SEEKING BETTER BANDWIDTH
Someone asked me to check the possibility of managing - or directory controlling - industrial process over the World Wide Web. There are several problems therewith. Security, response time, and band width to begin. If we are to "tele-operate," it's always been assumed we need fiber optic connections through out the country. The implied infrastructure capital costs are immense.
AS for bandwidth, every factory and warehouse will need T1 (1.5 megabits per second) performance to each drop or service location. And, the protocol would have to be interactive in real time, with existing interconnect systems preferred.
It turns out that telecommunications companies have been facing the same problem, and that ADSL (Asymmetric Digital Subscriber Line) is one possible answer. This revolutionary concept allows high-speed interactive bandwidth in existing installations of cooper loops such as are found in today's factories. And at low cost.
Use of ADSL could leapfrog that of modems and ISDN (Integrated Service Digital Network) - another possible means of high capacity data exchange. Modems are "stuck" at 33.6 kilobits per second, before data compression. Understand that Data compression is not the same as raw bandwidth. Data compression techniques are able to get a virtual bandwidth for times the raw bandwidth. This discussion will stick to the bandwidth story and leave data compression for another time.
ADSL's reported ability to approximate for T1 lines (about 6 megabits) over the installed copper twisted-pair met with initial skepticism from this reporter. But skepticism withered under testing and demo installations. ADSL is reliable, robust, and cost effective. It's blowing people's minds.
ANSI and other standards groups began work on ADSL in 1992. The first issuance considered a capacity of 6.2 megabits per second over 12,000 feet of 24-gauge twisted pair. Whew! Longer loops using "only" single T1 (1.544 megabits per second) can be used to 18,000 feet. The "modems," or processing units at the drops, require 1.5 million transistors.
Trial deliveries over copper have been impressive. "We ran ISDN, for TV channels, and POTS (plain Old Telephone Service) simultaneously without a glitch," said one participant. ADSL with four T1-capacity has also been heavily tested. Motorola, Analog Devices, and others are producing the hardware.
How does it work? ADSL's impressive transmission performance over long copper loops is achieved using advanced digital signal processing techniques implemented in VLSI (Very Large Scale Integration) technology.
The concept is that communication is asynchronous. A page of text is one to two kilobits. Compressed graphics is 10 to 100 kilobits. The control needs are only a fraction of the downlink requirements. One percent to 10 percent is normally the ratio of control to downlink mentioned. The "low speed" capacity of T1 uses an unlink control capacity of 64 kilobits per second. The downlink capacity of 6 megabits per second (four T1 "lines") uses 640 kilobits per second of control capacity. The basic modulation scheme uses discrete multiline modulation (DMT).
Present ADSL is expensive, about $2,000 per drop. The cost will soon be only about $500. And we can expect to se modem-like pricing by the turn of the century. Typical T1 costs are $200 per hour, while ADSL is projected at $15 for the same service.
What are the problems? The techies have "wired" every home and factory with "fiber" performance using the phone company twisted pair. But no single vendor has all the bits and pieces for the phone companies, and no one has looked at systems for manufacturing. Vendors are typically small and thinly financed. Fiber-optic and coaxial cable are coming on strong worldwide. Cultural and regulatory resistance can be expected to this or any innovative technology.
ADSL users untreated twisted pair loops to yield video and T1 performance. ADSL, or similar concepts, will change the installed copper loop capacity. Our factories and plants are wired for it today. It will be a quantum leap for out control and MIS needs. It works and is here now. No excuses. We have sufficient computer power, memory, software, and bandwidth beyond any immediate need. Time to get to work...
As appeared in Manufacturing Systems Magazine March 1996 Page 68
References - Table of Contents
|R. Morley Incorporated
614 Nashua Street, Suite 56
Milford, NH 03055-4992 USA
Tel: 603-878-4365 FAX: 603-878-4385