572 Chapter 8: Call Admission Control and QoS Signaling

Note the following attributes of measurement-based CAC mechanisms that are derived from their use of SAA probes:

•Because an SAA probe is an IP packet traveling to an IP destination, all measurementbased CAC techniques apply to VoIP only (including VoIP over Frame Relay and VoIP over ATM networks).

•As probes are sent into the network, a certain amount of overhead traffic is produced in gathering the information needed for CAC.

•If the CAC decision for a call must await a probe to be dispatched and returned, some small additional postdial delay occurs for the call. This should be insignificant in a properly designed network.

Service Assurance Agents

SAA is a generic network management feature that provides a mechanism for network congestion analysis, providing analysis for 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 capabilities to measure network delay and packet loss are useful as building blocks on which to base CAC features.

NOTE The SAA feature was called response time responder (RTR) in earlier releases of Cisco IOS Software.

SAA Probes Versus Pings

SAA probes are similar in concept to the popular ping IP connectivity mechanism, but are far more sophisticated. SAA packets can be built and customized to mimic the type of traffic for which they are measuring the network, in this case a voice packet. A ping packet is almost by definition a best-effort packet, and even if the IP precedence is set, it does not resemble a voice packet in size or protocol. Nor will the QoS mechanisms deployed in the network classify and treat a ping packet as a voice packet. The delay and loss experienced by a ping is therefore a very crude worst-case measure of the treatment a voice packet might be subject to while traversing the very same network. With the penetration of sophisticated QoS mechanisms in network backbones, a ping becomes unusable as a practical indication of the capability of the network to carry voice.

SAA Service

The SAA service is a client/server service defined on TCP or UDP. The client builds and sends the probe, and the server (previously the RTR responder) returns the probe to the sender. The