e BTS to the TRAU where the transmitted payload is compressed, and one between the TRAU and MSC where all the data is uncompressed. The TRAU is OVERVIEW OF MOBILE NETWORKS typically located between the MSC and BSC and should be taken into consideration when dealing with this interface. SS7 signalling is present on the A interface. Signalling LAPD m LAPDm stands for 'modified link access protocol for D-channel'. This is a modified and optimised version of the LAPD signalling for the GSM air interface. The frame structure consists of 23 bytes and is present in three formats: A-, B- and Abis. Both the A- and B-formats are used for both the uplink and downlink, while the Abis is used only for downlinking. SS7 provides the basis of all the signalling traffic on all the NSS interfaces. SS7 (signalling system no. 7) is a signalling standard developed by the ITU. This provides the protocols by which the network elements in the mobile (and telephone) networks can exchange information. It is used between the BSC and MSC. It is capable of managing the signalling information in complex networks. It is used for call setup and call management, features such as roaming, authentication, call forwarding, etc. It is not allocated permanently and is required only for the call setup and call release function. It is also known as Common Channel Signalling No. 7 (CCS#7). X.25 links the BSC to the O&M centre. It is an ITU-developed signalling protocol, allowing communication between remote devices. This is a packet-switched data network protocol that allows both data and control information flow between the host and the network. By utilizing connection-oriented services, it makes sure that the packets are transferred in order. 1.5 THIRD-GENERATION MOBILE NETWORKS Third-generation mobile networks are designed for multimedia communication, thereby enhancing image and video quality, and increasing data rates within public and private networks. In the standardization forums, WCDMA technology emerged as the most widely adopted third-generat