Measurement-Based Voice CAC 581

Example 8-14 Call Fallback Remote Site Configuration for Host Name Remote Site (Continued)

!Adds prefix in front of the dialed number route over the PSTN prefix 914085555

!

dial-peer voice 20 pots destination-pattern 9T port 3/0:23

!

rtr responder

!

With the configuration of Examples 8-13 and 8-14, 15 probes are placed in the priority queue of the host site router every 20 seconds. The responses received from the remote site router must not exceed 150 ms of delay or 5 lost packets for the host site routers to determine that this path will support the QoS necessary for a voice conversation. If this path is not used in 10,000 seconds, it is removed from the cache and a new set of probes have to be launched at the next request. The rtr responder command is enabled on the remote site router to enable responses to the probes.

PSTN Fallback Scalability

Examples 8-13 and 8-14 in the preceding section describe a simple network in which the remote site router acts as the terminating gateway for the voice conversation. The IP address of the remote site router and the network congestion values of the link between the remote and host router are held in the cache of the host site router. When the host site router receives the digits 12 followed by 2 additional digits, the host cache is consulted to determine whether the call can be successfully placed.

As the network becomes more complex, such as with the addition of IP telephony, it becomes necessary to designate a single terminating gateway to represent a number of IP destination devices. Consider the example illustrated in Figure 8-16. There are a large number of IP Phones at Site 6, each one having a unique IP address.

If Site 1 calls an IP Phone at Site 6, the cache at Site 1 does not need to contain an entry for each separate IP destination at Site 6. All IP call destinations at Site 6 can be mapped to the IP address of the WAN edge router at Site 6 so that a single probe from Site 1 to Site 6 can be used to probe CAC information for all calls destined to Site 6. The same principle applies if there were multiple terminating gateways at Site 6.

The probe traffic can therefore be reduced substantially by sending probes to IP destinations that represent the portion of the network that is most likely to be congested, such as the WAN backbone and WAN edge. This same scalability technique also provides a mechanism to support IP destinations that do not support SAA responder functionality.