Switching to VoIP

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Chapter 7: Replacing Call Signaling with VoIP

signaling protocol, then H.225 can establish the media parameters immediately, fore-

going the H.245 session after the called party has picked up the phone. In this event,

the RTP session will be open by the time the called party answers. This is useful in

minimizing the perception of lag in call startup when gateways are involved.

SoftPBX H.323 implementations

Because H.323 is such a distributed approach (gatekeepers, gateways, MCUs can all

be on different hardware), it’s tempting not to think of the standard as a way of

achieving software-based PBX functionality. But in reality, most softPBX makers

who’ve standardized on H.323 are putting gatekeepers, gateways, and MCUs all in

one box and calling it a softPBX. This is an accurate description of what a fully func-

tional H.323 network does—uses software to switch calls across multiple data links

using a variety of physical interfaces, the same purpose as a softPBX.

Avaya, Nortel, and others that support H.323 offer most of these functions on a sin-

gle Ethernet host—often a server blade that mounts inside their PBX chassis.

Cisco offers a consolidated approach in their media gateways. While they don’t offer

an MCU, Cisco’s media gateways do provide H.323 gatekeeper and gateway func- ...