Basic Call Setup and Teardown Example

This example demonstrates the steps involved in basic call setup and teardown. The example also shows ways you can use the ISUP protocol to connect and disconnect calls in the SS7 network. Figure 4-16 provides the network topology for this example.

Figure 4-16. Network for Call Setup and Teardown

Figure 4-16. Network for Call Setup and Teardown

In this example, the analog subscriber connected to SSP1 places a call to the analog subscriber connected to SSP2. The following steps occur when the subscriber on SSP1 places the call:

1. When the subscriber goes off-hook, call processing is initiated in SSP1. Call processing causes SSP1 to enter the originating call state and move to the collecting information state. SSP1 collects the information or dialed digits from the subscriber.

2. After all the digits are collected, the call enters the analyzing information state. SSP1 analyzes the dialed digits and determines that the call needs to be sent to SSP2. The call then enters the selecting route state, and SSP1 locates an available trunk in the trunk group to SSP2.

3. After SSP1 selects an idle trunk, it creates an IAM addressed to SSP2. The IAM identifies the originating exchange (SSP1), destination exchange (SSP2), called and calling number, and trunk selected. SSP1 sends the IAM (1) out one of the A-links toward the destination.

4. When STP1 receives the IAM, it reads the routing label and routes the IAM to SSP2. The IAM is received and SSP2 determines that it's the serving center. At this point, SSP2 verifies that the called number is idle and returns an ACM (2) to SSP1. At the same time, SSP2 connects the trunk back to SSP1, applies a ringing tone on the trunk, and rings the subscriber line of the called number. When

STP2 receives the ACM, it reads the routing label and routes the ACM to SSP1. The ACM indicates that the IAM was received and that SSP2 is the terminating exchange.

5. STP1 receives the ACM and connects the subscriber line of the calling party to the trunk. At this point, the calling subscriber can hear the ringing tone. When the called subscriber answers the call, SSP2 creates and sends an ANM (3) to SSP1. STP2 receives the ANM, reads the routing label, and routes the ANM to SSP1. SSP1 simply verifies that the subscriber and the trunk are connected.

6. If the calling party hangs up the call first, SSP1 creates an REL message (4) addressed to SSP2. The REL message identifies the trunk associated with the call. SSP1 then routes the REL message toward SSP2, where STP1 reads the routing label and routes the REL to SSP2.

7. When SSP2 receives the REL message, it disconnects the trunk from the subscriber line and returns the trunk to an idle state. SSP2 then creates an RLC (5) message, identifying the trunk used for the call. SSP2 addresses the RLC message to SSP1 and routes it toward SSP1. STP2 receives the RLC message, reads the routing label, and routes the message to SSP1. When it receives the RLC message, SSP1 idles the trunk identified in it.

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