VCM Channels

This form allows adjustment of the operation of any Voice Compression Modules (VCM’s) installed in a control unit.

Calls to and from IP devices can require conversion to the audio codec format being used by the IP device. For systems this conversion is done by voice compression channels. These support the common IP audio codecs G.711, G.723 and G.729a. For details of how to add voice compression resources to a system, refer to the IP Office Installation Manual.

These settings should only be adjusted under the guidance of Avaya support.

These settings are mergeable. Changes to these settings do not require a reboot of the system.

When are Voice Compression Channels Used

IP Device to Non-IP DeviceThese calls require a voice compression channel for the duration of the call. If no channel is available, busy indication is returned to the caller.

IP Device to IP DeviceCall 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.

Non-IP Device to Non-IP DeviceNo voice compression channels are required.

Music on HoldThis is provided from the system’s TDM bus and therefore requires a voice compression channel when played to an IP device.

Conference Resources and IP DevicesConferencing resources are managed by the conference chip which is on the system’s TDM bus. Therefore, a voice compression channel is required for each IP device involved in a conference. This includes services that use conference resources such as call listen, intrusion and silent monitoring. They also apply to call recording.

Page Calls to IP DevicePage calls require 1 voice compression channel per audio codec being used by any IP devices involved. The system only uses G.729a for page calls, therefore only requiring one channel but also only supporting pages to G.729a capable devices.

Voicemail Services and IP DevicesCalls to the system voicemail servers are treated as data calls from the TDM bus. Therefore calls from an IP device to voicemail require a voice compression channel.

Fax Calls These are voice calls but with a slightly wider frequency range than spoken voice calls. The system only supports fax across IP between systems with the Fax Transport option selected.

SIP Calls

  • SIP Line Call to/from Non-IP Devices: Voice compression channel required.

  • Outgoing SIP Line Call from IP Device: No voice compression channel required.

  • Incoming SIP Line Call to IP Device: Voice compression channel reserved until call connected.

T38 Fax Calls The system supports T38 fax on SIP trunks and SIP extensions. Each T38 fax call uses a VCM channel.

  • Within a multi-site network, an T38 fax call can be converted to a call across across an H.323 line between systems using the Fax Transport Support protocol. This conversion uses 2 VCM channels.

  • In order use T38 Fax connection, the Equipment Classification of an analog extension connected to a fax machine can be set Fax Machine. Additionally, the short code feature Dial Fax is available.

Measuring Channel Usability

The System Status Application can be used to display voice compression channel usage. Within the Resources section it displays the number of channel in use. It also displays how often there have been insufficient channels available and the last time such an event occurred.




Echo Return Loss (dB)

Default = 6dB. IP500 VCM, IP500 VCM V2 and IP500 Combination Cards. This control allows adjustment of expected echo loss that should be used for the echo cancellation process.

Echoes are typically generated by impedance mismatches when a signal is converted from one circuit type to another, most notably from analog to IP. To resolve this issue, an estimated echo signal can be created from one output and then subtracted from the input to hopefully remove any echo of the output.

The options are: 0dB3dB6dB and 9dB.

Nonlinear Processor Mode

Default = Adaptive. I

A low level of comfort noise is required on digital lines during periods where there would normally be just silence. This is necessary to reassure users that the call is still connected. These controls allow adjustment of the comfort noise generated by the nonlinear processor (NLP) component of the VCM. The options are:

  • Adaptive: Adaptive means the comfort noise generated by the NLP will try to match background noise.

  • Silence: Silence means the NLP will not generate comfort noise at all

  • Disabled: Nonlinear processing is not applied, in which case some residual echo may be heard.

NLP Comfort Noise Attenuation

Default = -9dB.

The options are: -3dB-6dB and -9dB.

NLP Comfort Noise Ceiling

Default =-30dB.

The options are: -30dB and -55dB.


For Fax relay, these settings allow adjustment of the TDM side operation applied to fax calls using VCM channels.

Tx Level (dB)

Default = -9dB. Range = 0 to -13dB.

CD Threshold

Default = -43dB, Options = -26dB, -31dB or -43dB.

No Activity Timeout (secs)

Default = 30 seconds. Range = 10 to 600 seconds.

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