GSM frame structure GSM Tutorial - RF Wireless World
The GSM standard  specifies a TDMA frame as a combination of 8 time slots. Each time slot has a duration of 3/5200 seconds (about 0.577 ms) and a time slot number (TN) from 0 to 7. The transmission timing of a burst within a time slot is defined in terms of the bit number. GSM frames use GMSK modulation, where one symbol is equivalent to one bit. The bit number (BN) refers to a particular bit period within a time slot. The bit with the lowest bit number is transmitted first.
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BN0 is the first bit period and BN156 is the last quarter-bit period. This figure shows time frames, time slots, and bursts for a GSM system . A TDMA contains eight time slots with each timeslot separated by a guard period. Available burst type are: normal burst (NB), frequency correction burst (FB), synchronization burst (SB), access burst (AB), or dummy burst .
The different burst types and the guard period are described in these next sections. The normal burst consists of these bit fields and can appear in uplink or downlink frames. Based on the specified training sequence code (TSC), the training sequence field contains one of eight possible training sequences. Bit Number Length Of Field Contents Of Field ___________ _____________ ________________________ "0 - 2" "tail bits" "3 - 60" "encrypted bits" "61 - 86" "training sequence bits" "87 - 144" "encrypted bits" "145 - 147" "tail bits" "148 - 156" "guard period (bits)" Bit Number Length Of Field Contents Of Field __________ _____________ ______________________ "0 - 7" "extended tail bits" "8 - 48" "synch.
sequence bits" "49 - 84" "encrypted bits" "85 - 87" "tail bits" "88 - 156" "guard period (bits)" The frequency correction burst consists of these bit fields and can appear in downlink frames only. Modulating all zeros with the GMSK modulator results in a constant phase rotation of -90 degrees for each symbol duration. Therefore, this burst generates an unmodulated carrier with a positive frequency offset of 1625/24 k Hz. Bit Number Length Of Field Contents Of Field ___________ _____________ _________________________________ "0 - 2" "tail bits" "3 - 41" "encrypted bits" "42 - 105" "extended training sequence bits" "106 - 144" "encrypted bits" "145 - 147" "tail bits" "148 - 156" "guard period (bits)" The GSM standard, , requires mobile stations to attenuate their transmission during the period between bursts. The ramp-up and ramp-down of the signal power level occurs during the guard periods. The useful part of a burst starts half way through the bit number 0.
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The useful part ends halfway through BN87 for ABs and BN147 for NBs, FBs, SBs,and dummy bursts. The plot shows 8 bursts in the frame, with guard periods between each burst. Изображения товара, включая цвет, могут отличаться от реального внешнего вида.
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Данное описание и количество товара не является публичной офертой. This page on GSM tutorial covers GSM frame structure including concept of slot, frame, multiframe, superframe and hyperframe.
It covers both 51 frame multiframe and 26 frame multiframe structure of GSM. Here each RF carrier frequency is shared between 8 users hence in GSM system, the basic radio resource is a time slot with duration of about 577 microsec. In GSM frequency band of 25 MHz is divided into 200 KHz of smaller bands, each carry one RF carrier, this gives 125 carriers. As mentioned each time slot has 15/26 or 0.577ms of time duration. As one carrier is used as guard channel between GSM and other frequency bands 124 carriers are useful RF channels. This time slot carries 156.25 bits which leads to bit rate of 270.833 kbps.
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This is explained below in TDMA gsm frame structure. For E-GSM number of ARFCNs are 174, for DCS1800 ARFNCs are 374. The GSM frame structure is designated as hyperframe, superframe, multiframe and frame. The minimum unit being frame (or TDMA frame) is made of 8 time slots. Each GSM superframe composed of multiframes (either 26 or 51 as described below). Each GSM multiframe composed of frames (either 51 or 26 based on multiframe type). Hence there will be total of 2715648 TDMA frames available in GSM and the same cycle continues. Fig.2 GSM Frame Structure As shown in the figure 2 below, there are two varients to multiframe structure. 26 frame multiframe - Called traffic multiframe,composed of 26 bursts in a duration of 120ms, out of these 24 are used for traffic, one for SACCH and one is not used. 51 frame multiframe- Called control multiframe,composed of 51 bursts in a duration of 235.4 ms. This type of multiframe is divided into logical channels. Always occur at beacon frequency in time slot 0, it may also take up other time slots if required by system for example 2,4,6. each ARFCN or each channel in GSM will have 8 time slots TS0 to TS7.
During network entry each GSM mobile phone is allocated one slot in downlink and one slot in uplink. Here in the figure GSM Mobile is allocated 890.2 MHz in the uplink and 935.2 MHz in the downlink.