Canon H323 manual Call Type Voice Compression Channel Usage

IP Office H323 IP Phones: Network Assessment

1.9 Voice Compression

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, IP500 Combination Cards and or IP400 Voice Compression Modules.

The voice compression channels are used as follows:

Call Type	Voice Compression Channel Usage
IP Device to Non-IP	These calls require a voice compression channel for the duration of the call. If no channel is
Device	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-	No voice compression channels are required.
IP Device
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	Conferencing resources are managed by the conference chip which is on the IP Office's TDM
and IP Devices	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	Calls to the IP Office voicemail servers are treated as data calls from the TDM bus. Therefore
IP Devices	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 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.

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.

H323 IP Telephone Installation	Page 17
IP Office Release 6	15-601046 Issue 16a (15 March 2010)