[TriLUG] OT: Home Depot and Cat 5

[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.  :)

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.

Aaron S. Joyner