4.3 Call reference

The purpose of the call reference is to identify the call or facility registration/cancellation request at the local user-network interface to which the particular message applies. The call reference does not have end-to-end significance across ISDNs.

The call reference is the second part of every message. The call reference is coded as shown in Figure 4-3. The length of the call reference value is indicated in octet 1, bits 1-4. The default maximum length of the call reference information element is three octets long.The actions taken by the receiver are based on the numerical value of the call reference and are independent of the length of the call reference information element.

At a minimum, all networks and users must be able to support a call reference value of one octet for a basic user-network interface, and a call reference value of two octets for a primary rate interface.

As a network option for a primary rate interface., the call reference value may be one octet also. In this case, a call reference value up to 127 may be sent in one or two octets.

The call reference information element includes the call reference value and the call reference flag.

Call reference values are assigned by the originating side of the interface for a call. These values are unique to the originating side only within a particular D-Channel layer two logical link connection. The call reference value is assigned at the beginning of a call and remains fixed for the lifetime of a call (except in the case of call suspension). After a call ends, or, after a successful suspension, the associated call reference value may be reassigned to a later call. Two identical call reference values on the same D-Channel layer two logical link connection. may be used when each value pertains to a call originated at opposite ends of the link.

Figure 4-3/Q.931

Call reference information element

The call reference flag can take the values"0" or "1". The call reference flag is used to identify which end of the layer two logical link originated a call reference. The origination side always sets the call reference flag to "0". The destination side always sets the call reference flag to a "1".

Hence the call reference flag identifies who allocated the call reference value for this call and the only purpose of the call reference flag is to resolve simultaneous attempts to allocate the same call reference value.

The call reference flag also applies to functions which use the global call reference (e.g. restart procedures).

NOTES

1. The call reference information element containing a dummy call reference is one octet long and is coded "0000 0000". The use of the dummy call reference is specified in Recommendation Q.932.

2. The numerical value of the global call reference is zero. The equipment receiving a message containing the global call reference should interpret the message as pertaining to all call references associated with the appropriate data link connection identifier. See Figure4-5.

Figure 4-4/Q.931

Dummy call reference

a) One octet call reference value

b) Two octet call reference value

Figure 4-5/Q.931

Examples of the encoding for global call reference