address field is used to indicate whether the message is a The call reference value in the third octet is used to identify the command or a response, a signalling or maintenance message call with which a particular message is to be associated. Thus a call and to which channel it refers. can be identified independently of the communications channel on The control field is one or two octets depending on the frame which it is supported. This feature is particularly important in type and carries information that identifies the frame and the connection with incoming call offering procedures on a passive bus Layer 2 sequence numbers used for link control. arrangement since the channel is only allocated to the called termi- 4. The information field contains the signalling or level 3 mess- nal after answer. The message type code in the fourth octet describes the intention 5. The Cyclic Redundancy Check (CRC) bits enable the receiver of the message (e.g. a SETUP message to request call estab- to determine whether any errors have been incurred during lishment). A number of other information elements may be included transmission. following the message type code in the fourth octet. The exact contents of a message is dependent on the message type, however In more detail the address field enables layer 2 multiplexing to be the coding rules are open ended and in principle it is a simple matter achieved by employing a separate Layer 2 address for each LAP in to include additional information elements to satisfy any require- the system. To carry the LAP identity the address field is two octets ment which may be identified in the future. long and contains a service identifier (SAPI), a terminal identifier In order to make an outgoing call request, a user must send all of (TEI) and a command/response bit. This address identifies the the necessary call information (i.e. called party number and sup- intended receiver of a command frame and the transmitter of a plementary service requests) to the network. Furthermore the user respons