ome interfaces and signalling involved in the GSM system. Here, we will briefly discuss interfaces and signalling. Interfaces Air Interface The air interface is the central and most important interface in every mobile system. The importance of this interface arises from the fact that it is the only interface the mobile subscriber is exposed to, and the quality of this interface is crucial for the success of the mobile network. The quality of this interface primarily depends upon the efficient usage of the frequency spectrum that is assigned to it. In the FDMA system, one specific frequency is allocated to one user engaged in a call. When there are numerous calls, the network tends to get overloaded, leading to failure of the system. In a full-rate (FR) system, eight time slots (TS) are mapped on every frequency, while in the half-rate (HR) system, sixteen TSs are mapped on every frequency. In the TDMA system, only impulse-like signals are sent periodically, unlike with FDMA where signals are assigned permanently. Thus, by combining the advantages of both the techniques, TDMA allows seven other channels to be served on the same frequency. As mentioned earlier, GSM uses the gaussian minimum phase-shift keying (GMSK) modulation technique. For the full-rate system, every impulse on frequency is called a burst. Every burst corre- sponds to a time slot. So: 1 TDMA frame = 8 bursts Uplink frequency band = 890-915 MHz Downlink frequency band = 935-960 MHz SECOND-GENERATION MOBILE NETWORKS (GSM) Frequency-division multiplexing divides each of the frequency ranges into 124 channels of 200 kHz width. Since this is required for both transmission and reception, 124 duplex communication channels are produced. Now, channel bit rate = D per unit time. As one TDMA frame has eight bursts, hence each burst has a bit rate d = D/8. This means that each time slot (TS) is of duration 577 microseconds (us). Thus, one frame has duration 4.615 milliseconds (ms). There are many types of frames in the air-interface, as shown in Table 1.1. T