/mcpConsequently, multiple CS-ACELP compression cycles quickly introduce significant levels of
distortion. This additive distortion effect is not as pronounced with adaptive differential pulse code
modulation (ADPCM) algorithms.
The impact of this characteristic is that in addition to the effects of delay, the network designer must
consider the number of CS-ACELP compression cycles in the path.
Voice quality is subjective. Most users find that two compression cycles still provide adequate voice
quality. A third compression cycle usually results in noticeable degradation, which can be
unacceptable to some users. As a rule, the network designer needs to limit the number of CS-ACELP
compression cycles in a path to two. If more cycles must be used, let the customer hear it first.
In the previous examples , it is shown that when a branch-to-branch connection is tandem switched
through the PBX (in PCM form) at the headquarters site, it experiences significantly more delay than
if it were tandem-switched in the headquarters C7200. It is clear that when the PBX is used to
switch, there are two CS-ACELP compression cycles in the path, instead of the one cycle when the
framed voice is switched by the central C7200. The voice quality is better with the C7200-switched
example (4.2), although there can be other reasons, such as calling plan management, that can
require the PBX to be included in the path.
If a branch-to-branch connection is made through a central PBX, and from the second branch the call
is extended over the public voice network and then terminates on a cellular telephone network, there
are three CS-ACELP compression cycles in the path, as well as significantly higher delay. In this
scenario, quality is noticeably affected. Again, the network designer must consider the worst-case
call path and decide whether it is acceptable given the users network, expectations, and business
requirements.
Considerations for High-Delay Connections
It is relatively easy to design packet voice networks which exceed the ITU generally accepted 150
ms one-way delay limit.
When you design packet voice networks, the engineer needs to consider how often such a connection
is used, what the user demands, and what type of business activity is involved. It is not uncommon
for such connections to be acceptable in particular circumstances.
If the frame relay connections do not traverse a large distance, it is quite likely that the delay
performance of the network is better than that shown in the examples.
If the total delay experienced by tandem router/gateway connections becomes too great, an
alternative is often to configure extra permanent virtual circuits (PVCs) directly between the
terminating MC3810s. This adds recurring cost to the network as carriers usually charge per PVC,
but it can be necessary in some cases.
Page 16 of 18
Cisco - Understanding Delay in Packet Voice Networks
7/31/2008
http://kbase.cisco.com/paws/servlet/ViewFile/5125/delay-details.xml?convertPaths=1