🚀 go-pugleaf

In article <01bdd698$fe212500$8028a8c0@jb.zedak.com>, jb@zedak.keepyourspam.com 
writes...
> Daniel Erskine <netsup75@edwardjones.com> wrote in article
> <35EACDBE.237D36B8@edwardjones.com>...
> > Aaron Blosser wrote:
> > > On Fri, 21 Aug 1998 11:07:04 -0500, Daniel Erskine
> > > <netsup75@edwardjones.com> said:
> > > 
> > > >> Using existing 75ohm coax as if
> > > >> it was 50ohm coax (with F to BNC adapters) is asking for trouble.
> > > 
> > > >  I'll inquire with my network staff for more research material, as
> > > >I'm curious as to why 75ohm cable would be that much of a problem?!?
> > > 
> > > I wonder as well.
> > > 
> > > For short runs of cable, I highly doubt that using 50ohm vs. 75ohm
> would
> > > make a difference.  That measurement is a measure of the cable's
> > > impedance over some distance (1000 ft?), so for a short bit of maybe
> > > 50-100 ft, the difference *may* be neglible.
> 
> Not so.  A cable's impedance is the ratio of the magnitude of a voltage
> wave form
> travelling down the cable, to the magnitude of the corresponding current
> wave.  Nothing
> to do with distance.  But everything to do with reflections at junctions
> where there is 
> impedance mismatch.  
> 
> > > Of course, this all depends on how senstive the receiving equipment is,
> > > and I can't make any claims to know how it would really affect the
> > > performance.
> > > 
> > > The reason you *should* use properly rated cable is because with
> > > impedance matching you can achieve maximum power output.
> 
> Not really.  Maybe you're thinking about amplifier power output, which
> peaks into
> load, when load impedance is equal to effective internal series resistance
> of amp
> final stage?  In fact, having impedance matching in a transmission line
> does two
> things: maximizes strength of received signal at receiver(s), minimizes
> signal 
> reflected back to source from junctions (which degrades ability to detect
> signal.)


The reason that using correct cable is important are as follows:

1 - Terminators and NIC's are 75 ohm, if the cable is not, you end up
    with reflections that play hell with the waveforms and thus, data 
    integrity.

2 - The reason distance is a factor (and why the IEEE specifies a 
    maximum length for 10base2 segments) is due to the rate of signal
    propagation.  Since 802.3 collision detection relies on a known rate
    of propagation and maximum segment length (tailoring it's "collision
    listen window" accordingly), if you run out-of-spec cabling you run
    the risk that collision-detection no longer works correctly, which
    causes a host of problems including runt packets, late-collisions, 
    etc.



Phil


-- 
Philip J. Koenig         The Electric Kahuna Organization          [see below]
-----------------Computers & Communications for the New Millenium-------------
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