Avaya Phone System

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Voice compression module (VCM)

The IP Office can be treated as having a telephony switching interface (its TDM bus) and a data networking interface (its Packet bus).
• TDM Bus: Used by devices attached to DS and POT ports (analog and digital telephones) and by external trunks. Also used by internal devices such as the Tone Generator, MOH source and Conference Chip.
• Packet Bus: Used by IP devices attached to the system’s LAN and WAN ports. This includes IP extensions(IP phones)and IP trunks.
Calls between the two buses will use available voice compression channels. When no voice compression channel is available, busy indication is returned to the calling device.

From/ To	Voice Compression Channels
IP Phone and Truck	Non-IP Phone or Trunk	1 voice compression channel is required for duration of call
IP Phone or Trunk	IP Phone or Trunk	1 voice compression channel is used during call setup. Once the call is connected: If the devices use the same audio codec, and are set to “Allow Direct Media Path”, the VCM channel is no longer used. If the devices use differing audio codes, a vcoice compression channel is required for each.
Non-IP Phone or Trunk	Non-IP Phone or Trunk	No channel used
Conference Calls		1 voice compression channel per IP phone. Conference calls are managed by the conference chip which is on the IP Office´s TDM bus. Therefore, a voice compression channel is required for each IP device involved in the conference.
Page Calls		1 Voice compression channel per audio codec being used by any IP devices involved
Voicemail Calls		Calls to the IP Office voicemail servers (Voice Mail Pro. Voicemail Lite and Embedded Voicemail) are treated as data calls from the TDM bus. Therefore the servers can be treated devices to determinate whether a VCM channel is required or not. On the Small Office Edition, each call to the embedded voicemail card uses a voice compression channel. This is addition to any voice compression channel required for the call to reach the TDM bus.