Specifications

66 IBM Voice Over Frame Relay Perform Guide
Figure 34. Frame Relay CIR
4.1.5 Bc (Committed Burst), Be (Excess Burst) and Tc (Calculated Time)
The committed burst (Bc) size is the maximum amount of data (in bits) that the
network commits to deliver during a calculated time (Tc) interval. The Tc is equal
totheBcdividedbytheCIR(Tc=Bc/CIR).Forexample,ifyousetaVC’sCIRto
9600 bps and the committed burst size to 14400 bits, the time period is 1.5 sec.
(14400 bits / 9600 bps = 1.5 sec). This means that the VC is allowed to transmit a
maximum of 14400 bits in 1.5 seconds.
This parameter is important because of the relationship between the committed
burst size and the maximum frame size. If the maximum frame size in bits is
greater than the committed burst size, the network may discard frames whose
size exceeds the committed burst size. Therefore, the committed burst size
should be greater than or equal to the maximum frame size. It should also equal
the burst size set up with the network provider.
The excess burst (Be) size is the maximum amount of uncommitted data the
router can transmit on a PVC in excess of the Bc during the Tc (Tc = Bc / CIR),
when CIR and Bc are not zero. The network delivers this excess data with a lower
probability of success than committed burst size data. Set the Be to a value
greater than zero only if you are willing to accept the risk of discarded data and its
effect on higher-layer protocol performance. The Be should equal the value set up
with the network provider.
Figure 35. Frame Relay Bc+Be
The excess information rate is rate at which uncommitted data is forwarded by the
network. Since this rate is in excess of the committed rate, the data may be
IBM 2212
BECN
Media Speed
64 kbps
CIR 16 kbps
IBM 2212
BC + BE
Frame
Relay
Network
Media
Speed
15000 kbps
Media
Speed
64 kbps
IBM 2212