Radio Interface

The most intresting interface in a GSM system is Um ,the radio interface ,as it coprises many mechanisms presented for multiplexing and media access.GSM implements SDMA using cells with BTS and assigns an MS to BTS .Futhermore ,FDD is used to seperate downlink and uplink as described.Medium access combines TDMA and FDMA .In GSM 900,124 channels ,each 200 khz wide are used for FDMA ,whereas GSM 1800 uses ,374 channels .Due to technical reasons ,channels are reserved for organisational data ; the reamaining 90 are used for customers .Each BTS then manages a single channel for organisational data and e.g ,upto 10 channels for used data .The following example based on GSM 900 systems ,but GSM works in a similar way at 1800 and 1900 Mhz.
Each of 248 channels is additionally seperated intime via a GSM TDMA frame ,i.e each 200 khz carrier is subdivided into frames that are repeated continously .The duration of a frame is 4.615 ms .A frame is again subdivided into 8 GSM time slots ,where each slot represnets a physcial TDM channel and lasts for 577 ms .Each TDM channel occupies the 200 khz carrier for 577 micro sec every 4.615 ms.
Data is transmitted in small portions ,called bursts , sometimes also called normal bursts as used for data transmission inside time slot.The bursts only of 546.6 micro sec long and contains 148 bits.The reamaning 30.5 micro sec are used as guard spaces to avoid overlapping with other bursts.