Information Technology Reference
In-Depth Information
Call Admission Control (CAC), as explained in the next section, should be imple-
mented, either CUCM location-based or Resource Reservation Protocol (RSVP)-
based. Regardless of the CAC method used, the goal is to prevent more calls than the
design specifies from being extended across the WAN, thereby protecting call qual-
ity for all IP calls.
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PSTN Backup Using CAC
CAC prevents IP calls from being extended across a WAN link, if the additional WAN
bandwidth required would exceed the Quality of Service (QoS)-allocated bandwidth for
concurrent calls. For example, if the design specifies a maximum of ten calls using G.729,
the QoS configuration would create a Priority Queue (LLQ) sized to serve those ten calls.
(The QoS configurations are beyond the scope of the CCNA Voice exam.) If an 11
th
call
were extended to the gateway, the additional bandwidth would overrun the input buffer
for the LLQ, and packets from all 11 calls would start to drop, causing unacceptable
packet loss and resultant deterioration of voice quality for all 11 calls.
Key
To p i c
With location-based CAC implemented, CUCM tracks how many calls are extended to a
given location, subtracting the bandwidth used for each concurrent call from the band-
width specified for that location. When the remaining bandwidth is less than the amount
used by a single call (which varies based on the codec used for the call, which is in turn
defined by the Region setting), the default behavior of CAC is to drop the call. Users get
re-order tone or an annunciator message indicating that the call has failed.
In most cases, dropping the call is not desirable, and Automated Alternate Routing (AAR)
is implemented to reroute the call across the PSTN. AAR is exclusively triggered by CAC;
when CAC prevents the call from extending across the WAN, CUCM checks to see if an
AAR Group is configured for the calling phone. If an AAR Group is configured, it speci-
fies what digit manipulation is required to retry the call with a full PSTN Dialed Number
Information Service (DNIS). The operation is transparent to the user (although it is possi-
ble that he might notice a difference in call quality using the PSTN instead of the WAN).
If the call is extended over the PSTN, the call-signaling path includes the PSTN gateways
at the main and branch locations, and the RTP voice streams are converted to the appropri-
ate PSTN transport.
Note:
Be clear about the difference between PSTN rerouting due to WAN failure versus
CAC and AAR being triggered by a lack of available WAN bandwidth for calls. If the
WAN fails, calls may be rerouted to the PSTN using the hierarchical design of the dial plan
and CFUR on the CUCM side, and SRST on the branch side. If the WAN has no available
bandwidth, CAC triggers AAR, which will redial the call with a full PSTN number, but
only if it is correctly configured to do so. Figure 10-6 summarizes the sequence of events
if the WAN fails.
Distributed Deployment Call Flow
In a distributed deployment, one CUCM cluster signals another CUCM cluster across a
WAN. The signaling flows from the calling phone to the local CUCM, then from the local
CUCM across the WAN to the remote CUCM, then from the remote CUCM to the
