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Measurement-Based Voice CAC 571

The following limitations apply to the LVBO feature:

It has local visibility only in current software (Cisco IOS Release 12.2); it monitors only Ethernet LAN interfaces (not Fast Ethernet), serial interfaces, and ATM interfaces.

It applies only to analog and CAS trunk types.

Table 8-9 evaluates the LVBO mechanism against the CAC evaluation criteria described earlier in this chapter.

Table 8-9

LVBO CAC Evaluation Criteria






Evaluation Criteria






VoX supported






Toll bypass or IP telephony

Toll bypass (calls originating from PBX and terminating to



IP telephony destinations)





Platforms and releases

Cisco 2600 and 3600 series routers, MC3810 multiaccess



concentrators; Cisco IOS Release 12.1(2)T





PBX trunk types supported

Analog and CAS





End to end, local, or IP cloud






Per call, interface, or endpoint

Per WAN, LAN, and telephony interface





Topology awareness






Guarantees QoS for duration of call






Postdial delay






Messaging network overhead





Measurement-Based Voice CAC

This section focuses on the following measurement-based CAC techniques:

Advanced Voice Busyout (AVBO)

PSTN fallback

These are the first of two types of CAC mechanisms that add visibility into the network itself, in addition to providing local information on the originating gateway as discussed in the preceding sections.

Before we discuss the actual features within this category, some background information on service assurance agent (SAA) probes is necessary, because this is the underlying technique used by the measurement-based CAC methods. SAA probes traverse the network to a given IP destination and measure the loss and delay characteristics of the network along the path traveled. These values are returned to the originating gateway to use in making a decision on the condition of the network and its capability to carry a voice call.