ISDN User Part Con't...

ISUP Part 1

Address Complete Message

An Address Complete Message (ACM) is sent in the "backward" direction to indicate that the remote end of a trunk circuit has been reserved.

The originating switch responds to an ACM message by connecting the calling party's line to the trunk to complete the voice circuit from the calling party to the called party. The terminating switch sends a ringing tone to the calling party's line.

Figure 11. ANSI and ITU-T Address Complete Message (ACM) Format

When the called party answers, the destination switch terminates power ringing of the called line, removes audible ringing tone from the calling line and sends an Answer Message (ANM) to the originating switch. The originating switch initiates billing after verifying that the calling party's line is connected to the reserved trunk.

Figure 12. ANSI and ITU-T Answer Message (ANM) Format

Release Message

A Release Message (REL) is sent in either direction indicating that the circuit is being released due to the cause indicator specified. An REL is sent when either the calling or called party "hangs up" the call (cause = 16). An REL is also sent in the backward direction if the called party line is busy (cause = 17).

Figure 13. ANSI and ITU-T Release (REL) Message Format.

Release Complete Message

A Release Complete Message (RLC) is sent in the opposite direction of the REL to acknowledge the release of the remote end of a trunk circuit and end the billing cycle as appropriate.

Figure 14. ANSI and ITU-T Release Complete (RLC) Message Format

Telephone User Part

In some parts of the world (e.g., China), the Telephone User Part (TUP) supports basic call processing. TUP handles analog circuits only; digital circuits and data transmission capabilities are provided by the Data User Part.

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