Note: This section is out of date and no longer maintained.
It is retained for the value of any residual information contained in it. distanceThe magic number for most DSL technology out there now is around 18000 feet. This is a function of the capability of copper to transmit high frequency over long distances. Past 18,000 feet, signal loss becomes increasingly unmanageable, bit errors rise fast, and therefore error-correction repetition of data means maximum reliable bandwidth is squeezed down to nothing. Thats the bad news.
The good news is that for the US, 18000 feet covers almost 80% of telephone subscribers (well, really, 80% of the US population). In addition, the phone company, in anticipation of the need for fast data rates and better quality lines, has been working on shrinking the average local loop lengths down in recent years, by installing smaller local hubs, known as DLCs or SLCs, that multiplex many lines onto optic fibre or other high quality lines that proceed on to the main switching centers. Most DLCs are not currently DSL capable, but the necessary hardware is out there, to upgrade them.
Many providers will not deal with you unless you are within 12000 feet.. this is to save the expensive of setting up a line to find that the quality precludes service, so being less than 18000 feet from your CO does not mean you can get DSL.
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When considering distance, the figures quoted by DSL equipment manufacturers assume good quality, thick, copper and under good conditions. For our area, even though our local loop is around 5000 feet, SDSL was limited to a maximum speed of 716k, half its theoretical maximum, probably due to oxidation and interference. (also see.. technical hurdles).
DLCDLC is A Digital Loop Carrier. An SLC is an AT&T variety, 'lightspan' is another.. you may come across either or any of these terms when hearing why you cannot get DSL.
Beginning in the 80s, to save money (copper loops are relatively expensive), Telcos began to install DLC frames around their existing wire centers, particularly in fast growing districts to provide for the flood of new lines required.
These local concentrators, although digital, and fibre optic, were designed before DSL was really available, and do not support it. They do support ISDN, however. It is estimated that nationally around 20% of US telephone lines are served from a DLC (the percentages vary slightly depending on which Telco you are talking about). Furthermore, about half of those DLCs are of an old type that do not support DSL. The newer half are installed capable of supporting DSL, but are only just being enabled for that..
All is not lost if you are on on either type of concentrator, the older ones can be retro-fitted with newer cards that will bring them to a level of DSL compatibility, but as of the end of 1999, the process has hardly started!
In summary, there is one chance in 5 that you are on a DLC, and at present cannot get DSL.
For or an interesting document on the pros and cons of ADSL vs Fibre vs DLCs, check out Extending ADSL services to remote DLC locations tutorial.
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