Technical Bulletin
201-014-1
October 16, 1989


                                Optimizing X.25 Transmission

______________________________________________________________________________
Introduction    This Technical Bulletin explains how to eliminate frame
                retransmission when using OSI or SNA Gateways (QLLC).
                Packet transmission in X.25 will degenerate when the
                interframe gap is less than 2 milliseconds.

______________________________________________________________________________
Eliminating     The most frequent cause of performance degradation in X.25
Retransmission  transmission is the retransmission of rejected frames issued
                on the link. Retransmission can be avoided by inserting a gap
                of at  least 2 milliseconds of processing time between frames.
                The interframe  gap can be created by inserting flags. The
                appropriate number of flags  to be inserted is calculated to
                make the packet equal to the smallest  possible frame size
                that can traverse the network. The smallest frames  are 4
                bytes long including the FCS (frame checking sequence), such
                as the supervisory frames RR,RNR and REJ.

------------------------------------------------------------------------------
                    Minimum Frame Size and Processing Time

            RR-Frame                                 RR-Frame
            --------                                 --------
|----- Smallest Frame Size -----|
 _______________________________         _______________________________
|         |         |     |     |       |         |         |     |     |
| Address | Control | FCS | FCS |       | Address | Control | FCS | FCS |
| Field   | Field   |     |     |       | Field   | Field   |     |     |
|_________|_________|_____|_____|       |_________|_________|_____|_____|

|---- Minimum Frame Processing Time ----|

------------------------------------------------------------------------------

                Note: This problem will not 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 software will send RRs in a piggyback
                fashion.

                _______________________________
                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.
                Therefore the interval which must be filled is:

                     2.0     Milliseconds required per frame

                   - 0.5     Milliseconds taken by one supervisory frame
                  -------
                   = 1.5     Milliseconds remaining


                One flag is equal to 1 byte, or an interval of 125
                microseconds (on a 64 Kbps line), therefore:

                     1.5     Milliseconds remaining to be filled

                   / 0.125   Milliseconds per flag
                  ---------
                  = 12.0     Number of flags required
