Bursting is one of the features that has made Frame Relay so popular. Bursting enables a user
to transmit data faster than the CIR for a short period of time. Figure 29.5 shows the difference
between the CIR and the access rate and how the burst traffic rate can increase beyond the CIR.
The network controls this bursting capability, and it usually does not result in any additional fees
on the user. There is a catch, though. Some burst traffic has the Discard Eligibility (DE) bit turned
on, indicating excess traffic above CIR. If a Frame Relay switch becomes congested, traffic with
the DE bit set (excess burst traffic) is the first to be dropped.
You will see the following symbols: Bc, Be, and Tc. Committed burst size (Bc)
and excess burst size (Be) are the two types of burst sizes. Each of these sizes is measured over
the committed rate measurement interval (Tc). Bc is the maximum amount of data that the network
can guarantee will be delivered during the time Tc. Be is the amount of traffic by which the
user can exceed the committed burst size.
For example, take a user who buys a Frame Relay circuit with the following characteristics:
1,544Kbps access rate
256Kbps committed information rate
Four-second committed time interval
The user is guaranteed a CIR of 256Kbps over a four-second period. The user could transmit
256Kbps for four seconds, and the network would ensure delivery. The user could alternately
send 1,024Kbps for one second, representing the committed burst. However, for the remaining
three seconds, there would be no guarantee of delivery for the excess burst traffic.