Practical VoIP Using VOCAL

Registration and Admission

In H.323, as in SIP, a distinction is made between call-related and non-call-related signaling. In the SIP world, the REGISTER message establishes the relationship between the UA and the registrar. In H.323, there are separate messages for discovery, registration, admission, and potentially location and bandwidth control. We did not implement location and bandwidth control in our translator.

The gatekeeper is the server: the intermediary brokering the calls. The gatekeeper’s clients are callable H.323 endpoints, each with a unique IP address that acts as their unique identifier. Most users are not interested in dialing IP addresses: they would rather dial an alias, like a phone number or name, and have it translated into an IP address. In the same way that DNS servers perform this translation on the World Wide Web, the gatekeeper performs this service for the H.323 network.

When a terminal comes online, it can send out a multicast message asking, “Are there any gatekeepers out there that will provide me with telephony service?” Hopefully, there is a gatekeeper that responds, “Yes.” If there are no gatekeepers, the terminals are forced into a point-to-point situation in which the users have to dial each other by IP address or use some other out-of-band mechanism for their addressing. The address of a gatekeeper can also be manually configured on H.323 endpoints.

A gatekeeper tells you that there are terminals out there that can accept calls. If you don’t use ...