Radio Interface

In the Public Land Mobile communication Network (PLMN), the MS is connected with the network via the radio channel. In this way, the subscribers can access the network and obtain communication services. To achieve the interworking between MS and BTS,  a set of standards are needed for signal transmission through the radio channel. This set of specifications which related to the radio channel signal transmission, aim at Um interface.

The Um interface is a kind of  radio interface. It is responsible for the communication between the mobile station and the BTS and provides the interworking link between the mobile station and  GSM system. Its physical connection is achieved via the radio waves. The Um interface is the most important interface among all the interfaces in  GSM system. First of all, the complete and normative Um interface realizes full compatibility between MS of different venders and different networks. That is fundamental conditions needed in global roaming of the GSM system; second, the radio interface determines the rate of frequency spectrum utilization of GSM system. The name “Um” is derived from the name of the interface between the client terminal and the network in ISDN , in which the “m” means “mobile”.