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.

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

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

CallManager

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 1C

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.

• This feature cannot busy back the local PBX trunk based on the state of the telephony trunk on the remote node; it monitors IP network only.

• SAA probe-based features do not work well in networks where traffic load fluctuates dramatically in a short period of time.

• As with LVBO, this feature can be applied only to analog and CAS trunks; CCS trunks are not yet supported.

Table 8-10 evaluates the AVBO mechanism against the CAC evaluation criteria described earlier in this chapter.

Table 8-10 AVBO CAC Evaluation Criteria

Table 8-10 evaluates the AVBO mechanism against the CAC evaluation criteria described earlier in this chapter.

Table 8-10 AVBO CAC Evaluation Criteria

Evaluation Criteria

Value

VoX supported

VoIP only

Toll bypass or IP telephony

Toll bypass (calls originating from PBX and terminating to IP telephony destinations)

Platforms and Releases

2600s, 3600s, and MC3810 with Release 12.1(3)T All router platforms with Release 12.2 Mainline

PBX trunk types supported

Analog and CAS

End to end, local, or IP cloud

IP cloud

Per call, interface, or endpoint

Per IP destination

Topology awareness

None

Guarantees QoS for duration of call

None

Postdial delay

None

Messaging network overhead

Periodic SAA probes

PSTN Fallback

PSTN fallback allows the originating gateway to redirect a call request based on the measurement of an SAA probe. The name PSTN fallback is to some extent a misnomer because a call can be redirected to any of the rerouting options discussed earlier in this chapter, not only to the PSTN. In the event that a call is redirected to the PSTN, redirection can be handled by the outgoing gateway itself, or redirection can be performed by the PBX that is attached to the outgoing gateway. For this reason, this feature is sometimes referred to as VoIP fallback.

Unlike AVBO, PSTN fallback is a per-call CAC mechanism. PSTN fallback does not busy out the TDM trunks or provide any general indication to the attached PBX that the IP cloud cannot take calls. The CAC decision is triggered only when a call setup is attempted.

Because PSTN fallback is based on SAA probes, it has all the benefits and drawbacks of a measurement-based technique. It is unusually flexible in that it can make CAC decisions based on any type of IP network. All IP networks will transport the SAA probe packet as any other IP packet. Therefore it does not matter whether the customer backbone network comprises one or more service provider (SP) networks, the Internet, or any combination of these network types. The only requirement is that the destination device supports the SAA responder functionality.

Although PSTN fallback is not used directly by IP Phones and PC-based VoIP application destinations, it can be used indirectly if these destinations are behind a Cisco IOS router that supports the SAA responder.

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