Enterprise QoS Solution Reference Network Design Guide

Chapter 3 WAN Aggregator QoS Design
CallManager CAC Limitations
Version 3.3
WAN Edge Link-Specific QoS Design
IP telephony in branch networks typically is based on the centralized call-processing model and uses
locations-based CAC to limit the number of calls across the WAN. Locations-based CAC currently does
not have any mechanism for tracking topology changes in the network. Therefore, if the primary link to
a branch goes down and ISDN backup engages, the CallManager remains ignorant of the occurrence. For
this reason, it is critical that the ISDN backup link be capable of handling the same number of VoIP calls
as the main link. Otherwise, CAC ultimately could oversubscribe the backup link.
The actual bandwidth of the primary link and the backup link do not need to be identical.
They just need to be capable of carrying the same number of VoIP calls. For example, the
backup link might use cRTP while the primary link does not, in which case, less bandwidth
is required on the backup link to carry the same number of calls as the primary link.
Because of these limitations, it is recommended that the 33 percent LLQ recommendation be relaxed in
this kind of dial-backup scenario. The LLQ could be provisioned as high as 70 percent (leaving 5 percent
for Voice control traffic over the ISDN link and 25 percent for Best-Effort traffic).
Voice and Data on Multiple ISDN B Channels
The Voice and Data design model over ISDN, illustrated in Figure 3-20, allows a service policy to be
applied to a bundle with multiple B channels. It takes advantage of the fact that LLQ bandwidth can be
expressed as a percentage instead of an absolute number. If cRTP is enabled, MCMP is required on the
ISDN links.
Figure 3-20 Voice and Data over ISDN
WAN Aggregator
ISDN Link
ISDN
Cloud
Branch Router
Cisco IOS provides two mechanisms for controlling how channels are added in response to demand.
The first mechanism commonly is referred to as dial-on-demand routing (DDR). With DDR, a load
threshold must be specified (as a fraction of available bandwidth). When the traffic load exceeds this
number, an additional channel is added to the bundle. The threshold is calculated as a running average. As
a result, there is a certain delay in bringing up additional B channels when the load increases. This delay
does not present a problem with data, but it is unacceptable with voice. This delay can be reduced to
around 30 seconds by adding the load-interval command to the physical ISDN interface, but even 30
seconds is too long.
The second mechanism is a more robust solution, which is simply to bring up all B channels immediately
and keep them up as long as the ISDN service is required. This is achieved by using the ppp multilink
links minimum command.
With two B channels available, the service policy can reserve (approximately) 90 kbps (70 percent of
128 kbps) for voice traffic. The total number of calls that can be transmitted depends on the codec and
sampling rates used.
Example 3-36 illustrates the configuration for enabling voice and data over multiple ISDN B channels.
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