Asymmetric Digital Subscriber Line

comments by Al Giovetti

Everyone is worried where the band width is coming from. Many people are thinking in traditional fashion that everything has to be designed to work together and that their is limited space on networks. They plan to extend their existing systems with fiber optic lines for one million dollar a mile costs.

Computers are not going to work according to the plans of those who are thinking this way. Computers are going to develop in ways as yet undreamed, or in ways which have always been just in the background ready to move into the light. What can be banked on in the area of computers is change and that change will unerringly move towards less expensive and more functional systems.

The internet and the intranet is hot today - everything is moving towards faster and wider applications of these technologies. The internet is a world wide group of protocols that allow for file transfer and shared applications, but more importantly the internet allows all of us to take part in what was, up until now only open to those who had spots on the net or the clout needed to get space on the system. The intranet is a regular wide area or local area network, affectionately called WAN LAN protocol which uses internet systems to run information.

Asymmetric Digital Subscriber Line (ADSL) was a technology that has been available for about 10 years, and in 1998 the ADSL technology will take over the marketplace. ISDN is expensive in the area of service costs, the phone companies seem determined to make a windfall on the technology. Dedicated fiber optic lines are expensive.

ADSL uses plain old telephone service (POTS) lines to achieve speeds currently of 4.4 Mbps (mega bits per second) in one direction, and eventually speeds as high as 8 to 50 Mbps. POTS modems only run as high as 33.6 Kbps (killobits per second) or 1,000 times slower than ADSL and Integrated Services Digital Network technology tops out at 128 kbps or four times faster than POTS and 250 times slower than ADSL.

The A in ADSL means asynchronous, which means that in one direction we see speeds up to 50 Mbps but in the other direction on 640 kbps is supported. The speed in the other direction is not needed with most applications since we will be downloading data most often and when we upload the system can reverse and send information the other way. ADSL is ideal for many applications including video on demand and internet access. Two ADSL phone lines which are simple POTS like twisted pair lines and cost the same as regular phone lines, can for almost no cost transmit both sides upload and download at up to 50 kbps.

One ADSL technology is available now, called HDSL or homogenous digital subscriber line, which also works over POTS twisted pair. HDSL employs inexpensive modems but moves at a much slower rate of 762 kbps over regular twisted pair which is 6 times faster than regular POTS lines.

What technology are the modem suppliers backing? PairGain Technologies in Tustin, California is working currently with HSDL. A survey showed that telcos predict that 58,000 xDSL lines will be installed by 1997 while original equipment manufacturers predict 421,000 lines. For 1998, telcos predict 483,000 while vendors predict 1.5 million lines. Dataquest predicts 3.7 million modems shipping by the year 2,000.

Why will cable modems probably not survive? The cost in most areas will be prohibitive at one million dollars per mile. This cost will have to be retrieved from the consumer in those areas like where Comcast is locally putting down an entire fiber optic network. Any while fiber is the best it is limited while twisted pair copper lines are already installed, operational, inexpensive, and already in support of the ADSL technology.

Making POTS lines carry the ADSL signal does not require ripping out and replacing the twisted pair lines with fiber optic, it requires a cheaper solution. Special modems need to be developed that will provide the communications speed boost needed to open up existing systems to this faster speed. Three companies, Analog Devices Inc. AT&T Paradyne, and Motorola Semiconductor, are currently testing and developing ADSL modems.

A new technology will adapt to wire gauge, wire length, severe bridge taps, or other variables throughout the worldwide telephone network. Called Rate Adaptive Digital Subscriber Line (RADSL) this system will run like a regular POTS modem in adapting speeds to local line conditions, which is needed with the diversity of telephone systems available in the world today.

Once the price of ADSL modems drops below $500 which most developers and suppliers agree will happen sometime in the next two years, everyone will have access to this cheap and efficient technology and this will turn into a very inexpensive way to perform teleconferencing and internet access at very fast speeds over a normal local line equipped with this new hardware on the end of the line

Reference: Barry Phillips, OEM Magazine, volume 4, number 30, August, 1996, pg. 44 - 51.