Switching to VoIP

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Chapter 7: Replacing Call Signaling with VoIP

9. When the called party picks up the phone, 3002 sends an H.225 Connect mes-

sage to the softPBX.

10. The softPBX sends a 200 OK SIP response to 3001.

11. 3001 sends a SIP ACK method to the softPBX.

Heterogeneous Signaling and the SoftPBX

Different vendors implement heterogeneous signaling differently, and if you’re really

interested in tracing the signaling steps taken during a particular multiprotocol call,

your best bet is to capture them using a packet sniffer or study the standards recom-

mendations themselves. In the meantime, Figure 7-13 should give you a working

idea of heterogeneous signaling.

Cisco AVVID/CallManager

Cisco’s approach to signaling is distributed, meaning that specialized, separate

devices are the preferred mechanism for signaling non-SCCP endpoint connections.

A media gateway such as the Cisco 2650 is a modular chassis that contains a variety

of interface hardware and IOS firmware that is specific to the type of signaling

desired—H.323, SIP, and so on.

These media gateways refer to endpoints or remote gateways as dial-peers regardless

of their individual signaling standards. A POTS link, an H.323 softphone, and a SIP

hardphone are all called dial-peers.

Independently, Cisco media gateways can be used as very basic ...