Eventually the world of communications will be glass – fiber optic cables. However, that will be a long and costly transition. Meanwhile, much of the world’s communications still runs on copper, the copper of POTS (the Plain Old Telephone System). Given that reality, one of the obvious things to do is squeeze as much transmission speed as possible out of the available copper lines. DSL (Digital Subscriber Line) is one very successful example. A new approach, recently demonstrated by Alcatel-Lucent, can be applied to homes and businesses that have two telephone lines (fairly common) to increase the speed from, say, 5 megabits per second up to 300 megabits per second (depending on distance from a communications hub).
This dramatic increase in speed, which begins to match those of advanced network and fiber optic systems, is achieved by combining three existing transmission techniques: Bonding, vectoring, and DSL phantom mode.
All forms of communication are subject to interference – noise – by magnetic fields and other forms of radiation. In fact, whenever more than one line of electric current is in proximity to another line, there is a probability of some interference or ‘cross talk.’ Faster transmission speeds are more vulnerable to line noise, so one of the problems of increasing the speed of a copper line system (DSL) is simultaneously reducing the noise. This can be done with vectoring, which can be most simply explained as matching the frequencies of noise with an equal and opposite counter frequency – noise cancellation. This is a common technique, for example, there are many earphone sets that have a noise cancelling feature.
Bonding refers to the technique of combining the signal of individual (copper) lines as if they were a single transmission. This can achieve speed increases almost equal to a multiple of the number of lines (e.g. 2X).
The third technique, DSL phantom mode was invented over a hundred years ago (1886) by telephone pioneer John J. Carty. Digital transmission is usually carried through two wires twisted together (a positive and negative line). Carty proved that it was possible to send a third signal on top of four wires separated into two twisted pairs. One twisted pair carried the positive side of the third (phantom) signal, and the other twisted pair carried the negative side. Processors at either end break up or combine the signal into the three ‘streams’ along the two pairs of lines.
Now, as mentioned, putting copper lines together increases the noise, so the application of vectoring techniques is a necessity for making DSL phantom mode work. The bonding techniques help with the combination of signals to get an optimum amount of transmission data through the available lines. Together all three techniques make it possible for DSL to achieve much higher speeds.
Alcatel-Lucent doesn’t believe it will roll out the combination technology until after 2011. Even so, that’s well ahead of the timetable for extending fiber-optic technology to all areas of the U.S.
One-hundred-megabit DSL is “what we can see” in the next five to 10 years, says Cioffi [CEO of ASSIA, a DSL management company]. That will be just in time to realize the Federal Trade Commission’s goal, announced in February, of rolling out 100-megabit-per-second broadband to 100 million U.S. homes by 2020.
[Source: Technology Review]
As ever, there are limitations to these advanced DSL techniques: They require at least a two-telephone line connection. Their installation requires new equipment to handle the processing. Users must have a different kind of modem. In some cases, more communications hubs are needed because of the loss of signal quality over distances greater than 400 meters. In short, squeezing more speed out of existing copper lines is effective, but it costs more.