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Introduction to GSM, 2nd ed.

Introduction to GSM Book

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GSM Frame Structure Diagram

Frame structure is the division of defined length of digital information into different fields (information parts). A GSM frame is 4.615 msec and it is composed of 8 time slots (numbered 0 through 7). During voice communication, one user is typically assigned to each time slot within a frame. The GSM system also combines frames to form Multiframes.

Multiframes are frames that are grouped or linked together to perform specific functions. Multiframes on the GSM system use established schedules for specific purposes, such as coordinating with frequency hopping patterns. Multiframes used in the GSM system include the 26 traffic multiframe, 51 control multiframe, superframe, and hyperframe.

Traffic Multiframe Structures - The 26 traffic multiframe structure is used to send information on the traffic channel. The 26 traffic multiframe structure is used to combine user data (traffic), slow control signaling (SACCH), and idle time period. The idle time period allows a mobile device to perform other necessary operations such as monitoring the radio signal strength level of a beacon channel from other cells. The time interval of a 26 frame traffic multiframe is 6 blocks of speech coder data (120 msec).

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

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

Hyperframe - A hyperframe is a multiframe sequence that is composed of 2048 superframes, and is the largest time interval in the GSM system (3 hours, 28 minutes, 53 seconds). Every time slot during a hyperframe has a sequential number (represented by an 11 bit counter) that is composed of a frame number and a time slot number. This counter allows the hyperframe to synchronize frequency hopping sequence, encryption processes for voice privacy of subscribers' conversations.

This figure shows the different types of GSM frame and multiframe structures. This diagram shows that a single GSM frame is composed of 8 time slots. When a radio channel is used to provide a control channel, time slot 0 and the other time slots are used for traffic channels. 51 frames are grouped together to form control multiframes (for the control channel). 26 frames are grouped together to form traffic multiframes (for the traffic channels). Superframes are the composition of 26 control multiframes or 51 traffic multiframes to provide a common time period of 6.12 seconds. 2,048 superframes are grouped together to form a hyperframe. A hyperframe has the longest time period in the GSM system of 3 hours, 28 minutes, and 53 seconds.

 

Introduction to GSM Book

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Introduction to GSM, 2nd ed.

This book explains the basic components, technologies used, and operation of GSM systems. You will discover why mobile telephone service providers have upgraded from 1st generation analog systems to more efficient and feature rich 2nd generation GSM systems. You will also discover how 2nd generation systems are gradually evolving into 3rd generation broadband multimedia systems.

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