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
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 originating switch
also 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
the ringing tone 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
Figure 12. ANSI and ITU-T Answer 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 =
Figure 13. ANSI and ITU-T Release (REL)
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.