Frame structure

Date: 2015-10-07; view: 448.

Radio operational features

The fixed part (FP) has an active channel for paging and synchroni­sation. An incoming call is paged on all FP and the PP responds on a channel chosen by the PP.

If channel quality deteriorates, PP moves out of range or due to increased interference, then handover is initiated to a more appro­priate FP. A base controller synchronises this re-routeing of calls. PP periodically scan all channels to update their list of free channels to ease handover.

Link quality is assessed from channel bit error ratios and received signal strength.

For handover the PP requests a second channel (perhaps on another FP) to he modulated with the wanted signal. The PP then decides when to switch channels and then informs the controller which channel to release.

Figure 19.12 (ETSI. I992b) shows the multiplex burst as it would appear for a normal telephony call and shows also the relationship between the PHL and MAC layer responsibilities. The 48 bits of the A field support the associated signalling channel requirements (C) the broadcast beacon transmission (Q) and the paging channel (P). The remaining 16 bits of the A field is a cyclic redundancy code (CRC) check used to protect the data.

The В field is used to transport the traffic information (1) and offers 320 hits per burst which is equivalent to 32kbit/s. The four X bits (which optionally can he extended to eight) are provided to detect collisions between bursts emanating from independent, and hence un-synchronised, systems. It should be noted of course that other multiplex structures are used in DECT particularly in the channel set up phase.