An Introduction to Power Line Communications

by Xavier Carcelle
02/15/2007

Power Line Communications (PLC) is the use of existing electrical cables to transport data, and it has been around for a very long time. Power utilities have been using this technology for many years to send or receive data on the power grid using the existing infrastructure. For instance, the electrical power utility in London used PLC to remotely control some of its equipment on the grid (such as high-voltage switches) in the 1920s. This technique is still employed by several utilities that use analog or digital devices to transfer 9.6 Kbits/s over many miles of electrical cable.

More recently, the development of home automation (the current notion of smart houses) and car automation (cutting cable costs and weight, and networking the devices in-car) has pushed the use of low bit rate PLC techniques with technologies such as X10 or LonWorks. Some major home-device manufacturers have announced in recent months that they will integrate PLC chips into their products to prepare the future network of the smart house.

But the real turning point for PLC came with the high bit rate technologies of chips that allow several Mbits/s over any typical in-home electrical network, with the universal Ethernet or USB interfaces. Now more and more electronics products integrate such interfaces, and PLC technologies allow them to be connected over an existing network of cables "in-wall" with a simple plug-and-play in any outlet.

PCs in homes today have two main requirements: wireless IP ubiquity and high bit rate IP services with the so-called multiplay service proposed by most ISPs (Internet, IP phone, IP Video …). The age of the wireless revolution has already started, but the generation of power-line communications is just beginning.

In-house, HD video streams require a stable, simple, and secure technology that is ready to use. The different Wi-Fi standards have demonstrated good bit rates from a physical-layer point of view (11 Mbits/s with 802.11b, 54 Mbits/s with 802.11g, and now 250 Mbits/s with 802.11n), but the reality has demonstrated some difficulties in achieving a good level of security, connecting systems through walls or floors, and providing stable data throughput for IP video applications such as IP TV or VoD.

It has taken some time for PLC to reach these kind of bit rates, but now HomePlug 1.0 supports 14 Mbits/s, HomePlug 1.1 (also called Turbo) supports 85 Mbit/s, and HomePlug AV supports 200 Mbit/s. One must remember that these specified bit rates are at the physical layer (also called theoretical), and should be roughly divided by three to represent the data throughput a user will experience with web browsing, file sharing, or other Internet usages. One must also remember that this data throughput is shared among all the devices connected on the electrical network, which is used as a shared medium and can be thought of as an Ethernet hub.

From a standardization point of view, several technological standards for PLC compete among many consortiums worldwide: HomePlug, OPERA (a European-based consortium), and CEPCA (a Japanese-based consortium). Each technology now proposes a 200 Mbits/s version on-chip, integrated by different OEMs worldwide for the different markets. HomePlug products have led the technology so far by shipping a significant number of chips, including reaching the million chip mark in 2005, which was a keystone in the maturity of PLC.

Today the need for PLC comes from ISPs that require an Ethernet link between the modem connected to the public network (could be cable TV, DSL, or FTTH) and the set-top box connected to the PCs, the IP phone, or the TV display. With the development of HDTV, this Ethernet link must deliver a high bit-rate stream that is stable with a high guarantee of service, and the latest flavors of PLC seem to be able to achieve these requirements. In fact, the last technical developments in PLC implement advanced OFDM and TDMA, with reserved time slots for each stream to guarantee good delivery of the IP packets in real-time--which is not built in with any of the Wi-Fi standards.

The security issues are also very important in PLC. First, the access to the medium (i.e., 110V/60Hz electrical cables) is difficult and dangerous. Second, the frames are encrypted with a network key and protected by an on-chip security.

Looking at the current market, some OEMs like Linksys are releasing starter kits like the PLE200, which allow several HD video streams from a set-top box to different displays in the home. Netgear and D-link are also working on similar equipment for the home-networking revolution, which has increased the anticipated number of PLC chipsets shipped in 2007 to 15 million.

Outdoors, Broadband Powerline (BPL--the name for PLC used for Internet access for the last mile on medium- and low-voltage electrical cables) is looking at major deployments, with agreements between TelCos and utilities worldwide. Current Technologies –- a subsidary of Google –- is deploying BPL around Cincinnati, OH; TXU is forecasting more than a million houses connected by 2008 in Texas; and BPL is beng deployed in developing countries like Brazil, Algeria, India, and Niger, for IP and VoIP-only services.

PLC is now mature and ready to be standardized worldwide within an international body of standardization. The three technological consortiums are working together to achieve this within the IEEE. Indeed, the working group P1901 at IEEE is about to propose drafts for PLC and BPL in 2007 and is also working on the issues of interoperability and coexistence of different PLC devices on a common electrical network.

Xavier Carcelle worked as CTO for a French Internet Portal in 2000, and then joined the EDF Group as a telecommunications expert in 2001. He's worked on several telecommunications projects including serving as architect for the first SDH Loop based on fiber optics wrapped around high-voltage lines, and as software engineer for a Californian subsidiary of the EDF Group working on Video Over IP and for another subsidiary specializing in PLC Networks.

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