[parisc-linux] C110 builtin nic slow?

[M. Grabert]

Sorry to reply myself, but think I might need to clarify some points
(I'm a little bit tired, so I have missed some things in my previous post)

On Sat, 15 Nov 2003, M. Grabert wrote:

[...]

> > So i will continue to see how to set it up in half-duplex (would it not
> > be the default in 10BT?)

Yes, usually it defaults to 10Mbit half-duplex and then (if the cards
support it) it tries to negotiate better connections (full-duplex, 100Mbit ...)

> Actually if one end point uses Full-Duplex and the other end uses
> Half-Duplex, there should be no connection possible.

This is true if the (forced full-duplex) network card test for full-duplex
capable connection and doesn't find it. However most (cheap?) cards will
skip the test for full-duplex if it was 'forced', and you can still connect.

Light traffic will work, but you will have problems with large traffic.
The explanation:

The half-duplex side will send a packet, and assumes the other end stops
sending if a collision occurs, ie. if the other side was sending something
at the same time (CSMA/CD).
The full-duplex side will however will always send/receive as fast as it
can as it doesn't do CSMA/CD, ie. it doesn't care about collisions.

The effect is that there will be a massive amount of collisions if both
sides want to send data, and only the half-duplex card will try to resend
the packets (causing even more collisions).
You will see slow traffic when sending from half-duplex, and dropped
packets when sending from full-duplex when both cards are sending.

This will be most likely your problem, since it fits your description
perfectly (forced full-duplex mode). Sorry, I overlooked that in
my first post.



> The same for the situation when one side uses 10Mbit and the other is
> set to explicitly use 100MBit.

This will definitely never work.

> The only situation where such a decreased network performance occurs
> is IMHO that your have a cheap network equipment.
> IE. a dodgy network cable that isn't properly shielded or doesn't use all
> 8 wires,

I mean in the context for 10BaseT, ie. all 4 wires (1/2,3/6) are connected,
but not the rest. There will be noise/crosstalk that will lead to
collisions.

[...]

> In this (worst case) scenario there will be a lot of packet drop on
> the physical layer and the network cards will re-send the ethernet
> packets autmatically (usually) without notifying you. You will see
> decreased network performace as you mentioned it.

The network card (only in half-duplex) will think it detects collisions
due to noise/crosstalk and will automatically resend the packets without
notification.


Slan,
  Max