Switching to VoIP

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Voice Channels

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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 ...