 fulfills the needs of the operator and the subscriber as the operator is able to maintain control over the subscription and the subscriber can protect his or her per- sonal information. Thus, the most important SIM functions include authentication, radio transmission security, and storing of the subscriber data. Base Transceiver Station (BTS) From the perspective of the radio network-planning engineer the base station is perhaps the most important element in the network as it provides the physical connection to the mobile station through the air interface. And on the other side, it is connected to the BSC via an Abis interface. A simplified block diagram of a base station is shown in Figure 2.4. The transceiver (TRX) consists basically of a low-frequency unit and a high-frequency unit. The low-frequency unit is responsible for digital signal processing and the high fre- quency unit is responsible for GMSK modulation and demodulation. 2.1.4 Channel Configuration in GSM There are two types of channels in the air interface: physical channels and logical channels. The physical channel is all the time slots (TS) of the BTS. There are again two types in this: half-rate (HR) and full-rate (FR). The FR channel is a 13 kbps coded speech or data channel with a raw data rate of 9.6, 4.8 or 2.6 kbps, while the HR supports 7, 4.8 or 2.4 kbps. "Logical channel' refers to the specific type of information that is carried by the physical channel. Logical channels can also be divided into two types: traffic channels (TCH) and control interface Filter Transmission Output/ System interface Input Figure 2.4 Block diagram of a base transceiver station RADIO NETWORK PLANNING AND OPTIMISATION Table 2.1 Control channels Channel Abbreviation Function/application Access grant channel Resource allocation (subscriber access authorisation) Broadcast common control Dissemination of general information channel (DL) Cell broadcast channel Transmits the cell broadcast messages Fast associated control FACCH For user network signalling channel (UL