🚀 go-pugleaf

For_Email@ddress.see.below_plea.se (Philip J. Koenig) writes:

> 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.

Terminators and NICs are 50 ohms for 10Base2 (Ethernet). 

> 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.

The reason for the limits on coax length is resistance. The time
budget is real, but only applies to repeated (multi-segment) networks
and fiber nets. Neither coax nor UTP lengths will allow for single
segments approaching the Ethernet time budget. (Thin net (10Base2) is
limited to 185 meters and thick net (10Base5) to 500
meters.)

While Vp varies from medium to medium, it is pretty safe to say that
time budget will allow for about a 2500 meter network diameter.

If you really want to get into all of this, read up on transmission
lines and see http://www.ots.utexas.edu/ethernet/. If you want to get
deeper into it, I recommend Charles Spurgeon's book, "Practical
Networking with Ethernet", ISBN: 1-85032-885-4.

And if you plan to make a living on Ethernet, get a copy of IEEE
802.3, the actual standard. (You need to be serious about Ethernet to
get that because it is NOT an easy read, assumes a fair amount of EE
background,  and costs about $200.)
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest Orlando Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman@es.net				Phone: +1 510 486-8634