Grandma's Phone, DSL, and the Copper They Share (hackaday.com)

szczys writes: DSL is high-speed Internet that uses the same twisted pair of copper wire that still works with your Grandmother's wall-mounted telephone. How is that possible? The short answer is that the telephone company is cheating. But the long answer delves into the work of Claude Shannon, who figured out how much data could be reliably transferred using a given medium. His work, combined with that of Harry Nyquist and Ralph Hartley (pioneers of channel capacity and the role noise plays in these systems), brings the Internet Age to many homes on an infrastructure that has been in use for more than a hundred years.

Comments

I worked on some of this.

[sbaker]

Many *MANY* years ago I was working as a software engineer at Philips Research in the early 1980's when they were looking into ISDN systems somewhat like DSL for the UK market - the business of sending anything over twisted pair copper is a nightmare. I wasn't directly working on the electronics (I was doing software) - but I shared an office with people who did...and they had a heck of a time characterizing the wires that their signals had to go down.

As I recall, the problems mostly come where one wire is spliced into another. Much of this infrastructure was put in the 1900's and it's horrible. Sometimes wires are just twisted together and capped, sometimes twisted and taped, sometimes twisted and just left open to the elements, sometimes they are soldered. Sometimes the places where the wires are joined gets wet when it rains. Sometimes the tightness of the twisted wire connection depends on the ambient temperature. The amount of cross-talk between wires is all over the map as different kinds of insulation was used (and much of it has degraded over the years). At the subscriber end, there were all kinds of phones being used - plus ugly stuff like "Party lines" (where two houses share a phone line!) that had been abandoned leaving extra wires in the ground that were still connected to the network.

All of those things affect the ability to get a decent amount of bandwidth down a wire that was never designed to do it. So the electronics has to be smart about the signal being reflected at each splice down the line and causing 'echoes', and designing affordable circuitry to detect and cancel those echoes was a nightmare. The amount of attenuation you'll get is all over the map - everything has to self- adjust and monitor to give it any chance of working.

So, as poor as DSL can be - it's a miracle it works at all over crappy old telephone wires.

    -- Steve

Re:Everyone's phone, DSL and copper

[bmo]

Not unless your grandmas phone was touch tone

Pulse dialing still works nearly everywhere. Indeed, some people are skilled enough to do pulse dialing by flashing the hook the required number of times.

Like me.

Get on my level.

--
BMO

HOWTO: http://www.oldskoolphreak.com/...