The concept of transmitting data and general communications services over the traditional mains wiring began in Switzerland in the late 1890s, at the same time as the paradigm shift from classical to quantum physics. This article addresses some of the developments in using the power line network for providing traditional and modern data communications services.

Since those early days power line communication has evolved from the low frequency to the high frequency transmission and reception, with applications ranging from switching on the lighting systems on motorways through to high speed data Internet traffic for the home and industrial user. The key countries involved in these developments include Europe, Japan and the USA. Although the concentration is directed to high-speed data for the Internet, much has been achieved in the low frequency part of the electromagnetic spectrum.

The frequency ranges used for the low and high frequency ranges vary according to the country of operation as follows:

LOW FREQUENCY RANGES

Europe: 3KHz to 148.5KHz CENELEC Standard EN50065-7
USA: 45KHz to 450KHz
Both these standards are well established. They operate at low data rates and low power, providing communication for automatic meter reading (AMR) lighting control, tariff control of electricity and load management.

Figure 1: Electrical characteristics of an LVDN cable

The success of these services over the power line led many companies and organisations to start investigating other potential services, in particular the connection of the computer to the Internet via the power line. To develop such a service using the ubiquitous power line, higher data capacity would be required, and therefore extended bandwidth. Examination of Shannon’s rule relating the data transmission rate over a communication medium with a defined bandwidth and a certain signal to noise ratio dictated that the frequency of transmission would need to be several orders of magnitude higher compared with the low frequency range, because of the higher bandwidth. The frequency of transmission was set to 1.6 MHz to 30MHz, with a partition into the access band of 1.6MHz to 10MHz and In-house band of 10MHZ to 30MHz.

The frequency bands are selected to take into account the attenuation effects of the power line as the frequency of operations increases.

Although there are considerable advantages to this technology, in particular the permanent wiring being in place for both homes and industrial premises, which will consequently lower the cost of installation, there are a number of technical problems to be investigated and resolved before a full roll-out can be guaranteed.

The transmission of high-speed data over the power line network requires an increased bandwidth and higher energy of transmission. Operating at these frequencies, the electrical power line behaves like an aerial, emitting part of the input differential signal that is converted to a common mode signal. This common mode signal is determined by the symmetrical properties of the cable, with a perfectly symmetrical cable being fully balanced and therefore not radiating a common mode signal. The more unsymmetrical a cable becomes, the greater the common mode signal and hence the increased radiation. Therefore the power cable may be considered as an aerial with a certain power efficiency or loss.

The electrical characteristics of the power cable for both the medium and the low voltage distribution network (LVDN) considerably affect the transmission and reception, due to the variable impedance parameters of capacitive reactance and inductive reactance. The length of the cables and the amount they are split at different junctions all contribute to the propagation conditions. An overriding factor is the age of the LVDN cable – it may be several decades old or a modern cable, which has improved electrical parameters (see Figure1).

The electrical architecture of cables in buildings, both residential and industrial, has an effect on the transmission quality. Generally the wiring architecture is of two types – tree and branch from the supply point, which is seen throughout the majority of building stock in continental Europe and the USA, and instances where the house wiring is based on the ring main layout, as seen in the UK and some parts of Europe. Radiated emission may take place, depending on the way the equipment is connected to the ring main. This can lead to interference of established radio services such as broadcasting channels, amateur radio, mobile communications, distress frequencies, military communications and radio astronomy, in adjacent buildings and homes.

Figure 2: Radiated emission standards

During 2001 the European Commission issued a mandate, M313, to the European Committee for standardisation, CENELEC, and the European Telecommunications Standard Institute (ETSI). Both these committees work in concert to develop high frequency power line standards and to determine the radiating emissions for such networks. Within these organisations, different countries and bodies had established their own particular radiated emission level (see Figure 2).

The different limits range from 60dB’s/uV/metre to 20dB’s/uV/metre, meaning that there has to be some compromise among the countries. Currently the EMC joint committee of CENELEC and ETSI is addressing these issues and making progress towards setting a limit. The IEC CISPR22 committee is also working on establishing radiated emissions, but only for products. The mandate M313, however, covers the network. It is only used where a complaint is received by a regulatory body that could potentially lead to the equipment being switched. These are key issues to be resolved if there is to be a radiated emission standard for the world.

Thoughout the world many trials have begun in recent years by leading manufacturers of modem equipment, in conjunction with electricity companies. (See Figure 3).

Figure 3: World trials of plt equipment

Key players in terms of equipment are ASCOM, MAINNET and DS2, who are providing successful trials in the UK with Scottish and Southern Energy and other utility companies in Europe and other parts of the world. In the USA, sound progress is being made with a number of companies including the Easy Plug Organisation, similar to the PLC forum in Europe and representing a number of companies.

The modem equipment is producing data rates ranging from 2 Megabits/sec upwards towards 45 Megabits/sec, which is comparable with DSL services and cable modems.

Despite these successes, however, there are still key problems to be solved, both theoretically and experimentally, before a full commercial service is rolled out throughout the world. The key issue is that of the radiated emission and setting the limits of the transmitted energy, and therefore the interference potential with established radio services. Within the home or building there is the connection of different manufacturers’ equipment to the mains wiring, with different impedance and electrical conditions.

A number of industrial companies and university research groups are being funded through the European 6th Framework Programme, known as OPERA, to investigate and resolve these issues, and thus to prove that power line technology is a recognised technology for world use.