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Local Voice CAC 569




Table 8-8

Trunk Conditioning CAC Evaluation Criteria





Evaluation Criteria






VoX supported

VoIP/H.323, VoFR, VoATM (connection trunk configurations








Toll bypass or IP telephony

Toll bypass applications only





Platforms and Releases

Cisco 2600 and 3600 series routers, and Cisco MC3810



multiaccess concentrators; Cisco IOS Release 12.1(3)T





PBX trunk types supported

Analog and CAS





End to end, local, or IP cloud






Per call, interface, or endpoint

Per telephony interface





Topology awareness






Guarantees QoS for duration of call






Postdial delay






Messaging network overhead

None; uses preexisting connection trunk keepalives




Local Voice Busyout

Several CAC mechanisms generate a busy signal to the originating PBX to indicate that an alternate route must be found to successfully place a call. The preceding section discussed trunk conditioning, which operates on connection trunk networks only. Similar functionality is needed for switched networks. Local Voice Busy-Out (LVBO) is the first of two features that achieve this.

LVBO enables you to take a PBX trunk connection to the attached gateway completely out of service when WAN conditions are considered unsuitable to carry voice traffic. This technique has the following advantages:

With the trunk out of service, each call will not be rejected individually and incur a postdial delay.

Prevents the need for hairpinning-rejected calls back to the originating PBX, using up multiple DS0 slots for a single call.

Works well to redirect rejected calls with PBXs that either do not have the intelligence or are not configured appropriately.

Prevents a third DS0 on the same T1/E1 circuit from accepting the original call if the call is hairpinned back to the gateway from the originating PBX. This condition is referred to as tromboning.

LVBO provides the originating gateway with the capability to monitor the state of various network interfaces, both LAN and WAN, and signal the originating PBX to use an alternate

570 Chapter 8: Call Admission Control and QoS Signaling

route should any of the monitored links fail. If any or all of the interfaces change state, the gateway can be configured to busy-back the trunk to the PBX. The reason this feature is called Local VBO is because only local links can be monitored. This feature has no visibility into the network beyond the link of the local gateway.

LVBO in current software works on CAS and analog PBX/PSTN trunks only. On CCS trunks, the cause code functionality can be used to inform the PBX switch to redirect a rejected call. LVBO can be configured in one of two ways:

To force individual voice ports into the busyout state

To force an entire T1/E1 trunk into the busyout state

Figure 8-12 illustrates the operation of the LVBO feature, and Example 8-11 shows the configuration necessary. In the example, the originating gateway is monitoring two interfaces, Ethernet interface e0/1 and WAN interface s0/1, on behalf of voice port 2/0:1, which is a T1 CAS trunk connected to a PBX. As shown in Figure 8-12, this feature is only applicable if the origination device is a PBX/PSTN interface, although the destination device can be anything, including an IP-capable voice device.

Figure 8-12 Local Voice Busyout


IP Network




Interface Failure










Example 8-11 Local Voice Busyout

Controller T1 2/0

ds0-group 1 timeslot 1-4 type e&m-wink-start

Voice-port 2/0:1

busyout monitor serial0/1 busyout monitor ethernet0/1