Manual
PSTN Fallback Enhancement
Feature Overview
2
Cisco IOS Release 12.2(2)XA
SAA is a network congestion analysis mechanism that provides delay, jitter, and packet loss information
for the configured IP addresses. SAA is based on a client/server protocol defined on the User Datagram
Protocol (UDP). UDP is a connectionless transport layer protocol in the TCP/IP protocol stack. UDP is
a simple protocol that exchanges datagrams without acknowledgments or guaranteed delivery, requiring
that error processing and retransmission be handled by other protocols. The SAA probe packets go out
on randomly selected ports from the top end of the audio UDP port range.
The information that the SAA probes gather is used to calculate the ICPIF or delay/loss values that are
stored in a fallback cache, where they remain until the cache ages out or overflows. Until an entry ages
out, probes are sent periodically for that particular destination. This time interval is user configurable.
With this feature enhancement, you can also configure codes that indicate the cause of the network
rejection; for example, packets that are lost or that take too long to be transmitted. A default cause code
of 49 displays the message “Quality of Service Unavailable.”
Benefits
The PSTN Fallback feature and enhancement provide the following benefits:
• Automatically routes a call to an alternate destination when the data network is congested at the time
of the call setup.
• Enables the service provider to give a reasonable guarantee about the quality of the conversation to
its Voice over IP (VoIP) users at the time of call admission.
• Provides delay, jitter, and packet loss information for the configured IP addresses.
• Caches call values from previous calls. New calls do not have to wait for probe results before they
are admitted.
• Enables a user-configurable cause code display that indicates the type of call rejection.
Restrictions
The PSTN Fallback feature has the following restrictions:
• When detecting network congestion, the PSTN fallback feature does nothing to the existing call. It
affects only subsequent calls.
• Only a single ICPIF/delay-loss value is allowed per system.
• A small additional call setup delay can be expected for the first call to a new IP destination.
• PSTN fallback is supported for H.323 VoIP calls only.
Caution Configuring call fallback active in a gateway creates an SAA jitter probe against other (target)
gateways to which the calls are sent. In order for the call fallback active to work properly, the target
gateways must have the rtr responder command (in Cisco IOS releases prior to 12.3(14)T) or the ip
sla monitor responder command (in Cisco IOS Release 12.3(14)T or later) in their configurations.
If one of these commands is not included in the configuration of each target gateway, calls to the
target gateway will fail.