For what are the prefixes in SupportedProtocols used? (Q113)

The information in this article applies to:

These prefixes, sometimes referred to as "technology prefixes," are used for special routing. For example, through registrations with a GK, EPs might associate

• "6" with reaching an outside voice network,
• "7" with reaching a SIP network (added in H.323v4),
• "5" with reaching an H.320 network, and
• "8" with calling site-to-site within a company.

So, for example, when the GK receives an ARQ with a destination alias of "64261883," it knows to route the call to the transport address that the EP registered for the "6" prefix. Note that the GK does not know the nature of the destination, e.g., whether it is a fax card or a SIP network.

In particular, these prefixes are not a means of registering a zillion aliases that start with those prefixes. (Support for "additive registrations" was added in H.323v4.) Unfortunately, the use and intent of the SupportedProtocols prefixes remain vague within the Recommendations, so an implementor might consider avoiding them.