/mcppropagation delays on the trunks within the network, and variable delays are from queuing delays
clocking frames into and out of intermediate switches. In order to estimate propagation delay, a
popular estimate of 10 microseconds/mile or 6 microseconds/km (G.114) is widely used. However,
intermediate multiplexing equipment, backhauling, microwave links, and other factors found in
carrier networks create many exceptions.
The other significant component of delay is from queuing within the wide-area network. In a private
network, it can be possible to measure existing queuing delays or to estimate a per-hop budget within
the wide-area network.
Typical carrier delays for US frame relay connections are 40 ms fixed and 25 ms variable for a total
worst case delay of 65 ms. For simplicity, in examples 6-1, 6-2, and 6-3, any low speed serialization
delays in the 40 ms fixed delay are included.
These are figures published by US frame relay carriers, in order to cover anywhere to anywhere
coverage within the United States. It is to be expected that two locations which are geographically
closer than the worst case have better delay performance, but carriers normally document just the
worst case.
Frame relay carriers sometimes offer premium services. These services are usually for voice or
Systems Network Architecture (SNA) traffic, where the network delay is guaranteed and less than
the standard service level. For instance, a US carrier recently announced such a service with an
overall delay limit of 50 ms, rather than the standard service's 65 ms.
De-Jitter Delay
Because speech is a constant bit-rate service, the jitter from all the variable delays must be removed
before the signal leaves the network. In Cisco router/gateways this is accomplished with a de-jitter
(Δn) buffer at the far-end (receiving) router/gateway. The de-jitter buffer transforms the variable
delay into a fixed delay. It holds the first sample received for a period of time before it plays it out.
This holding period is known as the initial play out delay.
Figure 5- 3 : De-Jitter Buffer Operation