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# Gsm physical and-logical_channels

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It explains the phsical and logical channels of the GSM system

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### Gsm physical and-logical_channels

1. 1. GSM: PHYSICAL & LOGICAL CHANNELS – AN OVERVIEW Prepared by Learntelecom.com
2. 2. www.learntelecom.com 1. GSM: PHYSICAL AND LOGICAL CHANNELS GSM uses a mix of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA). FDMA parts involves the division by frequency of the 25 MHz bandwidth in to 124 carrier frequencies (Also called ARFCN) spaced 200 KHz for GSM-900. For GSM-1800 frequency spectrum of 75 MHz bandwidth is divided in to 374 carrier frequencies spaced 200 KHz. TDMA further divides each carrier frequencies in to 8 time slots such that each carrier frequency is shared by 8 users. So in GSM, the basic radio resource is a time slot with duration of 577 µs. 8 Time slots of 577 µs constitutes a 4.615 ms TDMA Frame. GSM uses Gaussian Minimum Shift Keying (GMSK) modulation scheme to transmit information (data and signalling) over Air Interface. GSM uses number of channels to carry data over Air Interface, these channels are broadly divided in to following two categories:  Physical Channels  Logical Channels 2. PHYSICAL CHANNELS A physical channel is determined by the carrier frequency or a number of carrier frequencies with defined hopping sequence and the Time Slot number. 8 Time Slots (1 Time Slot = 1 Physical Channel) of 577 µs constitutes a 4.615 ms TDMA Frame. In GSM standard data on a time slot transmitted in bursts, so time slot is often expressed in BP (Burst Period). 1 BP represents 1 TS. TDMA frame (4.615 ms of 8 TS) further structured in to multiframes. There are two types of multiframes in the system:  26 TDMA Multiframe: Consists 26 TDMA frames with duration of 120 ms and used to carry the Logical Channels TCH, SACCH, FACCH etc.  51 TDMA Multiframe: Consists 51 TDMA frames with duration of 234.5 ms and used to carry the Logical Channels FCCH, SCH, BCCH, CCCH, SDCCH, SACCH etc. These multiframes further structured in to Superframe and Hyperframe. SUPERFRAME: Superframe consists of 51*26 TDMA frames with duration of 6.12 sec. This is corresponding to the smallest cycle for which the organisation of all channels is repeated. HYPERFRAME: Hyperframe consists 2048 superframes (2048*51*26 TDMA frames) with duration of 3 hrs, 28 min, 53 sec and 760 ms. It is in particular smallest cycle for frequency hopping, cyphering.
3. 3. www.learntelecom.com The frame hierarchy is used for synchronization between BTS and MS. Multiframes in GSM 3. LOGICAL CHANNELS Logical Channels are determined by the information carried within the physical channel. Logical channels used to carry data and signalling information. Different logical channels are mapped in either direction on physical channels. Logical channels divided in to following two categories:  Traffic Channels  Signalling Channels 3.1. TRAFFIC CHANNELS In GSM system two types of traffic channels used:  Full Rate Traffic Channels (TCHF): This channel carries information at rate of 22.8 Kbps.  Half Rate Traffic Channels (TCHH): This channels carries information at rate of 11.4 Kbps. 3.2. SIGNALLING CHANNELS Signalling channel carries control information to enable the system to operate correctly. There are three