• Radio subsystem As the name implies, the radio subsystem (RSS) comprises all radio specific entities, i.e., the mobile stations (MS) and the base station subsystem (BSS).

• A GSM network is composed of several functional entities, whose functions and interfaces are specified.The GSM network can be divided into three broad parts.

1. The Mobile Station is carried by the subscriber.
2. The Base Station Subsystem controls the radio link with the Mobile Station.
3. The Network Subsystem.

• 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 services Switching Center across the A interface.

Mobile Station:

• The mobile station (MS) consists of the mobile equipment (the terminal) and a smart card called the Subscriber Identity Module (SIM).

Base Station Subsystem:

• The Base Station Subsystem is composed of two parts:
• The Base Transceiver Station (BTS) and

• The Base Station Controller (BSC).

• These communicate across the standardized Abis interface, allowing (as in the rest of the system) operation between components made by different suppliers.

• The Base Transceiver Station houses the radio transceivers that define a cell and handles the radio-link protocols with the Mobile Station. In a large urban area, there will potentially be a large number of BTSs deployed, thus the requirements for a BTS are ruggedness, reliability, portability, and minimum cost.

• The Base Station Controller manages the radio resources for one or more BTSs. It handles radio-channel setup, frequency hopping, and handovers, as described below. The BSC is the connection between the mobile station and the Mobile service Switching Center (MSC).