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How will ADSL affect the future of ISDN?
ADSL is a great technology that promises data rates of up to 6Mbps over regular copper phone lines, the POTS installed in your home or business right now. ADSL works something like the cable in your cable TV line. By dividing the entire bandwidth of the physical copper lines into separate divisions of frequency ranges, called carriers, it is possible to carry several different signals on the same cable simultaneously. This is the same method your cable TV cable uses. With cable TV it is all the separate channels that come into your home (yes,they all come in at once, it is the converter on top of your TV that decodes the signal so you only see one at a time,) with ADSL it is separate channels all carrying different parts of the same data transmission at the same time. This is called frequency division multiplexing (FDM). It is analogous to having several modems all feeding the data to you at once, in effect a "bigger pipe." Each end must have the hardware to encode and decode the data signal, usually referred to as an active headend.
This is an elegant solution to a complex problem and it shows great promise. In fact several trials have already been done and it has proven to accomodate data and video conferencing at extremely high rates, all without even interrupting the telephone service (POTS) on the same line. (ADSL reserves a frequency range at the bottom of the frequency spectrum for POTS that has a guard band to protect it in case of ADSL failure.) Several phone companies are prepared to begin testing in earnest, including Bell Atlantic.
So, how will this affect ISDN? I'll bet some of you are wondering how I can put a positive spin on this <G>. First let me pose a question. Where is the big race leading us right now? There is currently a HUGE push by the cable companies to become your data providers. But the sad truth for them is that current wiring is very inadequate to do this. They either need to add wire to each home, or replace it with a different kind of cable (Fiber) that can carry a clean, two-way signal. Another big push is from phone companies who want to sell you fiber or T1 lines to carry data from your home office to their Central Office to carry frame relay or in the future ATM (remember that the hardest link to overcome is the final step from the phone company's central office to your computer ) . The third push is from the likes of Winstar who want to provide a short distance microwave link from your office to the phone company's central office.
Think. The beauty of ADSL is its ability to use the POTS copper wire. ISDN also uses the phone companies installed phone lines. This makes ISDN the perfect "scalable" ( the ability to upgrade technology in the most cost efficient way) choice for immediate needs until ADSL is standardized. Therefore these more expensive solutions are the ones who need to worry about ADSL, not ISDN (in the near tern anyway). Granted ADSL is much faster technology than ISDN. But it is still an immature technology. witness this quote from the ADSL forum:
"So the wait may not be long; indeed, we predict ADSL will be economical about the same time all the other parts of a multimedia communications system become economical, that is, available for the general market -- by the end of 1996. By then fiber/coax cabling will have reached, at best, 17 million homes worldwide, leaving only 560 million subscribers left on copper. Let the revolution begin -- the wiring is already in place. "
We have all witnessed how ATM has not quite fulfilled the expectations it had a few years ago. It's a great technology, but that alone does not make for a profitable product. Companies must evaluate the viability of any new technology they implement. It is well known among Info System managers that only suckers become "unpaid" beta testers (guinea pigs) for new technology. Everything they buy must be scalable to the future.
ISDN is here now, a very mature technology. And now it has the added advantage of being the perfect immediate logical step towards the ADSL system of the future.
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