June 2005
Intermediate to advanced
502 pages
21h 48m
English
Content preview from Switching to VoIP
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Voice Channels
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127
Project 6.1. Set Up Custom Codec Selection and Enable
an Independent Call Path
What you need for this project:
• Asterisk
• Two or more hard or soft SIP phones
• LAN
Within Asterisk, it’s possible to set preferences for each SIP channel as to which
codecs should be used, or allowed. This can be a necessity if a particular channel is
connected to the server using a low-bandwidth link. It would be prudent to limit that
SIP channel to codecs that preserve bandwidth. Figure 6-6 shows how calls across a
WAN might use G.729A, a low-bandwidth codec, while local Ethernet-based calls
might use G.711.
Codec selection is a function of the call setup routine, handled by SIP, H.323, or a
proprietary signaling protocol, but there are varying degrees to which you can tune
the process. Depending on vendor, the softPBX has some control over codec selec-
tion on a phone-by-phone basis. Some VoIP vendors call this per-peer selection.
Per-peer codec selection on Cisco media gateways
Cisco’s media gateway devices, themselves just simple VoIP softPBXs, allow the defi-
nition of endpoint and trunk peers using SIP and H.323 signaling configurations.
One of the parameters that can be assigned to each peer is a codec preference.
Figure 6-6. Codec selection schemes are often based on bandwidth availability ...
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