H323 Install Page 10
15-601046 Issue 14j (10 March 2009)IP Office 4.21.3 System Capacity
System capacity can be separated into two aspects; the number of configurable phone extensions and the number of
simultaneous IP phone calls.
The maximum number of H323 IP phones supported by an IP Office system is based on that system's maximum capacity
for extensions of any type as listed in the table below. To find the capacity for IP phones remove the number of physical
non-IP extensions installed on the system, ie. extension ports on the IP Office control unit and any external expansion
IP Office Unit
Small Office Edition
28 Total/16 IP
3 or 16
IP Office 500
1.The maximum extension capacity is 28 for all phone types but only 16 may be IP phones.
2.Fixed non-adjustable capacity.
There are a number of situations where the IP Office system needs to provide a voice compression channel in order for
an IP phone to make calls. These channels are provided by Voice Compression Modules (VCMs) installed in the IP Office
system. The number of VCM channels required and how long the channel is required will depend on a number of factors.
For further details see Voice Compression .
A simple summary is:
·A VCM channel is required during call setup.
·The VCM channel is released if the call is to/from another IP device using the same compression codec (the
supported VCM codecs are G711, G729 and G723a).
·The VCM channel is used for the duration of the call when the call is to/from/via a non-IP device (extension or
·It should be remembered that VCM channels are also used for calls from non-IP devices to IP lines if those are
configured in the IP Office system (IP, SIP and SES lines).
·Calls from IP phones to the IP Office voicemail server use a VCM channel.
·Note that on Small Office Edition systems with Embedded Voicemail, an additional channel is used for every
call to voicemail.