Editing ISDN User Part (section)

== Message types ==
An ISUP message contains a fixed header containing the [[circuit identification code]] and the ISUP message type, followed by a mandatory fixed-length parameter part, a mandatory variable-length parameter part, and an optional parameter part that are dependent on the type of message being sent. ISUP messages can be sent using the services of the [[Message Transfer Part]], or, less often, the [[Signalling Connection Control Part]]. These messages are transmitted in various stages of [[call setup]] and release. The most common messages are:{{Sfn |Russell|2002|pp = 362–419}}
*Initial address message (IAM) &mdash; First message sent to inform the partner switch that a call has to be established on the CIC contained in the message. Contains the called number, type of service (speech or data) and optional parameters.
*Subsequent address message (SAM) &mdash; For networks that support ''overlap'' dialing procedures, and then in the case that the IAM did not contain the full called number, one or more SAMs follow containing additional digits. This message is not supported by networks that only support ''en bloc'' dialing procedures.
*Address complete message (ACM) &mdash; Message returned from the terminating switch when the subscriber is reached and the phone starts ringing, or when the call traverses an interworking point and the intermediate trunk is seized.
*Call progress (CPG) &mdash; Contains additional information about the progress of a call. Normally sent after the ACM when the status of the call changes from that reported in the ACM.
*Answer message (ANM) &mdash; Sent when the subscriber picks up the phone, a resource is connected or answer supervision is returned by an interworking point. Normally charging starts at this moment. It is required that the call be cut through in both directions by this point.
*Connect (CON) &mdash; Sent when the call is answered by an automatic terminal. This message replaces the ACM, CPG and ANM for calls that are answered by automatic terminals.
*Release (REL) &mdash; Sent to clear the call when a subscriber goes on hook. This is also sent (in direct response to an IAM) if the terminating switch determines that the call cannot be completed. In either case, the terminating switch provides a cause value in the message to explain the reason for the release, ''e.g.'', "User busy".
*Release complete (RLC) &mdash; Acknowledgment of the release – the circuit is idle afterward and can be used again.