Protocol structure

Date: 2015-10-07; view: 393.

Signalling in СT2/СА1 is similar in form to that adopted in Layers 1, 2 and 3 of the Open Systems Interconnection (OSI) model. Layer 1 ensures that communication channels can be established and maintained. To achieve this three channels types are needed:

1. B-channel for speech or data.

2. D-channel for signalling and control.

3. SYN-channel for bit and burst synchronisation.

At various stages of a call set up these three are conveyed by the above mentioned multiplexes.

Layer 2 supports the ability of CT2/CA1 to communicate mess­ages between end points of the link. Such messages include those associated with the D-channel error detection and correction, mess­age acknowledgement, call set up and clear down messages etc. A Layer 2 message package consists of up to six words each formed from eight 8-bit bytes. The first word is the address code word and the remainder are data code words. Messages may be formed from a number of message packages.

Layer 3 messages are the information elements delivered error free by layer 2 to the end of the CT2/CAI radio link. These elements have meaning and can be translated into specific responses. For example keyed digits 0-9, star and square, handset display elements and many other messages that a manufacturer may wish to employ in his product.