Switching to VoIP

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Chapter 7: Replacing Call Signaling with VoIP

H.323

H.323, currently in Version 2, is the ITU-T’s recommendation for PBX-styled signal-

ing that supports a packet-based network. H.323 doesn’t have to be delivered

entirely using an IP network. Certain subrecommendations of H.323 allow for tradi-

tional telephony networks to be integrated with seamless signaling across all partici-

pating devices. For example, the H.323 suite allows for signaling over POTS on the

PSTN using recommendations in H.320 and H.324.

H.323 was originally proposed as a solution for video conferencing

using LANs. Later revisions saw it morph into a full-fledged PBX-

replacement plan.

H.323 is implemented in Microsoft’s NetMeeting conferencing software, as well as in

Avaya’s MultiVantage and Nortel’s Meridian IP-PBX systems. While quite mature

and well-documented by the ITU-T, H.323 has been implemented in vendor-specific

“flavors” that are, unfortunately, not totally interoperable. This hasn’t been a big

hindrance to acceptance of H.323 for VoIP applications, however, since most inte-

grators who choose this standard tend to choose a single vendor’s implementation

across their entire network.

Incompatibility of H.323 implementations is a problem only when trying to link

opposing vendors’ H.323 systems together. ...