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DSL - Section 3More detail.. DSL (Digital Subscriber Line) is a generic name for a family of standards allowing high speed data transfer over telephone lines (POTS), otherwise known as twisted pair. For carrying IP, DSL competes with cable modems, satellite digital data feeds, wireless digital data, ISDN, 56k modems and more expensive frame-relay lines etc.
Since the twisted pair lines used in the telephone network that are to carry DSL are not shielded from interference, there are limits on the distance DSL signals can travel before degrading the maximum DSL speeds available in all DSL standards.
This distance limitation of DSL is only one of many potential technical hurdles to pass before an installation is complete.
..take me back ---- tell me more...
POTSPlain Old Telephone System!
Here is a good story about POTS, specifically, POTS and Bell Atlantic.
The Bell Atlantic copper network is vast. We expect you imagine all these clever looking people in white lab coats, sitting in a mission control room, with fine HDTV overhead displays showing their network, colored symbols indicating (infrequent) problems, graphs, voices over the announcement system quietly announcing infrastructure problems and changes, something like, say, the space shuttle mission control?
Think .. paper. Think sheets of paper the size of table tops. Think 100s of thousands, maybe more, huge, 15 year old, sheets of paper.
Each sheet of paper represents a small area of the Bell copper system. Inscribed on these sheets (called plats, we believe), are the lines, and symbols and juntions, every last one of them, of the Bell Plain Old Telephone System.
These sheets are the only reliable documentation of what they have out there. They were put together around 15 years ago, by sending teams of convicts out to measure and note every aspect they could reach of the copper infrastructure that had built up on an as-needed basis since and before the war. Telphone pole locations, pole heights, wire gauge, box positions, trunks, distribution points. Every single little bit.
These sheets fit together to form a huge 2d sheet. The line up on the edges like a street directory from hell.
Ah!, we hear you say, What happens if they build up a new area and an old sheet becomes wrong and they need more space? Well, they re-write the sheet.. using finer and closer lines if need be, to fit the extra infrastructure in.
These sheets are stored in some enourmous, and hopefully somewhat fireproof, archive.
When there is significant work to be done, or troubles in an area, there is an internal sheet distribution service, that shifts the necessary sheet or sheets down to the local area for use. They get returned when not needed, hopefully without too many extra grubby stains on them.
All this is true.
A few years ago, it became clear to the hive brain that is Bell Atlantic, that they need to get this infrastructure computerized somehow. Imagine, they thought, if we can _computerize_ all these sheets, then... when there are several faults, we could have a program that would trace through these new electronic sheets, and probably isolate the problem to one junction or cable.!
Turning a million sheets the size of your dining room table into computer data is not something you want to do in MS Access, no matter how many wizards Bill provides. It was clear some lateral thinking was needed.
So, they embarked upon a program to send the sheets to India, where you can get data keyed in manually for 1 cent a K. Flown by the box load, they got the data on each keyed in, and returned on tape. This wasn't the whole sheet archive, just an initial experiment, for a small town. Call it Lake Falls, for want of a name, and it worked well! When Mr Smith the green grocer found a fault on his line, and so did Mr Jones the undertaker, the system chewed on it for a while and then correctly predicted it was likely to be a problem with junction box number 3A on sheet 351578617AU.
Unfortunately, the quality of the sheet data, and the keyed data from India was not the best, since the original convict data collectors did not really have their minds on the job... There was also the issue of sending 1000 sheets describing the phone system of the Whitehouse off to a foreign power possessing nuclear weapons, so a second project was started, this time to digitize (scan) the sheets so that they could at least slide and zoom around the network using computers, instead of receiving and delivering sheets.
This project is still underway, and we are told they don't expect to finish the job until 2004, and many don't believe it will ever be finished, nor all that useful if it is. The budget for all this is probably a good chunk of a billion bucks.
For those of you concerned about data backup of these sheets of paper, we can report that Bell arranged for huge photocopiers to be built, and these were employed to fast copy all the sheets for archive. So now at least they have a backup.
The newer fibre trunks and in better shape, data-wise, as the databases and locations were recorded accurately. So with Tiger land base maps, and this data, they can build their mission control just for the fibre backbones.
But for copper, over which DSL runs, we are on sheets of paper still, so now you know why its not easy to answer the question "can i absolutely get DSL", and "why does my phone line have a hum on it".
Think about this next time you order pizza by phone.
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