Call Setup and Teardown

ISUP capabilities are more easily understood after you understand the basics of setting up and releasing calls in the SS7 network. For the purposes of this exercise, assume that the call is destined for a remote end office, ISUP signaling is available end to end, only one intermediate switch is available, and the dialed digits do not require queries from a database.

The following describes a basic call setup and teardown in the SS7 network:

1. The subscriber initiates an off-hook, and the local end office sends the caller a dial tone. The caller dials the desired digits, and the local end office collects the digits dialed.

2. The local end office determines how to connect the call based on its routing tables. The routing tables identify the circuits available to establish an end-to-end connection. The originating office creates and sends an IAM to the switch that provides the first connection (the pass-along method) and indicates the circuit to be used.

3. When it receives the IAM, the intermediate switch responds by sending an ACM to the originating switch. The ACM is a confirmation that the intermediate switch reserved the same circuit that the originating end office designated in the IAM. The ACM also alerts the originating office to provide a ringback tone to the calling party.

4. While sending the ACM, the intermediate switch prepares to set up the next connection by creating an IAM containing the called and calling information provided by the originating end office. The intermediate switch forwards the IAM to the terminating office using its routing tables.

5. Upon receipt of the IAM, the terminating switch determines whether the called party is busy.

If the called party is not busy. the terminating switch responds by sending an ACM to the intermediate switch. Following the ACM, the terminating switch signals the subscriber's (called party) line by ringing the telephone. When the called party answers the call, the terminating office cuts through the voice path and sends an ANM along the same path to the intermediate switch.

6. The intermediate switch in turn cuts through the voice path and sends an ANM to the originating switch. Now the originating switch can connect the voice path and enable the conversation to begin.

7. For this example, the called party goes on-hook first and initiates the release procedures at the terminating exchange. The terminating exchange immediately sends a SUSPEND (SUS) message to the intermediate switch, and in turn, the intermediate switch sends an SUS message to the originating switch.

8. When the calling party goes on-hook, the originating switch sends a Release (REL) message toward the terminating switch using the same path as the other signaling messages. The intermediate and terminating switches acknowledge the release with a RELEASE COMPLETE (RLC) message. The RLC message also signifies that each circuit returned to an idle state.

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