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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.

582 Chapter 8: Call Admission Control and QoS Signaling

Figure 8-16 PSTN Fallback Scalability


SAA Probe Sent to R6 for All

Destinations in Site 6

Site 1



Site 4

IP Network



Site 2




Site 5



R6 Responds to




Probes for All




Destinations in Site 6

























Site 3



Site 6

PSTN Fallback Summary

PSTN fallback is a widely deployable, topology-independent CAC mechanism that can be used over any backbone. Consider the following attributes of PSTN fallback when designing a network:

Because it is based on IP probes, PSTN fallback applies to VoIP networks only.

PSTN fallback does not reroute calls in progress when network conditions change.

Measurement-Based Voice CAC 583

A slight increase in postdial delay will apply to the first call to a destination not yet in the cache.

No interaction occurs between the SAA probe timer and the H.225 timer setting: The SAA probe occurs before the H.323 call setup is sent to the destination, and the H.225 timer occurs after H.323 call setup is sent.

PSTN fallback performs well in steady traffic that has a gradual ramp-up and ramp-down, but poorly in quickly fluctuating traffic with a bursty ramp-up and ramp-down.

An erroneous CAC decision could be reached in a bursty environment based on noncurrent information due to the periodic nature of the probes.

Proxy destinations for the probes can be used by mapping destination IP addresses to a smaller number of IP addresses of the nodes located between the originating gateway and the terminating gateways.

No bandwidth measurements are taken by the probes, delay and loss measurements only.

MD5 keychain authentication can be configured for security to ensure that probes are initiated only by trusted sources, which will circumvent “denial-of-service” type attacks by untrusted sources initiating large volumes of probes.

Table 8-12 evaluates the PSTN fallback mechanism against the CAC evaluation criteria described earlier in this chapter.

Table 8-12 PSTN Fallback CAC Evaluation Criteria

Evaluation Criteria




VoX supported

VoIP only



Toll bypass or IP telephony

Toll bypass; however, calls originating from a PBX and


terminating to IP telephony destinations can be protected



Platforms and releases

Cisco 2600/3600, MC3810: Release 12.1.(3)T


AS5300: Release 12.2.(2)T


7200/7500 support SAA responder



PBX trunk types supported

All PBX/PSTN trunk signaling types (analog, digital CAS


and CCS)


For analog and digital CAS—alternate IP destination,




For digital CCS, reject the call to the PBX or PSTN for





End to end, local, or IP cloud

IP cloud



Per call, interface, or endpoint

Per active/cached IP destination