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Speeding up the local loops.
Voice frequency modems have hit the wall that was described by Claude Shannon of Bell Telephone Laboratories in 1948. Shannon found the theoretical capacity of a 3,000-Hz connection - the frequency of the worldwide voice network - to be about 30 Kbps, depending on the signal-to-noise ratio in the channel.
But if the connection is lim-ited to the twisted-pair loops of wire between your home and the nearest telephone company central office (ignoring the connection to the voice network), the 3,000-Hz restriction is removed. This means you can extend the data capacity of your line by a factor of 40 or more using modern digital modulation techniques.
Using technology developed in the 1980s, ISDN is the first service to exploit the bandwidth potential of twisted-pair loops with complex digital signal processing. ISDN uses electrical pulses with four voltage levels, and since there are four unique two-bit numbers (00, 01, 10, 11), each pulse represents two bits of data. By sending 80,000 pulses per second, ISDN can stuff a 160-Kbps signal through loops up to 18,000 feet long. ISDN also employs complex circuits called echo-canceler hybrids that allow signals to be sent simultaneously in both directions on one pair of wires.
The length of lines
The distance of 18,000 feet is as long as unaided twisted-pair loops are allowed to be for POTS (plain old telephone service) in the US, so it is the target distance for any company trying to increase the speed of data transmission through telephone loop pairs. About 75 percent of loops in the US are shorter than 18,000 feet. Longer loops require special engineering, even for regular phone service. ADSL provides the telcos with high bandwidth without necessitating a costly reengineering of the local loops.
Taking the high road
Signals on the ISDN "digital subscriber line," or DSL, and the similar, higher speed 784-Kbps HDSL (high bit-rate DSL) are called baseband signals because the frequency spectrum includes that used by direct current and POTS. If you tried to talk over a line carrying a baseband signal, you would hear an unacceptable level of noise.
Transmissions at frequencies above the band used for voice are called passband signals. ADSL (asymmetric DSL) technologies are examples of passband transmission techniques for use over telephone twisted-pair loops that don't disturb the POTS on the same pair.
Telephone companies developed ADSL in the late 1980s to provide video dial-tone services over the telephone loops. ADSL provided as much capacity as possible for the transmission of digital television signals from the network to end users and reserved less bandwidth for transmission from end users to the network. The North American standard for ADSL calls for data rates of as much as 640 Kbps upstream and as much as 6 Mbps downstream.
The need for higher speed Web access that avoids tying up telephone voice switches, as modems and ISDN do, is driving the commercial development of ADSL technology. Although Web surfing works well with asymmetric data transmission, the growing market for peer-to-peer communications would benefit from symmetrical capacity. Luckily, the two emerging passband data modulation techniques for ADSL - CAP and DMT - can also support symmetric transmission, or SDSL.
GlobeSpan Technologies and Bell Labs developed carrierless amplitude and phase modulation (CAP) for ADSL and SDSL. CAP translates bit combinations into symbols with different amplitude and phase combinations, like the quadrature amplitude modulation (QAM) used in most modems today. CAP-16, for example, transmits signals with 16 different combinations of amplitude and phase; each signal represents a different combination of four bits (there are 16 different four-bit numbers). If CAP-16 signals are sent 250,000 times per second, the resulting data signal carries 1 Mbps. The use of a complex coding scheme like CAP can extend data rates to more than 8 Mbps downstream and 1 Mbps up.
A standards race
GlobeSpan CAP is being used by telephone companies in trials of ADSL for high-speed Internet access, giving it a lead over the other popular modulation technique for ADSL: DMT (discrete multitone). DMT is the modulation technique chosen as the US video dial-tone standard and is used by companies like Amati Corporation and Aware Corporation.
DMT divides the frequency band for each direction of transmission into many sub-bands and uses QAM to modulate data bits into each of the sub-bands. By using sub-bands of approximately 4 KHz, a DMT modem acts something like dozens of voice modems operating simultaneously. For example, a DMT signal that occupies a 256-KHz bandwidth with 64 4-KHz sub-bands (each of which carry around 20 Kbps) could in aggregate transmit more than 1 Mbps of data.
And the winner is ...
Both CAP and DMT transceivers adapt their characteristics to the line when they are installed, and both are capable of providing a range of bit rates from a few hundred Kbps to more than 6 Mbps after adapting to the length and background noise of telephone twisted pairs. Both can be operated asymmetrically or symmetrically. Both can attain Mbit-per-second speeds to the home at distances approaching 18,000 feet with up to 384 Kbps allocated for the upstream communications at that distance. And both bypass the voice switch. Each approach has its advocates, and it's likely that factors such as time to mar-ket, power dissipation per line, physical size, and cost will determine the winner.
Of course, the ultimate winner will be the power Internet users who can expect megabit speeds on their own secure, low-cost data line - the same wires used for POTS. In the next year, we'll see the introduction of plain old data service using ADSL, joining the venerable POTS offered by the telcos.
George T. Hawley (email@example.com) is president and CEO of Diamond Lane Communications Corp., a start-up developing the Hitchhiker ADSL access system for telephone companies.