[TowerTalk] 600 Ohm Line

[Jim Lux]

At 02:46 PM 7/29/2004 -0500, Jim Brown wrote:


> >When I worked for a local phone company (AFTER the power company), we were
> >using ADSL over TP (duh) to attempt to deliver video.  The biggest hurdles
> >we encountered were not so much with the ADSL modems, but with the condition
> >of the outside plant (OSP) itself.
>
>I haven't worked in that part of the industry, but common sense and 
>Transmission
>Lines 101 says that those modems had better be treating that line as the 
>60-100
>ohm line that it really is of they are going to work at those data 
>rates!  That means
>matching each end to that characteristic impedance. There is, of course, 
>another
>wrinkle -- not all the cable in the loop has the same Zo, so there can be 
>bumps at
>each transition. And, of course, there the loss in the line at the higher 
>frequencies,
>and the noise.

Or, more properly, the adaptive equalizer in the modem had better 
compensate for the eternally changing lumps and bumps in the circuit, and 
not assume anything about the actual impedance (which probably varies 
widely with frequency over the more than decade frequency range).

   In the phone world, I don't think optimum power transfer (in the 
Thevenin sense) is really the issue.  You've usually got as much signal 
power as you need (although, this is why 56K modems don't really go 56K on 
a POTS wire... The Part 68(?) limits on Tx power).  It's things like group 
delay and differential effects that bite you.

That, and the 2wire to 4wire "hybrid" function (that modem has to separate 
the Rx and Tx signals somehow, and it certainly can't depend on there being 
a nice 600 ohm resistive impedance for a hybrid to look into).  At the CO 
end, they have a similar problem related to echo suppression.