ISUP message types

-       IAM

o   Initial Address Message (IAM) is sent in the forward direction by each switch in the circuit between the calling party and the destination switch of the called party.

o   Contains the called party number in the mandatory variable part and may contain the calling party name and number in the optional part.

-       SAM

o   Subsequent Address Message. In case the IAM did not contain the full called number, one or more SAMs may follow containing additional digits.

-       ACM

o   Address Complete Message is sent in the backward direction to indicate that the remote end of a trunk circuit has been reserved.

o   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.

-       ANM

o   Answer Message sent when the subscriber picks up the phone

o   Normally charging starts at this moment

-       CPG

o   Call Progress (CPG) can be sent by the terminating SSP after it has sent an ACM. For example, this could be used to signal that a call has been forwarded as a result of a busy line.

-       REL

o   Release Message is sent in either direction indicating that the circuit is being released due to a specific cause indicator

o   Is sent when either calling or called party hangs up the call (cause =16)

o   Is sent back to the calling party if the called party is busy (cause 17).

-       RLC

o   Release Complete Message is sent in the opposite direction of a REL to acknowledge the release of the remote end of a trunk circuit and to end the billing cycle.