Voice Compression Module (VCM) channels allow an IP500 V2 to convert media (for example speech) between analog/digital and IP. These are essential when routing analog/digital trunk calls to or from VoIP endpoints.

• It is important to note that in a Linux-based server network, media communication with any other server components requires the use of VoIP, including Primary, Secondary, other expansions, call recording, attendants, IVR, conferencing and voicemail.

• Local IP500 V2 conferences and music-on-hold use the digital domain; hence all VoIP parties (trunk or extension) require a VCM channel.

• VCM channels are also used to perform VoIP transcoding. Transcoding is used where the VoIP codec differs between two legs of a call; for example a VoIP endpoint supporting only G.729 calling a SIP trunk with only G.711. This case uses 2 VCM channels and should be avoided wherever possible.

The following table summarizes VCM channel usage.

Endpoint A	Endpoint B	Channels used [1]	Notes
Analog/Digital trunk or extension	Analog/Digital trunk or extension	None	Avaya Wireless DECT endpoints are classified as VoIP
	Local Conference	None	Conference hosted on the IP500 V2
	Local Music on Hold	None
	Embedded Voicemail	None	Includes voicemail, attendants, announcements.
Analog/Digital trunk or extension	VoIP trunk or extension	1
	Central Voicemail	1	Includes voicemail, IVR attendants, announcements
	Remote Conference	1
	Remote Music on Hold	1	Maximum of 3 MOH sources streamed from Primary Server using a maximum of 3 VCM channels
	Call recording	1	Using Voicemail Pro or ACCS.
VoIP trunk or extension	VoIP trunk or extension	None[2]	VoIP endpoints includes IP Office Line (SCN trunk), SM and H323 lines, DECT endpoints
	Central Voicemail	None[2]	Includes voicemail, IVR attendants, announcements
	Remote Conference	None[2]
	Remote Music on Hold	None[2]	Streamed from Primary Server
	Call recording	None[2]	Using Voicemail Pro or ACCS.
VoIP trunk or extension	Analog/Digital trunk or extension	1
	Local Conference	1	Conference hosted on the IP500 V2
	Local Music on Hold	1 per MOH source[2]	Maximum of 4 MOH sources. One VCM channel is used per codec type per source.
	Embedded Voicemail	1	Includes voicemail, attendants, announcements

1. Unless otherwise specified, the VCM channel is used for the duration of the call and the VCM resource is always local.

2. Assumes both endpoints’ VoIP codecs match, if they do not match 2 VCM channels are used.

Three base card types provide VCM channel capacity for the IP500 V2 control units:

• VCM 32

• VCM 64

• Combination card

Each base card can carry a trunk module, however the Combo card can only support BRI and analog. Hence, if more than two dual PRI cards are required, the VCM capacity is reduced. Also note that the type of trunk module fitted to the Combo card is fixed.

The following table shows various constructs and the resulting theoretical maximum. Not all the variants are listed. Only those variants that provide the maximum capacity are listed.

Base Card #1	Base Card #2	Base Card #3	Base Card #4	Maximum G.711 calls	Maximum G.729 calls	Maximum G.723 calls	Maximum G.722 calls
VCM 64	VCM 64	-	-	128	120	88	120
VCM 64	VCM 64	Combo	-	138	130	98	130
VCM 64	VCM 64	Combo	Combo	148	140	108	140

The capacity in the above table is for a bidirectional channel between a VoIP and an analog or digital endpoint and assumes the calls are of the same codec type. Differing codec types can be supported at the same time; the lowest channel figure should be used for calculations.

If VCM channels are used to convert SRTP media, a maximum of 40 calls per system are supported regardless of codec type.

The IP500 V2 control unit manages this common resource as efficiently as possible but if there are insufficient at any one time:

• Outgoing calls will not get connected (they do not receive dial tone)

• Incoming calls will queue until a VCM channel is free

• Transfers cannot be made