s emissions. Frequency hopping is incorporated into the GSM specification. When this facility is used, the mobile handset and the base station change frequencies or hop from one frequency to another. This is accomplished between transmitting and receiving the sequential TDMA time frames. In Cellular telecommunications 267 other words the mobile handset will receive on one frequency, say frequency A, in the receive band, transmit on frequency R in the transmit band, then receive on frequency B in the receive band, and SO forth. It can do this as it does not transmit and receive at the same time. The hopping algorithm allowing the mobile handset to know where to transmit and receive is broadcast on the Broadcast Control Channel. Frequency hopping has a number of advantages. Any interference on a given channel is present only for a short time, making the interference less noticeable, and any multipath effects are also reduced as multipath fading is dependent upon the frequency. Another feature of GSM is called discontinuous transmission (DTX). It is found that during a conversation a person speaks for less than 40 per cent of the time. This means that usage of the uplink and downlink channels is poor. To improve this, and to reduce the level of general interference, the transmitter can be turned off when no speech or other useful sound is being carried. The voice activity detection has to detect the difference between extraneous noise and speech and correctly interpret the difference otherwise the speech will sound very clipped. Alternatively if it does not switch off often enough then improvement gained by using DTX is considerably reduced. A further addition is that during the dead periods no sound at all would normally be heard and this would sound very unusual. To overcome this and give the user confidence that the connection was not dead, some 'comfort' noise matching the background noise is added at the receiving end. The DTX solution that is used today took a considerable amount of work during its development