Edgewater Networks 4200 Series - Enable CAC

4200 User Manual Edgewater Networks, Inc.
Version 3.0 85
Enable CAC
The 4200 uses CAC to limit the number of active voice calls over the WAN link. This
is necessary because a typical installation uses a ratio of 1:2 or 1:4 active voice calls
to voice devices on the assumption that 50% or 25% of all users are on the phone at
the same time. These ratios are guidelines only and at times the number of
concurrent calls may exceed the amount of WAN bandwidth available to process the
calls. In this instance existing phone calls will experience poor quality or be dropped
all together. To prevent this from occurring a typical voice installation will set a
threshold for the maximum number of concurrent voice calls supported by the WAN
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 4200 by enabling CAC. When the 4200 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 4200. Please check with your IT administrator to determine if this is the case.
PLEASE note that CAC is available in the 4200 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 4200 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 (1.544 Kbps) WAN is
calculated as follows:
(1544*.85)/85.6 = 15.3 or 15 total voice calls.
The maximum number of G.711 voice calls supported by a 768Kbps SDSL WAN is
calculated as follows: