Verilink 8100A (34-00237) Product Manual Bedienungsanleitung PDF herunterladen (Seite 293)

Applications Notes D-1

A PPENDIX

PPENDIX

PPLICATIONS

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.

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Verilink 8100A (34-00237) Product Manual Bedienungsanleitung Seite 293