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What are Voice Compression Modules (VCM's) for?

VCM's are required to support the following scenarios:

Internal phone calls between an IP device and a non-IP device.
Analog/digital phones to IP trunks (SIP/H.323) including managed Frame Relay and managed IP VPN (provides echo cancellation).
IP phones to ISDN or PSTN trunks (convert IP to TDM and vice-versa).

 

IP device to IP device
Call progress tones (for example dial tone, secondary dial tone, etc) do not require voice compression channels with the following exceptions:

Short code confirmation, ARS camp on and account code entry tones require a voice compression channel.
Devices using G.723 require a voice compression channel for all tones except call waiting.

 

When a call is connected:

If the IP devices use the same audio codec no voice compression channel is used.
If the devices use differing audio codecs, a voice compression channel is required for each.

 

"Direct Media" is a VoIP concept that circumvents resources (TDM bus, Gateway) within the system and improves the voice quality. If two IP devices are connected on the same system, a direct LAN connection between them will be established once the call has been set up (as long as they use the same Codecs). Direct media is supported between Avaya IP phones and SIP phones.

It is possible for an IP device to be physically located at one site while being registered at a different site. In this case, even for VoIP across the WAN the VCM would not be used, as long as the two IP devices involved in a phone call are registered on the same system.

 

© 2010 AVAYA
15-601041 Issue 20.f.-
18:14, 25 February 2010
(what_are_vcms_for.htm)

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Last Modified: 26/06/2009