Making broadband PLC a commercial reality16 November 2001
Powerline communications (PLC) - communications over the electricity distribution grid - has had its share of technological and commercial challenges. But in recent months it has once again become a hot topic, and for one Swiss company PLC equipment has now entered the mass production phase. Beat Hübscher, Ascom Powerline Communications, Berne, Switzerland
Although PLC technology has been in use for special applications for several decades - street lighting is frequently operated according to this principle, and the well-known baby intercom sends voice signals over the internal household power network - communication in these cases is exclusively in the narrowband range and transmission rates are correspondingly low.
Recent developments have been driven first by the growing demand for bandwidth, and second by the quest for a fast, efficient alternative to the "last mile", which in many countries is still protected from free market forces. High bandwidths are essential for the breakthrough to consumer-driven e-business. With powerline broadband, indoor networks, shared "internet access", peer-to-peer and other benefits become possible. Only when high-speed internet access becomes affordable and widely available, can electronic trading with end consumers really take off. Powerline aims to make this quantum leap possible using the existing infrastructure.
Indoors and outdoors
The idea of combining communications signals and electricity on a single transport medium is an obvious one. With a worldwide coverage of around 90 per cent, the electricity grid is the largest network bar none.
The PLC system (Figure 1) is divided into two areas: outside the home (outdoor); and inside the home (indoor). In the outdoor zone, the conventional telecommunications infrastructure is used to connect the relevant local network station with the telephone network or a specific internet backbone. Depending on distance and local conditions, the connection is enabled by radio, copper lines or optical cables. The local network station combines data and voice signals on the power grid and sends them as a data stream to any socket in connected households, ie to the end user via the low-voltage network.
The access point forwards incoming data streams to the indoor network, and an indoor master in the household controls and co-ordinates all (externally and internally) transmitted data signals. Intermediate adapters separate data and power at the socket and forward the data to individual applications. There is no need for separate telephone or data cabling since the socket, far from being a mere electrical point, becomes a powerful communications interface which bridges the last mile for high-speed internet access, thus enabling networking throughout the building or household.
The PLC network is divided into two cascading but independent systems for good reason. The public part of the network ranges from the network transformer to the individual access point in the home. This section is normally in the hands of, and the responsibility of, energy supply companies, who are ensure correct operation and high-level system quality. The electrical system from the access point to the individual socket is the householder's responsibility and is not subject to consistent checks. Another advantage of the division is that the outdoor system is used by several buildings or households for access to the backbone network and must therefore guarantee high-level reliability. By the same token, network traffic inside the household does not affect the outdoor zone. There are also solid technical reasons for the separation. Signal distribution in the public power grid behaves differently from signalling inside a building. Low frequencies are more suitable for the outdoor zone and higher frequencies for the indoor zone.
Powerline technology brings data streams and voice signals to sockets inside a building via the low-voltage network. The concept is based on a master-slave principle and uses a small number of standard units. The outdoor master acts as administrator for the outdoor system and as a gateway connecting the PLC system with the backbone network. The access point connects the outdoor and indoor systems. Outwardly it performs the functions of an adapter (slave), while inwardly it acts as the master and is responsible for administration of the indoor system.
The indoor adapter (Figure 2) provides the interface between the internal data network, PC, printers and telephones on the one hand, and the backbone network for Internet, telephony, video conferencing and similar applications on the other hand. Adapters which communicate on the outdoor system's frequency are also available for connection to the outdoor system. The adapters are equipped with standard interfaces (Ethernet, USB, analog a/b telephone interface). Repeaters amplify the signal over long distances.
The outdoor units (master, access point and repeaters) can be connected with all phases using fixed cables (Figures 3 and 4), and signalling can be phase optimised. Connection to the socket is via a conventional electric cable, with the signal connected between phase and neutral conductor.
Bandwidth and inteference
Powerline is a shared medium: it works on the point-to-multipoint principle. A local network station supplies power to a specific number of households and also delivers data or voice to several terminals. Users who communicate simultaneously share the available bandwidth. Currently this is 4.5 Mbit/s. But speeds of 20 Mbit/s are envisaged in the medium term. Flexible bandwidth management and packet-switched data transfer ensure sufficient bandwidth availability on the shared transmission medium for voice and data services, even during peak times.
Part of the bandwidth is prioritised for delay-sensitive traffic such as voice and video conferencing.
The equipment ensures the quality of various services such as voice and data with prioritisation, in compliance with Standard 802.1p.
High data volumes necessitate high bandwidths. The Ascom PLC system uses frequencies between 1.6 and 30 MHz. The various frequency bands are dynamically managed to produce a maximum overall throughput. Up to four frequency bands between 1.6 and 13 MHz are used for the outdoor zone, and up to three frequency bands between 15 and 30 MHz for the indoor zone. Carrier frequencies are determined on the basis of extensive measurements and frequency planning within the short-wave band, in line with the current standardising work being done by Cenelec (European Committee for Electrotechnical Standardization). The draft of a European standard (prEN59013) is currently being submitted for approval in individual member countries.
In principle the aim was to enable co-existence with radio frequency bands which are already used by key short-wave services. The signal is split into individual frequencies to enable switching from one band to another, if required, and deactivation in the event of local interference. In addition, lower frequencies were reserved for the outdoor system due to their lower signal attenuation, while at the same time measures were taken to ensure sufficient separation of parallel-operating outdoor and indoor systems and to prevent interference. Each of the carrier frequencies is comparable with an independent communications link, which provides users with a data speed of 750 to 1500 kbit/s, depending on connection quality.
For powerline installations, the optimum installation points also need to be determined, deployment plans must be drawn up for the available carrier frequencies, the IP addresses and VLAN identifications must be assigned, and the backbone connection set up.
Under normal circumstances the installation points are already in place: the outdoor master is sited in the transformer station, the access point beside the electricity meter, the adapters in a socket defined by the user. Determining the optimum distances is not quite as simple. These vary depending on power output, loss during power distribution, and the noise level at the receiving end. However, by applying the results of extensive measurements it is possible to predict the average distance in a concrete situation with sufficient accuracy (Figure 5). Without repeaters this can be between 200 and 300 meters, but only for public electricity networks with aerial or underground lines.
For indoor networks with a number of different interference sources, which in many cases are unstructured, the average values provide no useful indicators since different types of installation affect the transmission distance of up to 100 meters in very different ways. With increasing experience, however, reliable estimates can also be produced for indoor systems.
Connecting PLC systems into standard backbone architecture generally calls for close collaboration with the network provider and above all concerns aspects of system security, connection of PLC equipment to the network management system, IP addressing, the allocation of VLAN ID's, and integration of voice traffic in the system. Only when all this preliminary work is completed and the associated questions are clarified can the actual installation begin. A top-down approach is recommended: beginning with the backbone and proceeding to the outdoor master and access point, down to the adapter inside the building and the required settings on the user's PC.
From theory to practice
Proof that the powerline communications concept also works in practice was furnished by a series of field trials in 16 European countries, from Portugal to Scandinavia, as well as in Hong Kong and Singapore. These trials fully met expectations. The first installations are now already up and running or about to go live.
Users in Germany include the electricity companies RWE Energie, Essen, and EnBW Energie, Baden-Württemberg, while in Spain the energy and telecoms group Endesa is applying the technology.
Lina.Net of Iceland, a subsidiary of Reykjavik Energy, has just begun introducing PLC technology with the declared objective of providing private households with fast Internet access over the power grid rather than the telephone network and in Sweden, Sydkraft, one of the leading energy providers in Scandinavia, uses PLC for bridging the last mile as well as for networking inside buildings.
Internet browsing, simultaneous transmission of faxes and voice calls with first-class voice quality are only a few of the possibilities offered by Powerline technology. All equipment within a household - PC, printer, phone, fax - can be interconnected simply, flexibly and without additional cabling by using existing power lines. Electricity, gas and water meters can be read on-line, and alarm systems, heating and household appliances can be controlled over the internal power network and maintained via Internet.
Powerline is also opening up new possibilities in other areas - for instance care for the elderly and disabled.
For energy suppliers, the potential inherent in Powerline technology is enormous. Gaining direct access to end customers provides them with an opportunity to enhance their market image with individual, innovative offerings, and penetrate new business areas.
In view of the deregulation of the electricity market and the associated intense pressure on pricing and competition, this is a welcome development as well as a new way of using existing assets at a relatively low investment cost. A new business area is also being opened up for electrical installation companies: PLC offers them the opportunity to grow in their home market by expanding into data communications, and to play a direct role in the information economy.