What is the capacity of twisted pair phone lines?

The operative question is "how much information can we figure out how to get onto the old twisted pair?"

Personally, I don't know the answer. I do know that our local phone company runs a fiber-optic backbone down the main roads here, with little boxes that convert to twisted pair for the last mile. Are you sure that's not what your phone company is doing? It certainly gets more capacity out of a twisted pair of wires.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

I'm sure that twisted pair is not being run from the phone company's main office. There is a box that is about 1.25 miles from me that I am pretty sure my wires come from. Whidbey Telecom updated all sorts of boxes here on the south end of Whidbey Island and after all the work was done on the box closest to me I got an email telling me that TV is now available over my phone line. So there probably is fiber optic running from the main office to the boxes. Still, 1.25 miles seems like a long way for twisted pair when I think back about 15 years and how to get decent DSL speeds my brother had to switch to cable from the phone line. Eric

Think about capacity like spectrum, there's low frequency that pretty much gets through. At high frequency the 'skin effect' causes major loss. From memory, DSL instrumentation uses bands spread out along the possible spectrum as in, "don't put all your eggs in one basket" Thus 'holes' can get cut out and all pretty much works well. Now, thre is an excellent reference that gives models for losses versus distance for standard telco line 24 or 26 Qwg twisted pair that claim to be within 1% up to 10MHz. That band is 20Mbs. Now if you assign several layers at that speed you get

40Mbs, BUT there is a limit to stacking and noise and all. Look up Shannon's Theorem, or Limit. Very simple formula derived through observation.

"ADSL/VDSL Principles" by Dennis J. Rauschmayer, 1999 Macmillan Technical Publishing pp 28-42

Back slightly before 2000, this company was making DSL testers and about the maximum distance could go was 17,000 ft. Today 17k ft yields around

300 kbs, not fast, but better than dial-up.

DSL Test Instrumentation Sunrise Telecom Incorporated

302 Enzo Drive San Jose, CA 95138

I live in rural area north of phoenix and used to get calls from our telco service to get DSL. They promised up to 7MBs!! for only 39.95 per month. So I asked them WHAT exactly would I get here being 17,261 feet form switching house. Don't know, but they repeated,"you get up to 7MBs !! for only 39.95 per month." So, knowing what I know about DSL, I told them I would pay them proportionately for the speed, would you install it? [Now between you and me, that comes out to around .3/7 * 39.95 = 1.72] After telling them to go ahead and install, but I demand them to 'adjust' the cost to reflect what they actually deliver, they quit calling.

** The limitations you refer to above were * NOT * due to using twisted pair cables. Back then, a phone connection only allowed ONE voice quality signal to be transferred across the network - cos that was what the system was designed to do.

Some heroic efforts were made to get the most out of the available 3.4kHz bandwidth to carry data - ending with the 56K modem.

Since then, the former voice only network has changed out of all recognition to become a massive data transfer network instead, mostly using optical fibre. Luckily, it still carries voice signals for regular telephone users - as a courtesy.

The relatively shot run of twisted pair that arrives at your home can in theory carry massive amounts of data - limited firstly by the maximum power output of the modem sending it and secondly by the twisted wires melting or shorting from overvoltage at the sending end.

The huge losses that occur at high ( MHz) frequencies are compensated for by using high drive levels and sensitive receive end electronics.

If at some distance the data signal starts to disappear into background noise, then it is only necessary to add in-line boosters to restore it - as is normally done with undersea cables and optical fibre links.

So the answer to your question is *money* is the final limit. That limit occurs when it is cheaper to use another technology instead of twisted copper wires to do the job.

.... Phil

This is a good question.

I only know from experience that information quantity and distance are inverse proportional. DSL and other technologies use high frequency signals, actually it is nothing else than wire-bound radio transmission. The higher the maximum carrier frequency, the more information can be modulated on it. DSL uses the complete long, medium and shortwave radio band from Kilohertz up to 30 MHz. After a few miles, the HF signals are gone and so is DSL.

w.