Re: [OT] Network speed

["David Yeend" <david.yeend@xxxxxxxxx>]

There is no 'fall back' support in Ethernet, it actually has no
facility to
measure the link performance. In fact it is entirely possible for
auto-negotiation to allow selection of an operating mode which can not be
supported by the cabling. The auto-negotiation system operates at a
bandwidth lower than 10Mbps so it will allow link configuration at 100Mbps
on Cat3 cable. As Ethernet starts to make its way into the local loop
standards are in development which allow for operational diagnostic
facilities. Some chipset manufacturers have started to implement basic
functions but in my opinion those in low end kit are not of much value.

In my opinion, if the network is causing the problem then it is likely to
be
cable problems such as mis-wired pairs - remember the pairing is important
and while auto-crossover and auto polarity detection / correction exists to
conceal most common wiring issues split pairs can not be compensated for.
If
you switch supports stats then look for CRC and alignment errors (note on
half duplex links CRC is a side effect of collision and normal as long as
it
is not higher than collision count).  The wiring for 568B is 1:Wh/Or, 2:Or,
3:Wh/Gr, 4:Blu, 5:Wh/Blu, 6:Gr, 7:Wh/Br, 8:Br. Pin one on the RJ plug is on
the left as you hold the plug with the cable end pointing to the ground and
the locking latch away from you.

If the cabling is ok and you have a very poor quality switch typically
fitting the profile of very low cost purchased around 18 or more months ago
you could suspect this as being fit for the WEEE compliant recycling bin!
Some products that hit the market at that time had serious internal buffer
issues......by the way if you have the ability to turn off flow control on
the switch then do so. TCP was developed to do it over a very long time and
it works well. The Ethernet mechanism really gets in the way! If you have a
fancy switch that supports QoS and port rate limiting, unless you really
need it, turn it off.

Still associated with the network but more of a configuration issue is half
/ full duplex mis-matches. Autonegotiation only works properly if both ends
of a link are so configured (note that some low quality kit of a year or so
ago did not actually implement it properly in any case) If you manually
configure one end of a link for full duplex it is possible that
autonegotiation capability announcement will be disabled causing the link
partner to switch to a mode where it detects speed correctly but assumes
half duplex. (Some devices when manually configured still use
autonegotiation to announce the capability of the port - others don't. This
causes a lot of confusion.) This situation will cause the sort of behaviour
you talk about. Look at the link settings if one end is clearly manually
configured to any mode other then auto-negotiation then make sure the other
end is too. If you are looking at statistics a duplex mis-match would
result
in a high collision and late collision count at the half duplex end. The
full duplex end will think life is groovy!

Patch leads - particularly if you make your own (nothing personal) should
always be suspected. If you have installed solid cable and directly
terminated RJ-45s on to it for connection to your switch......you have my
sympathy as a break causing intermittent poor performance is easy to
achieve
and a pain to find.

One thing you could try is to link the devices between which you are trying
to transfer the data using a cross over patch lead (swap orange and green
at
one end). Re-run the test and see if it is faster. If not - suspect
configuration or hardware / OS issues.

Rambly as ever!

Regards,

David





On 7/4/06, Phil Harris <phil@xxxxxxx> wrote:
>
>
> > > I'll elaborate on this... are the cables wired in the
> > correct pair > order. High packet loss (test with ping -f or
> > the windows equivalent) > or falling back to 10Mbit would be
> > consistent with mismatched pairs.
> >
> > If this is the case, would the 100Mb lights on the switch
> > still be lit or not?
>
> They actually could be ... I had a network cable that was miswired
across
> pairs and it still lit the 100Mbit lights on a 3Com network card and
my
> NetGear switch. It even "worked" as such in that it
transferred data but
> just ridiculously slowly!
>
> Phil
>
>
>


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