The FCC has proposed a target of delivering 100 megabit per second Internet service to much of the US. Although this might be achieved using Cable TV or new fiber optic cables to every house, recent developments from Bell Labs suggest that the target can also be met with good old copper telephone wire. This paper describes the history and future of what is known as “telephone twisted pair”, the copper telephone wire you have in your house and see on telephone poles outside your window.
Plain Old Telephone Service (POTS) transmits your the sound of your voice as waves (an analog signal) over two telephone wires. The wires start at the microphone of your telephone, connect to the wall, then wires in your house collect in the basement where they connect to a pair of wires running out to the telephone pole and down the street to some phone company central office equipment.
When voice gets to the phone company it is digitized into a stream of bits or numbers. The phone company learned to digitize sound long before microprocessors were available to cheaply compress it, so the phone standards for voice telephone calls uses uncompressed 64,000 bits per second. Cell phones, by comparison, compress sound just as an MP3 file compresses the 700 megabytes of a CD down to a much smaller size. Once digitized, the signal of a phone conversation can be transmitted around the world on fiber optic cable with no loss of quality. At the other end, it is turned back into an analog signal and is transmitted over copper wire to the other phone (or compressed and transmitted wirelessly to a cell phone).
The phone company runs two pairs of copper wires to most homes and apartments. This allows you to install a second line for business or your kids, but it also provides a fallback in the typical single line house when the first pair of wires has a problem.
Any sufficiently long copper wire is an antenna. It receives radio and TV signals plus any noise from nearby electric equipment. It also “broadcasts” your phone converstation at low power. The phone company bundles dozens or even hundreds of phone wire pairs in the same cable, where they run next to each other for hundreds of feet. So your phone conversation is “broadcast” as radio waves to the next wire which acts as an antenna and “receives” the signal. This is called “crosstalk”. In old phone systems you could hear the faint sound of neighbors talking on their phone lines, and they could faintly hear what you were saying.
To avoid crosstalk, each pair of wires is twisted around and around each other. The phone signal is represented by the difference in voltage between the two wires. To make that difference, one wire is driven negative in voltage while the other is driven positive. When the two wires wrap tightly around each other, the positive and negative signals tend to cancel each other out. Any nearby wire may receive a slight signal from the positive wire, but then after a short distance and a half twist that wire is now close to the negative wire of the same pair with an opposite signal that cancels out the previous signal.
Over time phone standards have improved with better quality wire, insulation, and more twists per foot of cable. However, urban phone system have a mixture of new and old wire, and old houses and apartment buildings may have to deal with some very old low quality wire in the wall. When you wire a new house for Internet and use only Category 5e wire and modern cables and jacks you can (over a short distance) easily transmit data at a billion bits per second, but across all the circuits in town the phone company will average much lower speeds.
The maxium speed also depends on the distance the signal has to travel. The longer the path, the longer the antenna and the greater the opportunity for crosstalk and induced environmental noise. Rather than running the phone wire from your house all the way to the central office, it is now common for the local Phone Company to position a large box of electronic circuits in each neighborhood. This DSLAM (pronounced “Dee-Slam”) provides the phone company end of DSL Internet service. You can get 1.5 megabits per second if the DSLAM is within 15,000 feet (4.5 km) of your house, but to get much higher speed it has to be much closer (ideally within 2000 feet).
DSL transmits data over the ordinary (analog) voice telephone system. It does this by encoding the data as very high frequency signals, mostly beyond the range of normal hearing. It can produce some noise on the line, so the phone company provides subscribers with a filter that you put between the raw wires and any connection to a regular telephone.
How to Make this Faster
Now that we have the basic rules of traditional and modern phone service, what tricks can the phone company use to increase the Internet service speed?
Go All Digital
DSL carries data on top of a basically analog phone system. However, cell phones and voice-over-IP take the audio signal, digitize it, compress it, and then transmit it over what is basically a digital connection. To make this work, all the existing analog phones in your house have to be connected to some box that provides the digitizing and compression. Then the wires from your house can be used to carry an all digital signal with substantially higher bandwidth.
Adding an extra box in your home or apartment would be expensive all by itself, so the phone company may use this opportunity to offer extra services. The AT&T U-verse service combines conventional Internet DSL service speed (1-6 megabits) with competition for cable TV. Fiber optic cable supplies compressed network and premium channels as digital data to the DSLAM in your neighborhood. Using the box in your house, you select which TV “channel” you want to watch, and the compressed digital signal of that program is transmitted using some of the remaining bandwidth of the digital phone connection between the DSLAM and the box in your house. The phone company pays for the additional cost of the infrastructure by charging you more for a broader set of digital services.
Use More Than One
Internet traffic can use more than one wired connection by spreading the data across lines and then combing it at the other end. This technique is called “Bind”. Digital cable TV, for example, provides a signal of 40 megabits per second on any TV channel, but to achieve higher speeds the cable company can combine up to 4 TV channels of 40 megabits each into an effective 160 megabits per second of Internet connectivity (that you share with everyone in the neighborhood). The phone company already runs two pairs of wire to most households. If you add a second digital signal on the second line and then bind the two, you can double your Internet connection speed (and unlike the cable TV, this connection is dedicated to your individual location).
However, once you have more than one twisted pair of wires, there is an additional trick you can use. Signals are represented as a difference in voltage between pairs of wires. Two signals are sent down the two twisted pairs. However, a third signal can be generated by inducing a voltage difference between one twisted pair and the other twisted pair. This is called the “Phantom” signal. Traditionally, the phantom has not been used for high speed data. Sometimes it is used to provide power to devices, and in the 1980’s an old IBM technology called “Token Ring” used the phantom voltage simply to flip a relay from off to on. However, microprocessor technology has advanced a lot since then and Bell Labs appears to have figured out a trick for making the phantom circuit carry data.
The problem is that while each pair of wires twists around each other to cancel out crosstalk, any signal across two different twisted pair is more vulnerable. Bell Labs came up with a solution to use the computing power of modern chips to solve the problem. First, assume that you have a specific run of circuits through a cable bundle between a DSLAM and a group of houses. Once the wires are all in place, especially if every house has an all digital service like U-Verse with digital boxes terminating the connection in every house, it is possible to test the lines and determine just how much crosstalk any given circuit on one twisted pair of wires will induce on every other pair of wires in the bundle. If connections change you can repeat this test every so often. Since the DSLAM knows what signal it is about to put into every pair of wires in the bundle, it can calculate the likely crosstalk that each signal will generate in all the other pairs and the composite induced noise that will be generated in each individual pair of wires by the combined action of all the signals on all the other pairs. Then, just as noise canceling headphones generate “anti-noise” that cancels out the sound of the airplane engine while you are flying, the DSLAM can generate an anti-crosstalk signal at its end of each pair of wires that will exactly cancel out the induced crosstalk signal received at the other end in your home.
Computer disks are big and cheap today. However, if you want to spend five times more for the same disk, put it in your X-Box, PlayStation, or Tivo. Consumer electronics devices come with insanely small disks and then charge you a lot of money to upgrade to what is still an insanely small disk.
Dedicated personal bandwidth is expensive. Broadcast shared bandwidth is cheap. Cable TV broadcasts 130 channels of 40 million bits per second per channel 24 hours a day 7 days a week. What makes it cheap is that everybody gets the same signal at the same time. There are 142 million books in the Library of Congress. If each book on average is 10 megabytes of data, then at the aggregate rate of 675 megabytes per second Comcast could transmit essentially everything that has ever been written to everybody in a little more than three weeks.
Of course, you have to leave room for prime time TV and afternoon soap operas. However, there are not really enough insomniacs to use up all the pay per view bandwidth at 3:00 AM. Since the phone company runs a dedicated line to every house, that bandwidth is idle whenever you aren’t using it.
So the first step is to put a hard disk in the box that connects directly to the digital phone system. Then you need to add some type of subscription reservation system and a central broadcast mechanism. People could then subscribe to data that they want, but do not want to pay for dedicated bandwidth to view right now. The content will be delivered overnight using idle bandwidth and broadcast technology.
The Internet cannot support and cannot be designed to support a model where every American comes home at night and sits down to select an individual on-demand private viewing of streaming video content. However, the existing TV broadcast system has vastly more capacity than it needs to deliver its current content to every household provided that every house receives it at the same scheduled broadcast time. That is why people need digital recorders and in the long run cannot depend on Hulu or Netflix streaming. If we extend the broadcast model, we can probably deliver every commercial broadcast TV channel in every country on earth to anyone who wants to set up his recorder to receive this data during an otherwise idle period overnight.