Powerline Communications

From power grid to communications network
Highspeed Internet on the power grid

Powerline Communications (PLC) – communications over the electricity distribution grid – has become a hot topic in recent months. Although this 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. The latest development was driven firstly by the growing demand for bandwidth, and secondly 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. 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.

Separation of indoor and outdoor applications

The idea of combining communications signals and electricity on a single transport medium is an obvious one. With a world-wide coverage of approximately 95 percent, the electricity grid is the largest network bar none. Applications are divided into two areas: procedures which are performed outside the home (outdoor), and procedures 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 i.e. 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 coordinates 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.

Fig. 1 Overview of the PLC system 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 under 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.

System components

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 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.

Fig. 2 Ascom Powerline APA 45. This adapter is switched between the socket and terminal. The outdoor units (master, access point and repeaters) are normally connected with all phases using fixed cables. The PLC signal is switched between two of the three phases. As a result, signalling can be phase-optimised – an option not offered by the adapters. These are directly connected to the socket via a conventional electric cable, with the signal connected between phase and neutral conductor. At first glance this disparate linking may not appear particularly beneficial, but measurements have proved otherwise. Cabling inside a building produces a high level of crosstalk between the individual wires. The initially selected link counteracts this after a certain distance.

Fig. 3 Connection diagram for PLC equipmentFlexible bandwidth management

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 – so all users can derive equal satisfaction from performance.Part of the bandwidth is prioritised for delay-sensitive traffic such as voice and video conferencing. Ascom PLC equipment ensures the quality of various services such as voice and data without prioritisation, in compliance with Standard 802.p.

Installation aspects

As with other systems, good planning ensures efficient installation and optimal operation of the PLC system. In the case of 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. 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. Thanks to extensive field-trial experience, the Ascom installation crew is able to make reliable predictions of distance.

Fig. 4 Signal attenuation as a function of distance

Connecting PLC systems to the backbone is a more complex task. This 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 Ids, 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.

Practical deployment now follows field trials

Proof that the PLC 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 fulfilled all expectations of reliability, functionality and the practical applications of Powerline Communications. 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 uses PLC 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. 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.

New business areas

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. PLC components must be installed professionally and correctly before they are able to bring broadband communications into the home. Author’s addressBeat Hübscher, Ascom Powerline Communications AG, E-mail: beat.huebscher@ascom.ch.