mission there is no error detection operation the base and handset terminals alternately transmit and on the B-channel. The observed performance of the D-channel can receive and it is necessary to avoid unsuccessful call set up due to, be used to determine B-channel performance and if appropriate for example, both units transmitting at the same time. In multiplex instigate a radio channel change to a better quality channel during 3 the handset repeatedly transmits a sequence of preamble bits, the speech. Generally the channel change will be quick and go D-channel identity information and channel markers for a period of undetected by the user. 10ms. This is followed by a 4ms receive period in which a response from the base unit is listened for. This process is repeated for up to Figure 49.5 shows that the D-channel capacity can either be 5 seconds after which call set up is abandoned. This frame structure 1kbit/s or 2kbit/s according to manufacturer's choice, but this is designed to ensure that the base station, which is operating in the choice must be signalled between the base and handset at call set up. 1ms transmit and 1ms receive mode, has a maximum chance of Clearly a public telepoint base station must be able to support both. intercepting the initially un-synchronous handset transmission. It should be noted that the error corrected rates for each possibility After synchronisation call set up proceeds using multiplex 2. is about half the basic throughput. Multiplex 2 is used only when the base station is setting up a link 49.3.3 Protocol structure with its associated handset. At this stage the B-channel is not active but the base needs to transmit its identity to the required handset via the D-channel and synchronise that handset to its frame using the Signalling in CT2/CAI is similar in form to that adopted in Layers SYN-channel. The format is shown in Figure 49.6 (Evans, 1990). In 1, 2 and 3 of the Open Systems Interconnection (OSI) model. Layer this format the D-channel has a 16kbit/s capacity, the preamb