Circuit Suspend and Resume

In Figure 8-3, the originating subscriber goes on-hook first. The originator is normally considered in control of the call, so the circuit is released when the originator goes on-hook. If the terminator goes on-hook while the originator remains off-hook, there are two methods of handling the disconnection.

The first method is for the terminating exchange to release the call by sending a REL message to the originating exchange. This is no different than the scenario presented for a release initiated at the originating exchange; the originating switch responds with an RLC and the circuit is idled at each SSP.

The other method is for the terminating exchange to send a Suspend (SUS) message in the backward direction when it receives a disconnect indication from the terminating line. The SUS message provides notification that the terminating party has disconnected but that the circuit connection is still being maintained. Suspending the call allows the person who receives the call an opportunity to pick up on another phone extension.

When the SUS is received, the originating exchange starts a suspend timer (Timer T6, or Timer T38 in the case of an international exchange). If the terminating party reconnects (off-hook) before the suspend timer expires, a Resume (RES) message is sent in the backward direction, allowing the conversation to continue.

Figure 8-16 shows an example of a Suspend (SUS) and Resume (RES) being sent from the terminating exchange. If the suspend timer expires, a REL is sent in the forward direction. In the event that the originator goes on-hook during the time the circuit is suspended, the originating exchange sends a REL forward and normal call clearing takes place. The terminating exchange responds with a RLC.

Support for SUS/RES varies, based on factors such as the type of service and the local network policies. For example, in the United States, SUS/RES is only supported for non-ISDN service.