49. 3. 2 Frame multiplex structure

When a call is being initiated there is no synchronisation between the handset and base, and both parts will be scanning across the forty radio channels. In order for a call to be set up and maintained CT2/CAI employs three multiplex structures during the course of a call.

Multiplex 1 is used during the normal handset to base communi­cation phase when synchronisation and call set up have been achieved and the call is in progress. The structure is shown in Figure 49.5 (Evans, 1990) where it can be seen that two channels are supported.

The B-channel conveys the 32kbit/s speech signal (or perhaps 32kbit/s data) and the base and handset are conveyed through the D-channel. While D-channel errors are detected and corrected by requesting a re-transmission there is no error detection on the B-channel. The observed performance of the D-channel can be used to determine B-channel performance and if appropriate instigate a radio channel change to a better quality channel during the speech. Generally the channel change will be quick and go undetected by the user.

Figure 49.5 shows that the D-channel capacity can either be 1kbit/s or 2kbit/s according to manufacturer’s choice, but this choice must be signalled between the base and handset at call set up. Clearly a public telepoint base station must be able to support both. It should be noted that error corrected rates for each possibility is about half the basic throughput.

Multiplex 2 is used only when the base station is setting up a link 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 SYN-channel. The format is shown in Figure 49.6 (Evans, 1990). In this format the D-channel has a 16kbit/s capacity, the preamble consists of 1,0,1,0,1… reversals for bit timing, and the channel marker word (CHM) is used to set up frame synchronisation. Successful synchronisation is marked by the transmission of the SYNC word from the handset at which point it is able to transmit the same frame structure and forward its identification code in the D-channel.

If the handset needs to set up a call then a different frame structure, multiplex 3, is initially used by the handset in order to synchronise with the base unit. This is important because in multiplex 1 and 2 operation the base and handset terminals alternately transmit and receive and it is necessary to avoid unsuccessful call set up due to, for example, both units transmitting at the same time. In multiplex 3 the handset repeatedly transmits a sequence of preamble bits, D-channel identity information and channel markers for a period of 10 ms. 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 5 seconds after which call set up is abandoned. This frame structure is designed to ensure that the base station, which is operating in the 1 ms transmit and 1 ms receive mode, has a maximum chance of intercepting the initially un-synchronous handset transmission After synchronisation call set up proceeds using multiplex 2.