Verilink 8100A (34-00237) Product Manual - Manual (Page 293)

A p p l i c a t i o n s N o t e s D-1
A P P E N D I X
D
A PPENDIX D
A
PPLICATIONS
N
OTES
Frame Relay
ATM was originally intended for multimedia applications and, therefore, is
designed for delay-sensitive, real-time implementation. On the other hand,
Frame Relay was originally intended for interactive data applications, which
tend to be bursty and are delay tolerant since loss or errored frames can be
detected and resent. To maximize line utilization, Frame Relay uses variable-
sized frames that expand or contract based on traffic volume. For voice
applications, this can be a fatal problem; large frames on a slow link can be
addressed by constraining the frame size on voice DLCIs. The FRF.12
Agreement was formulated to address this problem. Fragmentation queueing
reduces both delay and delay variation by dividing large packets into smaller
packets for transmission and then reassembling them into original packets at
their destination.
Setting the Fragment (Maximum Frame) Size
A good rule to follow when setting the fragment size is to set the size (in bytes)
to the same value as the link rate (in kilobits). For example, if the link rate is
512 kbps, set the fragment size to 512 bytes. This ensures frame transmission
(serialization) delay stays at less than 8 ms and allows a deterministic value
when addressing network delay calculations. This rule works for any link rate as
shown below.
If “X” is the link rate in kilobits per second, then:
Fragment size = X in bytes
so that
Transmission delay = X bytes x 8 bits/byte
/
(X) x 1000 bits per second,
which = 8/1000 s,
which = 8 ms for any X.