ADSL's dirty little secret
ADSL has been around in concept for about 10 years. The idea is simple. Those same copper wire pairs that carry analog voice telephone calls can also be used to carry digital data. The voice signal uses only the frequencies up to about 4 kHz. Copper wires have a larger bandwidth than that.
But getting from concept to practice has been difficult. First, not all telephone wire pairs are equally suited for carrying data. Loading coils and bridged taps create reflections. A continuous metallic path is preferable. Second, the distance from the central office to the subscriber is limited. The distance limitation depends on the data rate and the power of the transmitter. A 1.5-Mbps signal can travel farther than an 8-Mbps signal. Some early tests suggested that the usable distance was too short, in many cases, to support a viable service.
And then there is the question of standards. There are two competing standards proposals. One is called Discrete MultiTone (DMT) modulation, and the other is called Carrierless Amplitude and Phase (CAP) modulation. CAP is being pushed by Lucent Technologies and Bell Atlantic. DMT seems to have the support of nearly everyone else.
The latest news is that the Bell and GTE telephone companies, and computer companies such as Microsoft, Compaq, Intel, 3Com and Cisco, have formed an ADSL Working Group to promote ADSL deployment. They plan to work out the standards dispute in the U.S., get the compromise standard adopted internationally by the ITU, develop the components and systems needed to get ADSL products to the market, and start offering ADSL services to subscribers. And maybe do this all within the next year, although there is no real timetable.RF emissions
Telephone industry subscriber distribution plant has largely employed baseband transmission technology over wire pairs. This has changed somewhat over time with the introduction of subscriber loop multiplexing technology. But in many neighborhoods, you can still see telephone technicians splicing huge bundles of wire pairs. Moreover, the drop wire and home wiring still uses baseband transmission of voice telephone calls. And this wiring is unshielded.
Meanwhile, cable plant has evolved to greater bandwidths over the years as coaxial cable technology has improved. Coaxial cable is shielded so that the signal does not leak out and cause interference.
Telephone drop wire and home wiring was installed to carry very low bandwidth voice telephone calls. Sure, it actually has a larger bandwidth than 3 kHz, and that bandwidth can be used to carry data. The higher the speed of the data, the greater the bandwidth that is needed. The data signals generate radio frequencies because of the DMT or CAP modulation. For a data rate of 1.5 Mbps, the frequencies on the wire might go up to 1 or 2 MHz.
ADSL signals on unshielded home wiring can leak, and can cause interference to AM broadcast reception. This was reported by an ADSL equipment manufacturer at a panel of the recent SCTE Conference on Emerging Technologies. The solution, he said, was to tear out the home wiring and replace it with shielded cable. Ha! Someone else said that nobody cares about interference to AM radio because nobody listens to it anymore. Meanwhile, the representative of the National Association of Broadcasters was busy taking notes.
In fact, the FCC cares about interference to licensed radio services. It cares about it a lot. Part 15 of the FCC Rules deals specifically with the issue of emission of RF signals from devices that do not intentionally generate and use RF. While Part 15 contains many technical specifications for these devices, the underlying requirement in Section 15.5 is that devices that generate RF signals must not cause interference to licensed radio services. That includes AM radio broadcasting. Moreover, LORAN, a radio system that provides position location information, operates at frequencies below AM radio.
It's one thing to turn on your PC and accept the interference that it causes to your radio reception. You can turn the computer off if you want, or you can accept the interference. It's another thing for the telephone company to start pumping RF into your home wiring, or down the wires hanging from their poles.
So the next time the FCC sends an inspector to your cable system to check for leakage, you might want to ask him to check the telephone plant for leakage as well, at AM broadcast frequencies.
Will ADSL become a real competitor to cable modems? Seems unlikely, given the problems. But ask me again next year.