Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз: Предмет: Файл:
DQOS Exam Certification Guide - Cisco press.pdf
12.7 Mб

Measurement-Based Voice CAC 575

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

Evaluation Criteria




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




Guarantees QoS for duration of call




Postdial delay




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

576 Chapter 8: Call Admission Control and QoS Signaling

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.

SAA Probes Used for PSTN Fallback

When a call is attempted at the originating gateway, the network congestion values for the IP destination are used to allow or reject the call. The network congestion values for delay, loss, or ICPIF are obtained by sending an SAA probe to the IP destination the call is trying to reach. The threshold values for rejecting a call are configured at the originating gateway. Figure 8-14 illustrates this concept.

Figure 8-14 PSTN Fallback










SAA Probe sent across the IP network.

















SAA Probe response. Congestions detected

















across the WAN; ICPIF or delay/loss

















exceeded threshold.



















IP Network



























































































































































Possible Destinations for Redirection:












Alternate IP Destinations

Gateway Connection to PSTN

• Hairpin Call to PBX











































(CAS and Analog)






































• Reject Call to PBX (ISDN)







• Reorder Tone















IP Destination Caching

Unlike AVBO, PSTN fallback does not require the static configuration of the IP destinations. The software keeps a cache of configurable size that tracks the most recently used IP destinations to which calls were attempted. If the IP destination of a new call attempt is found in the cache, the CAC decision for the call can be made immediately. If the entry does not appear in the cache, a new probe is started and the call setup is suspended until the probe response arrives. Therefore, an extra postdial delay is imposed only for the first call to a new IP destination. Figure 8-15 illustrates these possible scenarios.

Measurement-Based Voice CAC 577

Figure 8-15 PSTN Fallback Call Setup

Call Setup


Call Setup

2Scenario Call Reject / Hairpin

Call Setup 3Scenario


IP Network



Cache entry for IP address found; CAC values are acceptable.

Call Setup

Call Setup

Cache entry for IP address found; CAC values not acceptable and no secondary dial-peer exists.

Cache entry for IP address not found.

SAA Probe

SAA Response

Call Setup


Call Setup

Figure 8-15 demonstrates three possible scenarios. In all scenarios, a call setup message is send to router 1 (R1). R1 consults its cache to determine whether a path exists and, if so, that the ICPIF or delay/loss thresholds have not been exceeded. In scenario one, both conditions are true and the call setup message is forwarded to router 2 (R2) to connect the call. In scenario two, a path to the IP destination is found in cache; however, the ICPIF or loss/delay exceed the threshold for that path and the call is either rejected or hairpinned back to the origination PBX, depending on the interface type connecting the PBX with R1. In scenario three, a path to the IP destination is not found in cache. An SAA probe is sent to the IP destination to determine the ICPIF or loss/delay values. If the response shows that the thresholds have not been exceeded, the call setup message is forwarded on to router 2 (R2) to connect the call.

After an IP destination has been entered into the cache, a periodic probe with a configurable timeout value is sent to that destination to refresh the information in the cache. If no further calls

578 Chapter 8: Call Admission Control and QoS Signaling

are made to this IP destination, the entry ages out of the cache and probe traffic to that destination is discontinued. In this way, PSTN fallback dynamically adjusts the probe traffic to the IP destinations that are actively seeing call activity.

SAA Probe Format

Each probe consists of a configurable number of packets. The delay, loss, and ICPIF values entered into the cache for the IP destination are averaged from all the responses.

If an endpoint is attempting to establish a voice conversation using a G.729 or G.711 codec, the probe’s packet size emulates the codec of the requesting endpoint. Additional codecs use G.711-like probes.

The IP precedence of the probe packets can also be configured to emulate the priority of a voice packet. This parameter should be set equal to the IP precedence used for other voice media packets in the network. Typically the IP precedence value is set to 5 for voice traffic.

PSTN Fallback Configuration

PSTN fallback is configured on the originating gateway and applies only to calls initiated by the originating gateway. Inbound call attempts are not considered. PSTN fallback is configured at the global level and therefore applies to all outbound calls attempted by the gateway. You cannot selectively apply PSTN fallback to calls initiated by certain PSTN/PBX interfaces. The SAA responder feature is configured on the destination node, also referred to as the terminating gateway.

To apply PSTN fallback, enter the following global configuration commands:

Originating gateway: the call fallback command

Destination node: the rtr responder command

Table 8-11 lists the options and default values of the call fallback command.

Table 8-11 Call Fallback Command

Call Fallback Command









cache-size x

Specifies the call fallback cache size for network traffic



probe entries.





cache-timeout x

Specifies the time after which the cache entries of



network conditions are purged.





instantaneous-value-weight x

Specifies that the call fallback subsystem take an average



from the last two cache entries for call requests.





jitter-probe num-packets x

Specifies the number of packets in a jitter probe used to



determine network conditions.





Measurement-Based Voice CAC 579

Table 8-11 Call Fallback Command (Continued)

Call Fallback Command









jitter-probe precedence x

Specifies the priority of the jitter-probe transmission.





jitter-probe priority-queue

Assigns a priority queue for jitter-probe transmissions.






Specifies MD5 authentication for sending and receiving



SAA probes.





map subnet

Specifies that the call fallback router keep a cache table



by subnet addresses of distances for several destination



peers sitting behind the router.





probe-timeout x

Sets the timeout for an SAA probe for call fallback








threshold delay x loss y

Specifies that the call fallback threshold use only packet



delay and loss values.





Icpif x

Specifies that the call fallback use the ICPIF threshold.





Examples 8-13 and 8-14 demonstrate PSTN fallback between a host and remote site. SAA is configured on the remote site to answer the probes from the host site. When the number 1234 is dialed from the host site and congestion is observed on the link between the host site and the remote site, the call is redirected to port 3/0:23 and connected to the remote site over the PSTN.

Probes are sent every 20 seconds with 15 packets in each probe. The probes share the priority queue with the other voice packets. The delay and loss threshold command is configured with a delay threshold of 150 ms and a loss threshold of 5 percent and the cache aging timeout is configured for 10,000 seconds.

Example 8-13 shows the configuration for the host site router.

Example 8-13 Call Fallback Host Site Configuration

Hostname Host-Site


call fallback probe-timeout 20

call fallback threshold delay 150 loss 5 call fallback jitter-probe num-packets 15 call fallback jitter-probe priority-queue call fallback cache-timeout 10000

call fallback active


interface Serial1/0

ip address


interface Serial3/0:23 no ip address


580 Chapter 8: Call Admission Control and QoS Signaling

Example 8-13 Call Fallback Host Site Configuration (Continued)

no logging event link-status isdn switch-type primary-ni isdn incoming-voice voice no cdp enable


voice-port 3/0:23


dial-peer voice 100 voip destination-pattern 12.. preference 1

session target ipv4:


dial-peer voice 10 pots destination-pattern 12.. preference 2

port 3/0:23

!Adds prefix in front of the dialed number route over the PSTN prefix 9140455512


dial-peer voice 20 pots destination-pattern 9T port 3/0:23

Example 8-14 shows the configuration for the remote site router.

Example 8-14 Call Fallback Remote Site Configuration for Host Name Remote Site


interface Serial1/0

ip address


interface Serial3/0:23 no ip address

no logging event link-status isdn switch-type primary-ni isdn incoming-voice voice no cdp enable


voice-port 3/0:23


dial-peer voice 100 voip destination-pattern 5...

preference 1

session target ipv4:


dial-peer voice 10 pots destination-pattern 5...

preference 2 port 3/0:23