Radio Access Network

The radio access network comprises of the base transceiver stations (BTSs) and the controller element, which is called the base station controller (BSC). The BTSs are basically the radio elements (RF equipment) on the network side. Mobile terminals connect to the network via the BTSs. The BTS transmits system information over channels defined for broadcastting network specific information, and mobile stations tune in to these channels before performing access functions. A BTS is connected to a cell site, which hosts antennas atop towers or buildings. Cell sites can be of type macro, micro, or pico depending on the coverage radius. The size of a cell site is dependent on the transmit power level of the BTS.

The radio access network is the largest component of the mobile network, and a large number of base stations and cell sites are provisioned in order to provide coverage. Nationwide coverage of mobile networks requires the deployment of thousands of BTSs (coverage of the United States for example). The BTSs provide the channels for use on a dynamic basis to subscribers. Traffic and control channels are defined for the air interfaces depending on the type of technology used. The BTSs are controlled by the base station controller.

So from a relationship perspective, a single BSC controls many BTSs. The BSC is responsible for managing the radio resources at the BTSs. The BSC assigns channels to subscribers on a need basis. In addition, it is constantly aware of a mobile station's location and the state that it is in. It measures the signal strength (with the assistance of the BTS and the MS) and makes handoff decisions. In the case of CDMA networks, BSCs are also responsible for performing the macro-diversity-combining function required in spread spectrum systems. In addition, the speech coding function may be incorporated into the BSC in some cases.BSCs are connected to the BTSs over a wireline network using T1s and E1s. T1s and E1s are physical layer transmission technologies that are widely deployed by telecom operators. T1 is able to multiplex voice and data together in 24 user slots within a frame, as compared to E1, which has 30 user slots within each frame. Microwave links are also used for these connections. BTSs are normally deployed at the cell sites itself and hence are spread out geographically. The network connecting the BTSs to the BSC is referred to as a backhaul network. The BSC is normally at a central location such as a central office. The cost of connecting a large number of BTSs to the BSC is a major expense in radio networks.