames, super-frames and hyper-frames to give the required structure and timing to the data transmitted. This frame structure of GSM is used to enable the orderly passage of information and establishes for the predetermined use of timeslots. The basic GSM frame defines the structure upon which all the timing and structure of GSM messaging and signalling is based.

The fundamental unit of time is called a burst period and it lasts for approximately 0.577 ms (15/26 ms). Eight of these burst periods are grouped made a TDMA frame. One physical channel is one burst period allocated in each TDMA frame.

In GSM, the frames are grouped together to form multi frames and inthis way it is possible to establish a time schedule for their operation and the network can be synchronized.

According to the above format of frame structure. Frame structure is the division of defined length of digital information into different information parts.

1 time slot has 576.92 μsec or 0.577 msec burst period. And the group of 8 timeslots makes 1 frame.

According to the figure, the structure of normal burst is exactly defined and follows a common format. It contains data that provides a number of different functions:

3 tail bits: These bits at the start of the GSM burst give time for the transmitter to ramp up its power.

57 data bits: This block of data is used to carry information, and most often contains the digitised voice data although on occasions it may be replaced with signalling information in the form of Fast Associated Control Channel (FACCH). This type of data is indicated by the flag that follows the data field.

1 bit flag: This bit indicates the type of data in the data field of GSM burst.

26 bits training sequence: It is used as timing reference and for equalization. There are total 8 different bit sequences that may be used, each 26 bits long.

1 bit flag: Again this bit indicates the type of data in the data field of GSM burst.

57 data bits: Again this block of data within the GSM burst is used for carrying data.

3 tail bits: These bits are used to enable the transmitter power to ramp down.

8.25 bits guard time: At the end of GSM burst, there is guard time period which is introduced to prevent transmitted bursts from different mobiles overlapping.

Multiframes: These are the frames that are linked together to perform specific functions. Multiframes on the GSM system use established schedules for specific purposes, such as coordinating with frequency hopping patterns and also used in the GSM system include 26 traffic multi-frame, 51 control multi-frame, super-frame and hyper-frame.

Traffic multiframe structure: 26 traffic multiframe structure is used to send information on the traffic channel. It is used to combine user data (traffic), slow control signalling( SACCH) and idle time period. The time interval of a 26 frame traffic multiframe is 6 blocks of speechcoder data (120msec).

Control Multiframe structure: the 51 control multiframe structure is used to send information on the control channel. 51 frame control multiframe is sub divided into logical channels that include the frequency correction burst, the broadcast channel (BCCH), the paging and access grant channel (PAGCH), and the stand alone dedicated control channel (SDCCH). PAGCH is logically sub divided into PCH and AGCH.

Superframe: A superframe is multiframe sequence that combines the period of 51 multiframe with 26 multiframes 9 6.12 seconds). The use of superframe time period allows all mobile devices to scan all the different time frame types at least once.

Hyper-frame:  Itis multiframe sequence that is composed of 2048 superframes nand is the largest time interval in the GSM system. Every time slot during a hyperframe has a sequential number that is composed of a frame number and time slot number. This counter allows the hyperframe to synchronize frequency hopping sequence, encryption processes for voice privacy of subscriber’s conversations.