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Measurement-Based Voice CAC 573

SAA probes used for CAC go out randomly on ports selected from within the top end of the audio UDP-defined port range (16384 to 32767); they use a packet size based on the codec the call will use. IP precedence can be set if desired, and a full RTP/UDP/IP header is used like the header a real voice packet would carry. By default the SAA probe uses the RTCP port (the odd RTP port number), but it can also be configured to use the RTP media port (the even RTP port number) if desired.

SAA was introduced on selected platforms in Cisco IOS Release 12.0(7)T. With the release of 12.2 Mainline IOS, all router platforms support SAA; however, the IP Phones do not currently support SAA probes or respond to SAA probes.

Calculated Planning Impairment Factor

The ITU standardizes network transmission impairments in ITU G.113. This standard defines the term calculated planning impairment factor (ICPIF), which is a calculation based on network delay and packet loss figures. ICPIF yields a single value that can be used as a gauge of network impairment.

ITU G.113 provides the following interpretations of specific ICPIF values:

5: Very good

10: Good

20: Adequate

30: Limiting case

45: Exceptional limiting case

55: Customers likely to react strongly

SAA probe delay and loss information is used in calculating an ICPIF value that is then used as a threshold for CAC decisions, based either on the ITU interpretation described or on the requirements of an individual customer network.

Advanced Voice Busyout

AVBO is an enhancement to LVBO. Whereas LVBO provides for busyout based on local conditions of the originating gateway, AVBO adds the capability to trigger an SAA probe to one or more configured IP destinations. The information returned by the probe, which can be either the explicit loss and delay values, or the ICPIF congestion threshold, is used to trigger a busyout of the TDM trunk connection to the PBX.

AVBO therefore introduces the capability to busy out a PBX trunk, or individual voice ports, based on the current conditions of the IP network. Figure 8-13 illustrates this capability.

574 Chapter 8: Call Admission Control and QoS Signaling

Figure 8-13 Advanced Voice Busyout

 

 

SAA Probe sent across the IP network.

 

 

 

SAA Probe responds. Congestions detected

 

 

across the WAN; ICPIF or delay/loss

Busy-Out This Trunk

 

exceeded threshold.

 

 

 

 

 

R1

IP Network

R2

 

SW1

 

SW2

IP

 

IP

CallManager

 

CallManager

Example 8-12 shows a sample configuration of AVBO on a T1 CAS trunk connected to a PBX.

Example 8-12 Advanced Voice Busyout

controller T1 2/0

ds0-group 1 timeslots 1-4 type e&m-immediate-start

!

voice-port 2/0:1 voice-class busyout 4

!

voice class busyout 4 busyout monitor Serial0/1 busyout monitor Ethernet0/1

busyout monitor probe 1.6.6.48 codec g729r8 icpif 10

When using AVBO, remember the following restrictions and limitations:

Busyout results based on probes (measurement based) are not absolute. Some conditions, such as fleeting spikes in traffic, can cause a false positive to happen.

The IP addresses monitored by the probes are statically configured (as shown in the configuration example). It is necessary to ensure, manually, that these IP addresses are indeed the destinations to which calls are being made. There is no automatic coordination between the probe configuration and the actual IP destinations to which VoIP dial peers or a gatekeeper may direct calls.

The destination node (the device that owns the IP address to which the probe is sent) must support an SAA responder and have the rtr responder command enabled.