Calls to and from IP devices require conversion to the audio codec format being used by the IP device. For IP Office systems, that conversion is done by voice compression channels as detailed in the table below.

For IP500 V2A control units, channels can be added using IP500 VCM cards and IP500 Combination Cards. These support the common IP audio codecs: G.722, G.711, G.723, G.729a and G.729b. Note: G.729b is not supported on Linux-based IP Office systems.

The voice compression channels are used as follows:

Call Type

Voice Compression Channel Usage

IP Device to Non-IP Device

These 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 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 G723 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 Device

No voice compression channels are required.

Music on Hold

This is provided from the IP Office's TDM bus and therefore requires a voice compression channel when played to an IP device.

Conference Resources and IP Devices

Conferencing resources 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 a conference. This includes services that use conference resources such as call listen, intrusion, call recording and silent monitoring.

Page Calls to IP Device

IP Office 4.0 and higher only uses G729a for page calls, therefore only requiring one channel but also only supporting pages to G729a capable devices.

Voicemail Services and IP Devices

Calls to the IP Office 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. IP Office only supports fax across IP between IP Office systems with the Fax Transport option selected. It does not currently support T38.

T38 Fax Calls

Each T38 fax call uses a VCM channel.

Within a Small Community Network, a T38 fax call can be converted to a call across an H323 SCN lines using the IP Office 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, a new short code feature Dial Fax is available.

Measuring Channel Usage

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.

On IP500 VCM cards, channel usage is also indicated by the LEDs (1 to 8) on the front of the IP500 VCM card.