You may configure SGAC rules and thresholds so that the
high-priority voice (911) traffic receives an exclusive share of bandwidth.
Because the average call volume for Emergency 911 traffic may not be very high,
the fraction of bandwidth reserved for Emergency 911 calls may be small. In the
case of regional emergency, the call volume of Emergency 911 calls may surge.
In this case, it may be necessary to preempt some of the normal voice traffic
to make room for surging Emergency 911 calls.
The Cisco CMTS software preempts one or more normal-priority
voice flows to make room for the high-priority voice flows. SGAC provides the
command-line interface (CLI) to enable or disable this preemption ability.
SGAC preemption logic follows the following steps:
- When the first pass of
admission control fails to admit a high priority PacketCable flow, it checks if
it is possible to admit the flow in another bucket configured for normal
PacketCable calls (applicable only if the PacketCable normal and high-priority
rules are configured for different buckets). If the bandwidth is available, the
call is admitted in the normal priority bucket.
- If there is no room in
normal priority bucket, it preempts a normal priority PacketCable flow and
admits the high priority flow in the bucket where the low priority flow was
- If there is no normal
priority flow that it can preempt, it rejects the admission for high-priority
flow. This usually happens when both normal and high-priority buckets are
filled with 911 flows.
is effective only for PacketCable high-priority flows.
When a downstream low-priority
service flow is chosen for preemption, the corresponding service flow for the
same voice call in the opposite direction gets preempted as well.