Explaining the ISUP Release message (REL)

Release Message (REL)

As shown in Figure 8-15, the REL message indicates that the circuit is being released. When a RLC has been received in response, the circuit can be returned to the idle state for reuse. The REL message can be sent in either direction. It contains a single mandatory Cause Indicators field to indicate why the circuit is being released.

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 REL Message Format

Cause Indicators specify the cause information associated with the circuit being released. The Cause Indicators contain the general location in the network (such as local, remote, or transit) in which the circuit was released. The Coding Standard indicates which standard is used for decoding the Cause Value (such as ANSI, ITU). ANSI and ITU define some cause values differently, and ANSI also has additional values the ITU does not include.

The Cause Value contains an integer that represents the reason the circuit is being released. This value can be further decomposed into a class and a value. The most significant three bits of the Cause Value field represent the class. Each class is a general category of causes; for example, binary values of 000 and 001 are normal event class, and a value of 010 is resource unavailable. So, a cause value of 1 (unallocated number) is in the normal event class and a cause value of 34 (no circuit available) is in the resource unavailable class. Appendix M, "Cause Values," contains a complete list of the ITU and ANSI cause values.

The Diagnostics field is only applicable to certain cause values. It provides further information pertaining to the circuit release (for example, Transit Network Identity, Called Party Number [CdPN]) for those cause values.

Release Complete Message (RLC)

The RLC message is sent to acknowledge a REL message. Upon receipt of an RLC, a circuit can return to the idle state.