Switching to VoIP

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

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SIP versus H.323: the great debate

H.323 is older than SIP, and more considerate of legacy infrastructure, too. It defines

standard procedures and best practices for interfacing with old-school telephony

technology—practices like the use of gateways with built-in support of legacy proto-

cols like ISDN and FXO/FXS.

SIP makes no such provisions, but is still the favorite among those schooled in Inter-

net thought: it’s extensible and reusable and does a whole lot more for telephony

apps, ultimately, than H.323. SIP allows multiple endpoints to register the same alias,

allows freedom from E.164, and enables other, non-voice applications like instant

messaging and presence. For these reasons, some in the industry have pronounced SIP

the winner of the battle for telephony signaling.

SIP doesn’t define gateways for interaction with the PSTN, so a legacy-

aware signaling system like MGCP, MEGACO, or H.323 is often used

alongside SIP in order to facilitate legacy gateways.

The fact is, both protocol suites are very much necessary. Parts of the H.323 recom-

mendation—RTP and PSTN gateways in particular—are in use by SIP networks so

the importance of H.323’s features is obvious.

One day, SIP may contain some legacy interfacing of its own, but for now, H.323 fills

that role quite aptly.