Sun Microsystems Sun Quad User Manual

Page 40

Sun Quad FastEthernet PCI Adapter Installation and User’s Guide • August 1997

TABLE B-3

lists the default values and allowable values for the inter-packet gap (IPG)

parameters,

ipg1

and

ipg2

By default, the driver sets

ipg1

to 8-byte time and

ipg2

to 4-byte time, which are

the standard values. (Byte time is the time it takes to transmit one byte on the link,
with a link speed of either 100 Mbps or 10 Mbps.)

If your network has systems that use longer IPG (the sum of

ipg1

and

ipg2

) and if

those machines seem to be slow in accessing the network, increase the values of

ipg1

and

ipg2

to match the longer IPGs of other machines.

Defining an Additional Delay Before Transmitting
a Packet Using

lance_mode

and

ipg0

The PCI Fast Ethernet and Expansion (PFEX) ASIC supports a programmable mode
called

lance_mode

. The

ipg0

parameter is associated with

lance_mode

After a packet is received with

lance_mode

enabled (default) an additional delay is

added by setting the

ipg0

parameter before transmitting the packet. This delay, set

by the

ipg0

parameter, is in addition to the delay set by the

ipg1

and

ipg2

parameters. The additional delay set by

ipg0

helps to reduce collisions. Systems

that have

lance_mode

enabled might not have enough time on the network.

lance_mode

is disabled, the value of

ipg0

is ignored and no additional delay is

set. Only the delays set by

ipg1

and

ipg2

are used. Disable

lance_mode

if other

systems keep sending a large number of back-to-back packets.

You can add the additional delay by setting the

ipg0

parameter from 0 to 31, which

is the nibble time delay. Note that nibble time is the time it takes to transfer four bits
on the link. If the link speed is 10 Mbps, nibble time is equal to 400 ns. If the link
speed is 100 Mbps, nibble time is equal to 40 ns.

For example, if the link speed is 10 Mbps, and you set

ipg0

to 20 nibble times,

multiply 20 by 400 ns to get 800 ns. If the link speed is 100 Mbps, and you set

ipg0

to 30 nibble-times, multiply 30 by 40 ns to get 120 ns.

TABLE B-3

Read-Write Inter-Packet Gap Parameter Values and Descriptions

Parameter

Values
(Byte-time)

Description

ipg1

0, 255

ipg1

= 8 (default at initialization)

ipg2

0, 255

ipg2

= 4 (default at initialization)