Zhone 2000 ユーザーガイド

A

PPENDIX

 C

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.

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.