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Practical VoIP Using VOCAL by Luan Dang, Cullen Jennings, David Kelly

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MGCP Translator

The MGCP translator acts as a SIP User Agent that talks SIP to a SIP proxy on one side and acts as an MGCP Call Agent (CA) that speaks MGCP to control MGCP gateways on the other side. This allows MGCP gateways to be controlled by the MGCP translator’s CA, but servers within the SIP world control the routing decision. A call can be made from an MGCP gateway to a SIP entity or from a SIP entity to an MGCP gateway. A call may also be made from an MGCP gateway back to the same MGCP translator via a network of SIP servers, which in turn will attempt to contact another or the same MGCP gateway.

Although two MGCP connections can be made through the same CA and gateway, the translator sends all messages through to the VOCAL network. There is no hairpinning permitted through the translator (where the signal receives responses from the translator without the participation of the VOCAL servers).

The translator sends SIP registration messages on behalf of all the endpoints it controls. It can make outgoing SIP calls and terminate incoming SIP calls on the MGCP gateways. Besides basic calls, it supports transfer, conference, and call waiting. It has limited redundancy support: the backup MGCP translator is able to support connected calls from a failover. For most of these features , detailed call flow diagrams later in this chapter illustrate their operation.

Startup and registering

The MGCP translator sends SIP REGISTER messages on behalf of all the MGCP endpoints that ...

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