Information Technology Reference
In-Depth Information
We could have applied a similar efficiency for emergency calling. Every ephone-dn in our
scenario had the ability to make emergency calls. This was granted to them because every
ephone-dn had the 911 tag assigned to their incoming COR list. However, in light of Rule
1, we could have
not
assigned an outgoing COR list to dial-peer 10 (which was used for
emergency calls). At that point, any of the ephone-dns would be able to use the dial-peer
regardless of their incoming COR list.
Tip:
Even though the previous two rules can make your configuration more efficient, it
might help other administrators who not as familiar with CME to add the extra configura-
tion shown in Examples 6-24 through 6-27. Assigning all calling/called entities a COR list
makes it less likely that one would “slip through the cracks.”
Quality of Service
Quality of Service (QoS) is a topic that has been hinted at in nearly every chapter of this
book. For a VoIP network to operate successfully, the voice traffic must have priority over
the data traffic as it traverses its way from one end of the network to the other. The Cisco
definition of QoS is as follows:
Quality of service is the ability of the network to provide better or special service to
a set of users and applications at the expense of other users and applications.
That sounds exactly like what the voice traffic needs as it crosses the network: better or
“special” service than the typical data traffic, such as web browsing, FTP transfers, e-mail
traffic, and so on. The voice traffic needs this not so much because of bandwidth require-
ments (VoIP uses very little bandwidth compared to most data applications), but rather
delay requirements. Unlike data, the time it takes a voice packet to get from one end of the
network to the other matters. If a data packet crossing the network experiences delay, a
file transfer bar might take a couple more seconds to complete or a web page might take a
half second longer to load. From a user's perspective, this is not a big deal. However, if
voice traffic crossing the network experiences delay, conversations begin to overlap (a
person begins speaking at the same time as another person), the conversation breaks up,
and, in some extreme cases, the voice call drops.
To combat these issues, you need to ensure not only that there is bandwidth available for
VoIP traffic, but that the VoIP traffic gets the first bandwidth available. This means if a
bottleneck is in the network where a router queues traffic before it is sent, the router will
move the waiting voice traffic ahead of the data traffic to be sent at the first available in-
terval. Accomplishing this is the job of QoS. QoS is not a tool in itself, but rather, a cate-
gory of many tools aimed at giving you complete control over the traffic crossing your
network. There might be times when you just use a single QoS tool aimed at decreasing
the delay of traffic. Other times, you might employ multiple QoS tools to control delay,
reserve bandwidth, and compress data that is heading over the WAN. How and when you
use each of the QoS tools depends on the network requirements of your traffic and the
characteristics (such as bandwidth, delay, and so on) of the network supporting the traffic.
