[Beowulf] 1. Re: Jury rigged ethernet? (Robert G. Brown, et al)

[Ed Karns]

Again, I would suggest simply trying the CAT3/5 wiring and get as many  
"clean" pairs as possible into use and/or as redundant circuits, and  
ignore what your impedance meters may tell you. Four (two twisted  
pairs) would do the trick and you get decent performance, maybe even  
100baseT levels. The theoreticians and white paper writers usually  
don't take into account the irregularities of the real world. Truly it  
is almost impossible to accurately predict the real impedance of a  
cable set that is "dark". Modern impedance matching techniques built  
into modern ethernet circuits and chip sets can sometimes handle a very  
broad range of impedance mismatches once the wires are "heated up".

Try it ... test the available wiring with a couple of laptops.  
Interestingly, the use of "mismatched" or "off brand" ethernet adapters  
(PCI or PCMCIA cards) will sometimes work better than the name brand  
originals, which are often manufactured with "tighter" specs. than the  
no name clones.

" ... Honestly, I don't think that a 30 foot run (ten meters, barely  
longer than their five meter limit in toto) will be too much of a  
problem, although one might have to experiment a bit and get very clean  
connections to the terminators on both ends. ..."

... hear, hear ... (As mentioned off line, I have seen coat hangers  
work in a pinch.)

Ed Karns
FireWireStuff.com



On Tuesday, June 28, 2005, at 12:00  PM, beowulf-request at beowulf.org  
wrote:
>
> Today's Topics:
>
>    1. Re: Jury rigged ethernet? (Robert G. Brown)
>    ....
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 27 Jun 2005 16:52:19 -0400
> From: "Robert G. Brown" <rgb at phy.duke.edu>
> Subject: Re: [Beowulf] Jury rigged ethernet?
> To: Ed Karns <edkarns at firewirestuff.com>
> Cc: beowulf at beowulf.org
> Message-ID: <cone.1119905539.89650.5767.500 at lilith>
> Content-Type: text/plain; charset="us-ascii"
>
> Ed Karns writes:
>
>> ... below ...
>>
>>
>> On Monday, June 27, 2005, at 10:22  AM, Reuti wrote:
>>
>>> Hi,
>>>
>>> point to mention is the most likely unknown wave resistance of the
>>> existing wires. You have
>>>
>>> 150 Ohm for IBM Type 1 .... possible to use.
>>> 100 (sometimes 105?) Ohm for CAT3/5 twisted pair ... The best
>>> candidates ... if you have enough pairs between this cabling and  
>>> above
>>> ...
>>
>> ..........
>>
>> These cables below are not recommended. Using coax will only make your
>> connections unreliable unless you can make or use a breakout box with
>> the proper connection converters (RG type coax conversion to RJ type
>> connectors = bad news)  ...
>
> As I also pointed out to several people in offline remarks, impedance
> matching is a known problem with known solutions for at least some of
> the more common STP to UTP situations.  The biggest problem occurs  
> where
> one goes from STP (say, the lines into said "tight space") to UTP (say,
> a UTP/Cat 5 patch cable).  You tend to get reflections at these
> junctures that can increase error rates if the UTP and STP runs are of
> comparable length and the run lengths are "bad" lengths relative to
> carrier wavelengths (where standing waves can be established via the
> reflections).
>
> One solution is to avoid such junctures and use STP end to end, primary
> and patch cables alike. However according to this cisco white paper
> (that examines the issue from the point of view of organizations  
> needing
> to reuse old e.g. token ring or phone wiring):
>
>    
> http://www.cisco.com/en/US/netsol/ns340/ns394/ns74/ns149/ 
> networking_solutions_white_paper09186a00800b4249.shtml
>
> it isn't a big issue for short runs, either of the primary STP cable or
> short patch cables on the ends.  To quote:
>
>   ...short lengths such as 90 meters of Token Ring A-suffix STP cable
>   and 5 meters of UTP patch cords on each end should not cause
>   significant problems.
>
> at least through 100BT.
>
> They do discuss using impedance matching adapters for various cable
> types, finding that in a lot of cases they don't really help much.
>
> Honestly, I don't think that a 30 foot run (ten meters, barely longer
> than their five meter limit in toto) will be too much of a problem,
> although one might have to experiment a bit and get very clean
> connections to the terminators on both ends.  Termination per se is
> likely to be the biggest issue, with or without adapters or baluns or
> impedance matching as empirically determined to be necessary.
> .......