[TriLUG] OT: Home Depot and Cat 5

[Rob Lockhart]

Quoting "Aaron S. Joyner":

> Jim Ray wrote:
> 
> >>Do note that a traditional, old school, hub does not "reform" the
> >>packets in the manner that you're attempting to do, to extend an
> >>Ethernet segment.  You need to use a switch in that situation.
> >>    
> >>
> >[JR>] signals propagating in active digital transmission circuits whether
> >hub or switch go through transistors that snap 0's and 1's back into shape.
> >
> >  
> >
> Where "shape" is defined as a square wave on an oscilloscope, sure.  
> Degradation of "shape" or "height" in that respect are all byproducts of 
> attenuation, and as I mentioned earlier a hub will correct for 
> attenuation related problems.  What it won't correct for is timing 
> issues.  Consider a 100m cable run, operating at 100MBits / second.  
> Remember that both ends have the potential to attempt to talk at the 
> same time, this isn't a frame or token based network where everyone 
> talks in an agreed-upon order.  So let's say one end raises the voltage 
> to 5v (I don't recall the actual voltages on Ethernet, these are for 
> example purposes) - the other end won't see that voltage raise due to 
> propagation across the copper for _at_best_ 334 nanoseconds (assuming 
> the speed of light in a vacuum, in practice it's much slower, thus 
> longer).  If the voltage ever goes to 10v, it means that two stations 
> tried to talk at once and a collision occurs, and everyone has to try 
> again.  The farther / longer that signal has to travel before being 
> received by everyone else on that unswitched segment, then the higher 
> your chances of collisions.  As a final note, if you only have two 
> machines on the segment, and they're operating in full duplex mode, this 
> problem is really a moot point - but if that's the case it's highly 
> unlikely that you're also going to have a classic hub involved.  :)

100Mbps isn't exactly a square wave, I believe it's 3 level signalling with
shaping to prevent high frequency components (of square waves).

> I'll freely admit that I'm stretching the limits of my knowledge at this 
> point, but if someone with a stronger physics or EE background wants to 
> step in and clarify the velocity (to borrow an RF term) of Cat-V cable, 
> feel free.

I have MSEE and coursework of MSPhysics but generally what you're looking for is
the propagation velocity; approximately 0.7c (c = 3x10^8 m/s).  Optically it
goes as c/n (n=index of refraction) but electrically you have to look at the
problem as a waveguide.  More details follow, gotta pick up my son from daycare :-)

  -Rob