[NTLUG:Discuss] Bandwidth?

[Jim Mitchell]

>>> patrick at patrickparks.com 04/07/03 05:57PM >>>
On Mon, 2003-04-07 at 17:49, Jim Mitchell wrote:

> What about duplex settings, or lack thereof.  If you have a duplex mismatching, that would certainly account for the slowness


Well, that is part of what I am trying to find out. I dont know how or
where to look and see if that is the case, and I am assuming your
talking about the nic?


Yes, I'm talking about the NIC.  I'm really not that much of a linux guru, (more of a lurker on this list, trying to learn more) but I do know a few things about ethernet, so if someone else knows more on this subject, please feel free to correct me.  I beleive an "ifconfig" command will tell you what you need to know as far as the workstation is concerned.  

One of the parameters tracked and reported from this command is the collision count.  Assuming the machine has been up for a while and has had a load placed on the NIC (medium to large sized file copy should suffice) then if the NIC is set to half-duplex, it should report at least a few collisions.  So if you see a collision count of 0, then you can probably assume that the link is set to full-duplex.  If you see a non-zero value for the collision count, then the workstation is certainly set to half-duplex.

The hard part of this is the switch.  You said earlier that it is a linksys switch.  I don't know those at all, so you're kinda on your own.  Sometimes you configure those through a HTTP session, other times you use telnet, and sometimes their not manageable at all.  You'll have to poke around in the switch and on the linksys website to get more information there.  

I would bet that if this is the problem that is causing the slowness on the laptop, then here's what things look like:

-NIC in laptop is auto-negotiating to half-duplex (incorrectly)
-switch interface is auto-negotiating to full-duplex

Therefore the switch is ignoring all collision signals generated by NIC in workstation, and just flooding the interface with packets whenever its sees one to send.  Those packets are lost and not recovered at the Ethernet level, and must be resent by a higher layer protocol.

Try manually setting the NIC to full-duplex.  I beleive the setting is "full_duplex=1" but I haven't the slightest idea where it is set.  Perhaps someone else on the list can help with that part.

If that doesn't work, there is another possible bottleneck on the laptop, and that is the PCMCIA card.  If the PCMCIA card isn't a CardBus card, it will be operating on a 16-bit bus.  That too could cause a bottlenck, but not the kind you described.  That would also basically require replacing the card.