June 2005
Intermediate to advanced
502 pages
21h 48m
English
Content preview from Switching to VoIP
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H.323
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137
Sometimes, MCUs are referred to as conference bridge servers or DSP farms, because
it is via digital signal processing that all parties are able to hear the other parties on
an MCU-hosted conference call. DSP is used to converge, or interleave, participants’
audio signals into a new, combined audio signal that is sent back out to the partici-
pants. (Traditional telephony may also use DSP for conference mixing—this charac-
teristic isn’t unique to VoIP.)
The software element within the MCU that actually performs the DSP function in
order to aggregate media channels into a conference call is known as an MP, or mul-
tipoint processing, element. This is a part of the MCU that is multicodec aware and
can handle transcoding if necessary. MCUs are also RTP hosts, as RTP itself is
designed to support mixer-controlled conferences.
The second component of an MCU is the MC, or multipoint controller—this is the
part that is signaling-savvy and can negotiate with the endpoints using H.225 and H.
245 to get them added, and removed from, conference calls as needed.
The H.323 Call-Signaling Process
There are five general steps in the H.323 signaling process for each leg of a call path:
setup/teardown, capabilities negotiation, open media channel, perform call, and
release.
Setup/teardown
To
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