Switching to VoIP

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Chapter 7: Replacing Call Signaling with VoIP

H.323 Terminal

Each H.323 terminal—either a software endpoint or a hardphone—contains a stack

of software elements that cover different aspects of the calling process:

• H.245, which provides capabilities negotiation in order to make sure there’s a

compatible application and codec running on the calling terminal and the receiv-

ing one.

• H.225, which provides billing and monitoring functions necessary for reliable

call setup and carrier-grade call accounting.

• RTP, the IETF standard for transmitting packets of encoded real-time media.

• A selection of one or more audio codecs.

• Optionally, an H.323 terminal may offer T.120, a protocol for enabling interac-

tive data applications such as white-boarding. A great example of this is Net-

Meeting’s shared drawing board feature.

H.323 Gateway

The purpose of a gateway is to interface the IP-based voice channels to legacy signal-

ing and transport technologies such as FXO, FXS, E&M, ISDN BRI and PRI, T1, and

DID. A great example of this device is Cisco’s modular media gateways, which can

be equipped with interfaces that can support all of these legacy technologies and

more. An H.323 gateway is required only when an interface to a legacy voice net-

work is necessary—which is usually the case.