Measurement-Based CAC Mechanisms 129
Measurement-Based CAC Mechanisms
This section describes the following measurement-based CAC techniques:
Advanced voice busyout
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 outgoing gateway as discussed in the
preceding sections.
Before covering the actual features within this category, some background information on
Security Assurance Agent (SAA) probes is necessary, as this is the underlying technique
employed 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 outgoing gateway to use in making a decision
on the condition of the network and its ability to carry a voice call.
The following attributes of measurement-based CAC mechanisms are derived from their
use of SAA probes:
Because an SAA probe is an IP packet traveling to an IP destination, all measurement-
based CAC techniques apply to VoIP only (including VoFR and VoATM networks).
As probes are sent into the network, there is a certain amount of overhead traffic
produced in gathering the information needed for CAC.
If the CAC decision for a call has to await a probe to be dispatched and returned, there
is some small additional post-dial delay for the call. (This should be insignificant in a
properly designed network.)
Security Assurance Agents
Security Assurance Agents (SAA) is a generic network management feature that provides
a mechanism for network congestion analysis. It also underlies a multitude of other Cisco
IOS features. It was not implemented for the purpose of accomplishing CAC, nor is it a part
of the CAC suite. But its abilities to measure network delay and packet loss are
tremendously useful as building blocks on which to base CAC features. The SAA feature
was called Response Time Responder (RTR) in earlier releases of Cisco IOS Software.
9 Post-dial delay None
10 Messaging network overhead None
Table 4-11 Summary of Local Voice Busyout (Continued)
Evaluation Criteria Value

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