VCM channels

Last Updated : Nov 03, 2015 |
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IP Office uses voice compression modules (VCMs) to convert media traffic between IP and TDM. Some examples of when VCM channels are required in the branch are:
  • Each call between an IP endpoint or trunk and a TDM endpoint or trunk
  • Each call between an IP Office analog or digital endpoint and a device behind the SM Line, including calls to phones in other sites, centralized phones within the branch, and centralized trunks or voice mail systems through the Avaya Aura® Session Manager
  • Each call between an IP Office ISDN or analog PSTN trunk and a device behind the SM Line, including calls between centralized SIP users within the branch local ISDN or analog PSTN trunks.
Calls between IP Office users with IP endpoints and entities behind the SM Line are considered IP device to IP device calls. These calls do not typically require VCM channels unless no common codec can be selected on the two sides.
The following table describes general usage of VCM channels within IP Office environments.
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, and so on) 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 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 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
For page calls, IP Office 4.0 and later can use any codec, except G722. IP Office requires one channel for each codec type used by the target phones.
Voice mail services and IP devices
Calls to the IP Office voice mail servers are treated as data calls from the TDM bus. Therefore calls from an IP device to an IP Office voice mail system require a voice compression channel. This does not apply to central voice mail systems over the SM line.
T38 fax calls
In order to use T38 fax connection, fax tone detection is performed if the analog extension connected to the fax machine is set as “Standard telephone.” If the fax machine does not include an attached handset that is used to make/receive voice calls, then the Equipment Classification of an analog extension connected to the fax machine can be set to Fax Machine, which will result in T38 fax connection without fax tone detection and respective signaling renegotiation. 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 channels in use. It also displays how often there have been insufficient channels available and the last time such an event occurred.
For the VCM cards, the level of channel usage is also indicated by the LEDs (1 to 8) on the front of the VCM card.

Calculation guidelines

On the IP Office, you can configure the following number of VCM modules:
  • Up to 2 optional VCM 32 modules or VCM 64 modules, providing 32 and 64 VCM channels each
  • Up to 2 combination cards, providing 10 VCM channels each
With 4 modules, the maximum VCM channel capacity is 148.
The calculation of how many VCM channels are required in a particular deployment depends on the type and number of phones in the branch, the types of trunks used, the traffic patterns or typical call flows, as well as typical call frequency and length. For example, if most phones are centralized SIP phones and most calls are made to local ISDN or analog PSTN trunks, then VCM channels will be needed for most calls. However, VCM channels will not be needed as often if most phones are analog or digital phones and most calls are made to local ISDN or analog PSTN trunks.
As a general rule, the number of required VCM channels is equivalent to the number of IP Office IP phones (not including centralized phones) divided by three, plus the number of SIP trunk channels used for simultaneous calls over the SM Line. However, this calculation rule can vary depending on the deployment and traffic patterns of the enterprise.
Related links
Capacity and scalability