/mcpThe initial playout delay is configurable. The maximum depth of the buffer before it overflows is
normally set to 1.5 or 2.0 times this value.
If the 40 ms nominal delay setting is used, the first voice sample received when the de-jitter buffer is
empty is held for 40 ms before it is played out. This implies that a subsequent packet received from
the network can be as much as 40 ms delayed (with respect to the first packet) without any loss of
voice continuity. If it is delayed more than 40 ms, the de-jitter buffer empties and the next packet
received is held for 40 ms before play out to reset the buffer. This results in a gap in the voice played
out for about 40 ms.
The actual contribution of de-jitter buffer to delay is the initial play out delay of the de-jitter buffer
plus the actual amount the first packet was buffered in the network. The worst case is twice the de-
jitter buffer initial delay (assumption is that the first packet through the network experienced only
minimum buffering delay). In practice, over a number of network switch hops, it is probably not
necessary to assume the worst case. The calculations in the examples in the remainder of this
document increase the initial play out delay by a factor of 1.5 to allow for this effect.
Note: In the receiving router/gateway there is delay through the decompression function. However,
this is taken into account by lumping it together with the compression processing delay as discussed
previously.
Build the Delay Budget
The generally-accepted limit for good-quality voice connection delay is 200 ms one-way (or 250 ms
as a limit). As delays rise over this figure, talkers and listeners become un-synchronized, and often
they speak at the same time, or both wait for the other to speak. This condition is commonly called
talker overlap. While the overall voice quality is acceptable, users sometimes find the stilted nature
of the conversation unacceptably annoying. Talker overlap can be observed on international
telephone calls which travel over satellite connections (satellite delay is in the order of 500 ms, 250
ms up and 250 ms down).
These examples illustrate various network configurations and the delays which the network designer
Page 10 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