User manual

4300T User Manual Edgewater Networks, Inc.
Version 1.7 39
access link. New call requests in excess of this threshold will receive the equivalent
of a “fast busy” and the WAN link will not become oversubscribed.
For IP Centrex installations the maximum number of concurrent voice calls is usually
configured in the 4300T by enabling CAC. When the 4300T is deployed in IP PBX
applications the maximum number of concurrent calls could be configured in the IP
PBX. If the PBX is responsible for this setting you do not need to configure CAC in
the 4300T. Please check with your IT administrator to determine if this is the case.
PLEASE note that CAC is available in the 4300T for the MGCP and SIP VoIP protocols
only.
Determining the maximum number of concurrent calls
The maximum number of concurrent calls that can be supported by the WAN access
link is calculated using the following formula:
Max calls = (Maximum WAN upstream bandwidth * .85)/VoIP codec rate
where,
Maximum WAN upstream bandwidth = value entered in step D above (in Kbps)
VoIP codec rate = 85.6Kbps for G.711 voice devices or 29.6Kbps for G.729 voice
devices.
The maximum WAN upstream bandwidth is multiplied by .85 in the formula above to
reduce the total bandwidth available for voice calls by 15%. This reduction is
necessary because the 4300T automatically reserves 15% of the total WAN
bandwidth for low priority data traffic so that it is not starved completely. Starving
data traffic completely would increase the number of retry attempts and exacerbate
congestion on the link during periods of peak usage.
Examples
The maximum number of G.711 voice calls supported by a T1 (1.536 Kbps) WAN is
calculated as follows:
(1.536*.85)/85.6 = 15.3 or 15 total voice calls.
The maximum number of G.711 voice calls supported by a 768Kbps fractional T1
WAN is calculated as follows:
(768*.85)/85.6 = 7.6 or 7 total voice calls
The maximum number of G.729 voice calls supported by a 256Kbps fractional T1
WAN is calculated as follows:
(256*.85)/29.6 = 7.4 or 7 total voice calls
After determining the maximum number of voice calls CAC is enabled as follows:
F. Select the Enable Call Admission Control checkbox.
G. Enter Maximum number of calls allowed as calculated above.