ADSL - a concept for high-speed transmission on copper lines

By Knut C Aas

Not too many years ago, it was envisioned that most homes in the near future would be connected to high-speed transmission lines making interactive multimedia services available to the general public. Electronic libraries containing video, audio and text could be accessed from the TV-set at the touch of a button. Fibre-optic lines were thought to be the means by which these services were to be delivered into each home. In the meantime, telecommunications companies have become more realistic. Though fibre has become the preferred medium for transmission between local exchanges, the final few kilometres between exchanges and residences consist of old-fashioned twisted-pair copper lines, which lack the large bandwidth of fibre-based lines that makes high-rate transmission simple. The cost of replacing these transmission lines on a large scale runs into billions, placing such investments far into the next century. Moreover, there is as yet no demand for broadband services existing today only on the drawing board. It has become evident that a transition to fibre will have to be gradual and be supported by an increasing demand for such services. It may appear that multimedia home-access has been relegated to the distant future.

But now prospects are looking up once again. In 1989 Bellcore introduced a concept that may put the information highway within the reach of this decade, at reasonable costs to telecommunications companies and consumers alike. The secret lies in taking advantage of digital transmission and compression techniques using the existing copper-based infrastructure to deliver digitised information at high rates. A key idea is that the envisioned services require high-rate transmission of video and audio in only one direction: from the exchange to the subscriber. Upstream transmission is required only for simple control signals initiated by the user, such as for selecting programming and services from a menu, which can take place at much lower rates. This consideration together with advances in digital signal processing and VLSI technology, have put transmission via copper lines within the realm of the possible, giving rise to the Asymmetric Digital Subscriber Line (ADSL).

Copper lines are subject to several impairments. Signals propagating on such a medium are attenuated by an amount that increases with the length of the line. This limits the distance a signal can travel without being regenerated, which in any case would not be economically feasible for large-scale deployment. As high frequencies of a signal are attenuated more than low frequencies, the effective transmission bandwidth is limited to about 1 MHz, only a fraction of the bandwidth available on fibre. Moreover, the transmission loss is frequency-dependent and not easily taken into account even at lower frequencies. Performance is also adversely affected by traffic on adjacent copper lines in the same cable interfering with the transmitted signal; an impairment known as crosstalk. Such distortion cannot simply be eliminated by increased transmission power, as the interference would grow in proportion. Occasional impulse noise generated from external electronic appliances and AM radio broadcasts further degrade transmission quality. It is in such a sharply band limited, distorted and noisy environment that high-rate signal transmission has to take place. Needless to say, this places high demands on the new technology.

A proposed standard for ADSL splits the available transmission bandwidth into three parts, illustrated in figure a. The low-frequency end will as now be reserved for ordinary analogue telephone service, occupying the lower 4kHz of the spectrum. Alternatively, a slightly larger piece of the spectrum may be used for ISDN basic rate access. The main bulk of the bandwidth in the range from 50kHz to about 1MHz is reserved for digital transmission of data in the downstream direction, i.e. from the exchange to the subscriber. The upstream control channel occupies the 40kHz band in between, a figure which includes ample separation between the three bands. The signals in each channel can be extracted with an appropriate band-pass filter.

To deal with the frequency dependent loss and noise in a typical copper line, the downstream channel will likely employ a modulation technique known as discrete multitone (DMT). This technique divides the available bandwidth into 256 sub-channels of width 4kHz each, and allocates transmission power to the individual channels depending on the noise power and transmission loss in each band. Each channel carries multilevel pulses that can represent up to 11bits of data, while the poorer sub-channels carry fewer bits and can even be shut down entirely (figure b). The modulators can be implemented digitally in software or in digital logic, keeping production costs low. DMT has been used successfully in commercial high-rate telephone-voice-channel modems. To deal with occasional bursts of impulse noise, the digital signals can be encoded with error-correcting codes similar to those employed on compact disks that will enable transmission at bit error rates of 10-7bits/s or better.

A commercially successful implementation of ADSL places minimum requirements on transmission capacity and penetration. While most customers lie within 3.5km of an exchange, a small percentage may require transmission over distances of up to 6km. Currently, transmission rates of 1.5 to 6Mbit/s are being discussed for service over distances in this range. Advances in digitising and compressing video information make it possible to supply full-motion-video of acceptable quality at rates down to 1.5Mbit/s. For instance, video encoded at 2Mbit/s according to the MPEG2 standard compares favourably to traditional VHS recordings. Digital subscriber lines operating at this rate will be able to supply multimedia services to nearly all households in a region. Transmission rates of 6Mbit/s over shorter distances will allow several channels to be viewed simultaneously at a single subscriber location. The upstream channel can be configured for transmission of up to 384kbit/s. A few customers living in industrial areas with severe electrical interference may not be able to receive these services with the proposed design specifications.

For video-on-demand to be a success, the price for consumers must be competitive with video-rental, satellite, and cable-TV services. While the technology is still being developed, it seems likely that mass-produced ADSL transmitters and receivers will be sufficiently inexpensive for this requirement to be met. In that case, multimedia services may not be very far away. ADSL would allow for a gradual build-up of demand for more exotic broadband services. Ultimately, it may prove to be the means by which a transition to fibre in the subscriber loop is made possible.