Nortel VoIP Gateway, Configuration Manual

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22 Key IP telephony concepts
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Gatekeepers
A gatekeeper tracks IP addresses of specified devices, and provides authorization for making and
accepting calls for these devices. A gatekeeper is not required as part of the network to which your
VoIP Gateway is attached, but Gatekeepers can be useful on networks with a large number of
devices.
Codecs
The algorithm used to compress and decompress voice is embedded in a software entity called a
codec (COde-DECode).
Two popular Codecs are G.711 and G.729. The G.711 Codec samples voice at 64 kilobits per
second (kbps) while G.729 samples at a far lower rate of 8 kbps. For actual bandwidth
requirements, refer to
“Determining the bandwidth requirements” on page 153 , where you will
note that the actual kbps requirements are slightly higher than the label suggests.
Voice quality is better when using a G.711 CODEC, but more network bandwidth is used to
exchange the voice frames between the telephones.
If you experience poor voice quality, and suspect it is due to heavy network traffic, you can get
better voice quality by configuring the IP telephone to use a G.729 CODEC.
The VoIP Gateway supports these codecs:
• G.729
• G.723
• G.711-uLaw
• G.711-aLaw
Note:
The VoIP Gateway does not contain a gatekeeper application. If you want to put a
gatekeeper on your network, it must be put on a separate gatekeeper server. The VoIP
Gateway is compatible with RadVision and CSE 1000 gatekeepers.