Eicon Networks C21 Przewodnik Instalacji Pobierz pdf (Strona 187)

Line Speeds and Performance 187

HDLC Window Size

When modifying X.25 Window Size, you must also consider HDLC Window

Size. X.25 packets are contained within HDLC Frames, and you can modify

the HDLC Window Size to optimize transmission. Yet as the Window Size

increases, so does the amount of data which must be re-transmitted in case of

data error. The increased traffic from error recovery with a very large window

may outweigh the efficiency gained by reduction of the effect of propagation

delay.

If you transmit HDLC frames intermittently, in bursts, or receive many RNRs

(Receiver Not Ready signals), you may need to change the HDLC Window

Size. Try increasing the HDLC Window Size first. If this is unsuccessful, try

reducing the Window Size. Unfortunately, most X.25 networks do not allow

much leeway in modifying Window Size.

Note

The DTE and the DCE must have the same Window Size when the host

and the gateway are connected directly through a null-modem cable, when one

end of the cable is DTE and the other end is DCE. If the connection is through

an X.25 network, the HDLC window must match that of the network (DCE),

not that of the remote host.

Buffer and Packet Size

Packet switching time constitutes a large share of the total processing time on

X.25 equipment. The switching time rises with the number of packets

processed. Increasing packet size improves performance by reducing

transmission overhead because fewer packets are required to transmit a given

amount of data. Processing time at each node in the circuit is also reduced.

Note

If the line quality is poor, and a large packet is rejected, you may

consider reducing the packet size. If a large packet is rejected, it means that

there is more data to reassemble and re-transfer. A smaller packet would have

less data to reassmble and re-transfer, and would transmit faster.

For optimal performance of the Eiconcard, the size of the data buffer ideally

should be a multiple of the X.25 packet size. If the buffer size is not a multiple

of the X.25 packet size, some X.25 packets will not be filled to their capacity.

The buffer is set by the application you are using, but most applications do not

let you modify the data buffer size.

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Eicon Networks C21 Przewodnik Instalacji Strona 187