How to verify transmission quality & cables on gigabit network?
fedora at drussell.dnsalias.com
Fri Apr 28 21:56:17 UTC 2006
> I have a cluster, master + 8 nodes, connected via
> a Netgear Gigabit Ethernet Switch (model JGS516).
> The cables are of CAT5 type.
I assume you mean Cat 5e UTP. If you really mean Cat 5, no, I wouldn't
expect it to work well in a gigabit environment.
Cat 5e, on the other hand supports gigabit rates.
> Elsewhere I read that for a gigabit network, one
> should use CAT6 type cables, to avoid bad
> transmissions. Is that so?
*Should* is rather subjective, but as you see in many RFCs, *should*
does not mean *must*. :-)
Cat 5e is adequate for Gigabit rates, pay attention to cable paths and
termination. i.e avoid running near and parallel to power lines etc.
When terminating the cables keep the "twist" as intact as you can...
don't unwind more than actually required to punch the wire-ends down.
> I already have done basic speed checks, that indicate
> a throughput of about 0.5 Gbps. Hence the network
> appears to operate on half of its spec. speed.
Well.... there's more to network speed than the cabling.... are you
certain your hardware can deliver a sustained rate of a gigabit/second?
Can the receiving end collect it at that rate?
If you transfer a large file and divide file size by time, that's not a
true indication because it doesn't take into account the packet
overhead. It is also an average rate. And, unless the average is the
actual sustained rated, there are times when the rate was higher than
the average and of course times when it was lower.
> Question 1:
> Should I worry about this reduced speed?
Are you noticing any negative effects with your applications that need
such high data rates? Are you sure everything is working in full duplex?
What protocols are you using to test the throughput? Does every packet
require an ACK? Or just an ACK every n packets to imply "all OK so far..
keep it coming"? In that case, perhaps increase "n".
> Question 2:
> How to check the transmission quality?
> Should I inspect the output of "/sbin/ifconfig eth0"
> for errors/dropped/overruns/collisions etc. in the
> RX/TX lines?
I don't think collisions is what you want here... that's more a measure
of network congestion, not transmission quality.
Transmission quality could be perfect with a high number of collisions,
or quality could be crap with no collisions at all... so what is the
collision count really telling you? Do you have collision count data
from your pre-gigbit set up? With your new set up I'd expect the
collision rate to drop because the "in use time" of any given packet
transmission is shorter, thus making a collisionless likely.
You want to look at errors and retransmissions, that sort of thing.
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