ICW is a service that enables subscribers to receive notification of an incoming voice call on their PCs while connected to their ISP. Subscribers are notified of the incoming call through a screen-pop on their PCs, at which point they can do the following:

• Receive the call on the PC using H.323 software (VoIP).

• Drop the Internet session and receive the call on the telephone (PSTN).

• Ignore the call (provide a busy signal or let it ring).

These enhanced services afford benefits to both the service provider and the customer. The service provider can leverage its existing infrastructure to provide more services, and it can have an existing potential customer base with its dial-up customers. The service provider can provide this new service without having to become an official telecommunications provider (such as a Competitive LEC, or CLEC).

The customer benefits in that he does not miss incoming calls while online, does not have to pay for a second line from the telephone company just for Internet access, and can handle incoming calls in many ways. He can still have access to caller-ID, for instance, and he can set up variables such as forwarding to voice mail, ignoring the call, or transferring the call to a cell-phone.

Figure 6-11 details a call-flow between a PSTN (in this case, a Signaling System 7 [SS7] network) and ICW.

Figure 6-11. ICW Call-Flow

Figure 6-11. ICW Call-Flow

Figure 6-11 shows the Initial Address Message (IAM), "SS7 setup," being passed through to an ISDN setup message. The end office switch has call forwarding turned on so that when the line is busy, the number is forwarded to the AS 5300. The AS 5300 is a VoIP gateway and forwards the call through an Admission Request (ARQ) to an application server.

This application server notifies the PC (using a screen pop), at which point the customer decides whether he wants to accept the call by clicking appropriately. The application server also acts as a gatekeeper and responds to the AS 5300 with an Admission Confirm (ACF) containing the IP address of the PC to which to terminate the call. The PC then verifies that it is okay to talk to the AS 5300 by sending an ARQ and an ACF to the application server, and then it completes the call successfully with a connect message. The messages used in this call-flow are defined in detail in Chapter 10, "H.323."

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