How to verify transmission quality & cables on gigabit network?

[Robin Laing]

Mike McCarty wrote:
> Rob wrote:
> 
>> Hello,
>>
>> I have a cluster, master + 8 nodes, connected via
>> a Netgear Gigabit Ethernet Switch (model JGS516).
>>
>> The cables are of CAT5 type.
>> Elsewhere I read that for a gigabit network, one
>> should use CAT6 type cables, to avoid bad
>> transmissions. Is that so?
> 
> 
> Well, there is some debate about this. First, note that
> nominally CAT5 is rated for gigabit speeds. Second, note
> that gigabit cable does not really carry gigabit rate
> transistions, but "only" 250MHz. However, ISTM that
> even that is a bit much for simple twisted pair, even if
> it is impedance controlled.
> 
>> 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.
> 
> 
> That appears quite good to me. 50% occupancy is not
> bad.
> 
>> Question 1:
>> Should I worry about this reduced speed?
> 
> 
> No, I don't think so. What have you got filling the pipe?
> Also, what protocols are you using? If you have to wait
> for an ACK from time to time, then you are going to get
> less than full BW. Also, protocols have other overhead,
> like headers on messages and CRCs and whatnot.

The last time I did anything with this, I learned that the maximum 
bandwidth for actual data is ~75% of the rated bandwidth.  This 
basically says 750Mb of data should take ~1sec.  And this depends on all 
of the above stated points.  If you really want to learn about network 
bandwidth testing, look at testing network operations for Beowulf clusters.

Install NetPipe and test your network.  At least that is what I used on 
our cluster when we installed it.



-- 
Robin Laing