able 1.1 GSM frame hierarchy multiframe, (51) = 51 TDMA frames multiframe (26) = 26 TDMA frames superframe, 51 multiframes superframe, (26) = multiframes hyperframe = 2028 superframes. Each frame is of 577 us and carries a communication channel in which a message element called as packet is transmitted periodically. The physical channel of the TDMA frames carry logical channels that transport user data and signalling information. Thus, there are traffic channels and signalling channels in the TDMA frame. The traffic channels (TCH) transmit either data or voice signals and can be HR or FR. HR provides a bit rate for coded speech of 6.5 kbps and FR has double that capacity, i.e. 13 kbps. The signalling channels may contain the information such as broadcast channels (e.g. BCCH, SCH, FCH); common control channels (e.g. AGCH, PCH, RACH), dedicated channels (e.g. DCCH, SDCCH), and associated channels (e.g. FACCH, SACCH, SDCCH). Abis Interface The Abis interface is the interface between the BTS and the BSC. It is a PCM interface, i.e. it is defined by the 2 Mbps PCM link. Thus it has a transmission rate of 2.048 Mbps, having 32 channels of 64 kbps each. As the traffic channel is 13 kbps on the air interface, while Abis is 64 kbps, hence multiplexing and transcoding do conversion from 64 kbps on the Abis interface to 13 kbps on the air interface. Four channels are multiplexed into one PCM channel while traffic channels are transcoded up to 64 kbps. TCSM are usually located at the MSC to save the transmission costs but can also be located at the BSC site. LAPD signalling, along with TRX management (TRXM), common channel management (CCM), radio link management (RLM) and dedicated channel management (DCM) form a part of this interface. A Interface The A interface is present between the TCSM and MSC or physically between the MSC and BSC (generally, TCSM are physically located at the MSC). This interface consists of one or more PCM links each having a capacity of 2048 Mbps. There are two parts of the A-interface - one from th