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                    The ISDN User Part or ISUP is part of the Signaling System #7 which is used to set up telephone calls in Networks. To establish call between two mobile subscribers which involving two or more Telephone switch then ISUP plays an important role in Call setup.

The telephone exchanges are connected via E1 or T1 trunks which transport the speech from the calls. These trunks are divided into 64 kbit/s timeslots, and one timeslot can carry exactly one call. Each timeslot between two switches is uniquely identified by a Circuit Identification Code (CIC) that is included in the ISUP messages. The exchange uses this information along with the received signaling information (especially the Called Party Number) to determine which inbound CICs and outbound CICs should be connected together to provide an end to end speech path.
Initial Address Message (IAM) — 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 and calling number, type of service (speech or data) and many more optional parameters.
Subsequent Address Message (SAM) — In case the IAM did not contain the full called number, one or more SAMs may follow containing additional digits.
Address Complete Message (ACM) — Message returned from the terminating switch when the subscriber is reached and the phone starts ringing.
Answer Message (ANM) — Sent when the subscriber picks up the phone. Normally charging starts at this moment.
Release (REL) — sent to clear the call when a subscriber goes on hook.
Release complete (RLC) — Acknowledgement of the release
A subscriber     telco switch A    telco switch B     B subscriber
Off hook
Dial digits --->
                            -- IAM -->
                                                -Ringing ->
                            <-- ACM --
                                                       Off hook
                            <-- ANM --
-----------------------    Conversation    -----------------------
On hook
                            -- REL -->                  On hook
                            <-- RLC --