The main part of Network Subsystem is the Mobile services Switching Center, which performs the switching of calls between the cell phone and other fixed or cell phone network users, as well as management of mobile services, such as authentication. The Mobile Station and the Base Station Subsystem communicate across the Um interface, also known as the air interface or radio link. The Base Station Subsystem communicates with the Mobile service Switching Center across the Abis interface. The Mobile Station (MS) consists of the physical equipment and a smart card called the Subscriber Identity Module (SIM). The SIM provides personal mobility, so that the user can have access to all of their subscribed services, no matter where they are or what device they are using. By inserting the SIM card into another GSM cellular phone, the user is able to receive calls at that phone, make calls from that phone, or receive other services they are subscribed to.
The Base Station Subsystem, as mentioned earlier, is composed of two parts, the BTS and the BSC. These communicate across the specified A ¬bis interface. The BTS houses the radio transceivers of the cell and handles the radio-link protocols with the Mobile Station. The BSC manages the radio resources, such as channel setup, frequency hopping, and handovers, for one or more BTS's. The BSC is the connection between the mobile and the MSC, by communicating with the MSC across the Signaling System Number 7 (SS7) interface.
The central component of the Network Subsystem, the MSC is like a telephony switch plus everything for a mobile subscriber: registration, authentication, handovers, call routing, and connection to fixed networks. Each switch handles dozens of cells. The MSC also provides the connection to the PSTN across the SS7 interface. The components of the switching system mentioned earlier work with the MSC. The HLR is a database of all users and their current location, one per network.
