ntrol, error detection and multiplexing of the several transport connections on one network connection. Layer 5: Session Layer This layer enables synchronisation between two applications. Both nodes use layer 5 for coordination of the communication between them. This means that it does the application identification but not the management of the application. Layer 6: Presentation Layer This layer basically defines and prepares the data before it is sent to the application layer. Layer 6 presents the data to both sides of the network in the same way. This layer is capable of identifying the type of the data, and changes the length by compression or de-compression depending upon the need, before sending it to the application layer. OVERVIEW OF MOBILE NETWORKS Layer 7: Application Layer The application layer itself does not contain any application but acts as an interface between the communication process (layers 1-6) and the application itself. Layer 1 is medium- dependent while layer 7 is application-dependent. 1.4 SECOND-GENERATION MOBILE NETWORKS (GSM) Of all the second-generation mobile systems, GSM is the most widely used. In this section we will briefly go through its important constituents. It is divided into three major parts (base station subsystem, network subsystem, and network management system) as shown in Figure 1.4. 1.4.1 Base Station Subsystem (BSS) The BSS consists of the base transreceiver station (BTS), base station controller (BSC) and transcoder sub-multiplexer (TCSM). The latter is sometimes physically located at the MSC. Hence the BSC also has three standardised interfaces to the fixed network, namely Abis, A and X.25. Base Transceiver Station (BTS) This manages the interface between the network and the mobile station. Hence, it performs the important function of acting as a hub for the whole of the network infrastructure. Mobile terminals are linked to the BTS through the air-interface. Transmission and reception at the BTS with the mobile is done via omnidirectional or directional antennas 