Eicon Networks C21 Installationsanleitung PDF herunterladen (Seite 188)

Line Speeds and Performance 188

Eliminating Retransmission

The most frequent cause of performance degradation in X.25 communications

is the retransmission of rejected frames. Retransmission can be avoided by

inserting a gap of processing time between frames. The size of the gap,

measured in milliseconds, depends on your line speed.

This gap between frames can be created by inserting flags. This is usually not

possible unless you have your own X.25 switch and can configure this

parameter. The idea is to insert enough flags to simulate the smallest frame

possible. The Eiconcard, and other X.25 devices, will then have sufficient time

to process the previous frame before the next one arrives. The smallest frames

are 4 bytes long including the FCS (frame checking sequence), such as the

supervisory frames RR, RNR and REJ.

Note

This problem cannot be reproduced in a testing environment where two

Eiconcards are linked by a null modem cable to simulate an X.25 network

connection. In the testing environment, Eicon Networks software sends RRs in

a piggyback fashion if the data to transmit is in I-Frames. Otherwise, explicit

RRs are sent.

Calculating the Number of Flags

Calculation of the number of flags to be inserted between frames is illustrated

in the following example:

If we assume a line speed of 64 kbps, or 8000 bytes per second, transmission

of 1 byte takes 125 microseconds. Thus the interval that must be filled is:

2.0 - 0.5 = 1.5

Minimum Frame Size and Processing Time

RR-Frame RR-Frame

Smallest Frame Size

Address

Field

Control

Field

FCS FCS Minimum Frame

Processing Time

Address

Field

Control

Field

FCS FCS

Milliseconds required between

frames. This depends on the line

speed.

Milliseconds taken by one

supervisory frame

Milliseconds remaining

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