Calls to and from IP devices can require conversion to the audio codec format being used by the IP device. For IP Office systems this conversion is done by voice compression channels. These support the common IP audio codecs G.711, G.722, and G.729a.
For the IP500 V2 control units, channels can be added using IP500 VCM cards and IP500 Combination Cards.
IP Office Server Edition systems provide their own voice compression channels through software without requiring additional hardware.
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.
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.
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
IP Office 5.0+ supports T38 fax on SIP trunks and SIP extensions. 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 H.323 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.
Note:
T3 IP devices must be configured to 20ms packet size for the above conditions to apply. If left configured for 10ms packet size, a voice compression channel is needed for all tones and for non-direct media calls.
Measuring Channel Usage
The IP Office 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.
For IP500 VCM cards, the level of channel usage is also indicated by the LEDs (1 to 8) on the front of the IP500 VCM card.
