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IP Office Installation Page 56
15-601042 Issue 20b (16 July 2009) IP Office
2.30 Voice Compression Channels
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 G711, G723
and G729a.
For IP400 control units channels can be added by fitting IP400 Voice Compression Modules (VCMs) . For the IP500 control
units, channels can be added using IP500 VCM cards and licenses or IP400 Voice Compression Modules.
When are Voice Compression Channels Used
The voice compression channels are used as follows.
•
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 except for Small Office Edition Embedded Voicemail access.
•
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
Page calls require 1 voice compression channel per audio codec being used by any IP devices involved. 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.
•
On the Small Office Edition, embedded voicemail uses voice compression channels for audio conversion.
Therefore all calls to Small Office Edition embedded voicemail require a voice compression channel and calls
from IP devices require two voice compression channels.
•
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.
•
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
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, an T38 fax call can be converted to a call across 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.