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Appendix F
Gateway Application Call Control
- Introduction
- Accepting Calls
- Rejecting Calls
Introduction
- Gateway applications perform call control for calls that must be switched between separate trunks. They perform a switch-like function such as directing inbound calls from the PSTN to appropriate addresses on an internal network (the application may also be embedded in the PSTN itself). Typically, these applications receive inbound calls, analyze the incoming addresses, and then place calls to the specified addresses.
- Accepting and rejecting calls with gateway applications can pose problems because the decision to accept or reject an incoming call depends on the status of the associated outbound call. However, the time available in the incoming call protocol to accept the call can be shorter than the setup time for the corresponding outgoing call.
- The sections that follow describe how to accept and reject incoming calls with gateway applications.
Accepting Calls
- A gateway switching application goes through the following phases when accepting a call:
- The inbound side of the application receives a call.
- The call is "put on hold" (by accepting the call with nccAcceptCall and outputting silence on the line) and the outbound side of the application dials the received address.
- When the outbound side of the application receives an indication that the called terminal is free and ringing, it connects the voice path so that the caller hears the ringing tone from the called equipment.
- When the outbound call is answered, the application also answers the inbound call with no rings. This instructs the TCP to send the answer signal to the network immediately. Both calls are now in a Connected state.
- Figure 4 shows how gateway applications accept inbound calls:
Figure 4. Accepting Inbound Calls with Gateway Applications
Rejecting Calls
- When a gateway application places an outbound call (in response to inbound calls on another trunk), the call may be rejected for a variety of reasons. When this happens, the application must also reject the corresponding inbound call.
Figure 5 illustrates how this process works:
Figure 5. Rejecting Calls with Gateway Applications
- The way a gateway application rejects calls is similar to the way it accepts calls. Initially, the TCP accepts the incoming call and puts it "on hold" by sending silence to the line. When the outbound call is rejected, the application calls nccRejectCall. The TCP immediately starts to perform the action requested by nccRejectCall. It can play a busy tone, or let the application play a reject message. Then TCP then relies on the caller side to tear down the connection.
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