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Network and wireless analysis: Power line networks (March 2010)

Network and wireless analysis

At a glance

  • The electricity grid infrastructure, as a widely distributed network of copper wiring, seems an obvious candidate for use as a data network.
  • A high-frequency data signal can be imposed on the normal alternating current, which will propagate between the phases in a building but not through power transformers in substations or connected to consumer hardware.
  • Modern digital signal processing equipment can drive data at 200Mbps on household mains and as fast as 2.5Mbps on higher voltage distribution cabling.
  • Mains interference from switching and high power equipment can also propagate through buildings and local electricity networks, hampering transmissions.
  • A number of power line communication specifications are competing for adoption as the 'de facto' standard, but they also have to prove their advantages compared to other wired and wireless technologies.
  • Monitoring and control of 'smart grid' meters and devices may prove to be suited to power line networking.

Network power

The electrical grid has long held out a promise of pervasive networking based on widely installed infrastructure, but the associated practical limitations have hindered implementation. Mains electricity comes to almost every building in the country and is wired through every room, but the grid was never designed with data transmission in mind.

The public's perception is largely governed by an understanding that mains electricity is dangerous. Such a perception may suggest to people that they should never connect networking equipment, let alone expensive audiovisual (AV) devices, to the mains to transfer data. However, the electrical theory underpinning such connections is perfectly valid and can be safely implemented. 'Baby monitors' plugged into mains sockets generally use this method of communication with little immediate risk.

Many power companies exploit the grid infrastructure for signalling and telemetry. Relatively low frequencies are used (in the kilohertz range), so only limited data can be carried, but this is adequate for network monitoring and control. Historically, these systems have used analogue signalling equipment, but modern high-speed analogue to digital converters and inexpensive digital signal processors have opened up opportunities for broadband communications in the home and over the grid.

The mains solution

Power line communications (PLC) is being promoted as a solution for home media networking. According to PC Advisor, ABI Research predicts that revenue from networked home media devices will more than double to $243 billion (about £160 billion) in the three-year period to 2012.

Household wiring can be used to create a data network by overlaying a data signal on the alternating current waveform of the electrical mains. (This description and diagram from HD-PLC illustrates the point.) In practice, mains wiring often transmits significant interference caused by devices turning on and off and harmonics (a kind of resonance) generated by connected equipment. Some sensitive hardware, such as computers, need surge protection and filtered supplies to operate reliably, but most appliances, such as washing machines, are not affected by 'dirty' mains supplies.

A further problem for all levels of power line communications is that high-frequency signals are effectively filtered out by power transformers, whether on the mains lead for an LCD monitor or at a local electricity substation. This means that home devices that use PLC must either be directly connected to the full 240V supply or via an Ethernet or USB lead that 'bridges' across any transformer.

A variety of industry groupings, including the HD-PLC Alliance, the HomePlug Powerline Alliance and the Universal Powerline Assocociation, are promoting a range of related power line technologies. Taking HomePlug AV as an example, the system delivers 200Mbps at the physical level, although error correction, 128-bit AES security and other overheads reduce the effective data rate to 150Mbps. In the US domestic market, as in many UK educational establishments, two or three phases of the electricity supply are connected to a single building. This HomePlug whitepaper explains how parallel supply wiring and a common bus permit the relatively high the frequency (2-28MHz) signals to cross between phases.

HomePlug AV was one of a number of technologies examined in the initial stages of developing a draft PLC standard, known as IEEE P1901. This standard is intended to specify systems that can deliver 100Mbps or more using frequencies up to 100Mhz, whether in a building (transmission distances under 100m) or 'last mile' connections to buildings (under 1,500m). The standard should ensure 'fair coexistence' between devices, but the draft specifies two alternative physical layers based on incompatible modulation techniques, which would limit full interoperability unless devices support both. (Modulation affects the way that digital data is represented as an analogue signal.)

Home alternatives

Power line networks are competing with a mature Wi-Fi ecosystem, a range of wired technologies and specific wireless standards for networked media devices. Many consumers have broadband routers with 802.11g or 802.11n built in, capable of maximum (theoretical) throughputs of 54Mbps and 450Mbps respectively. The latest wired HDMI v1.4 specification supports video resolutions above full 1080p HD at over 8Gbps, while the current WirelessHD and the forthcoming WiGig  standards have maximum throughput of 4Gbps and 7Gbps respectively. However, these HD standards are all relatively short range due to the cabling used or high (60GHz) frequencies chosen.

Many homes, especially in the US and newer developments in Europe, already have co-axial cabling installed in every room for distribution of television signals. The Multimedia over Coax Alliance (MoCA) has a proprietary standard that currently delivers 175Mbps and promises 400Mbps to come. Meanwhile, the HomePNAAlliance 's latest specification delivers up to 320Mbps across telephone wiring and coaxial cabling.

The HomeGrid Forum aims to unite these cabled infrastructures (but not necessarily all the groupings) into a single G.hn specification, which is currently being ratified by the International Telecommunications Union (ITU) as 'Recommendation G.9960'. Covering co-axial, telephone and power line cabling, the G.hn standard is effectively complete, with devices expected later this year. As reported in TechNews 01/09, G.hn is designed to deliver 400Mbps over co-axial cables and 200Mbps on power lines.

Tom's Hardware, a respected online publication, compared the performance of 802.11n wireless, a HomePlug AV compatible device and a MoCA unit in a typical house. The review concluded that power line technology had the lowest throughput figures and that it was not "suitable for anything beyond a single HD video stream". The hardware was all sourced from the same manufacturer, so it is possible that other units would compare differently. Also, building materials in other dwellings might adversely affect Wi-Fi, which came second in their tests.

The last mile and beyond

Broadband over power lines (BPL) aims to apply similar principles for connecting homes back to internet service providers (ISPs). Due to the structure of local power networks connected through transformers at substations, each network would operate independently and would need to be bridged together at substations and elsewhere in the grid. Each local network would need an addressing structure that accounted for both buildings and the devices within those buildings, since they all share what is effectively a single data bus. In the UK such a local network might cover 200 to 300 houses and interference from appliances in any of those properties would be transmitted across the whole network. Such issues have limited the development of BPL in the UK.

High voltage overhead power cables, which could be used for the 'backhaul' between a local network and an ISP, effectively act as large antennae, so a number of pilot schemes in the US and elsewhere have been criticised for generating radio frequency interference. (Typical data transmission frequencies range from 1.6 to 80MHz, potentially affecting shortwave and FM transmissions.) Both BPL and home PLC modems can use protocols that 'notch' transmission frequencies, either according to fixed lookup tables or by sensing radio transmissions in the area, to avoid interfering with broadcast signals.

The electrical grid could provide 'last mile' connectivity between premises and high bandwidth copper or fibre optic connection points. However, depending on the transmission frequency used, BPL currently offers up to 2.5Mbps, which is hardly competitive in areas where standard ADSL or cable services are available and often packaged as part of a phone deal. In the US, where local substations serving only a few buildings are mounted on the poles for power lines, BPL could provide backhaul for 'metro' Wi-Fi networks. (See Wireless mesh networks in TechNews 11/09.)

A new discovery, if its promise is fulfilled, could provide last mile access in areas with overhead power distribution cables. A transverse-magnetic surface wave can be generated in the field lines around an unshielded cable, which, with regular amplification, can propagate over significant distances. The developers of the new 'E-Line' system claim that the hardware is relatively simple and inexpensive to install, and could be combined with 'nano-cells' to provide mobile phone access. Their test system used 2GHz frequencies, although they suggest anything from 200MHz to 20GHz will work. The white paper states that " practical systems have been built with five to ten amplifiers per mile of line which have supported more than 2Gbps information capacity".

Connecting the smart grid

To deliver significant energy efficiencies, smart grids will require two-way communication between consumers and power supply companies. (See TechNews 01/10.) Standards bodies, such as US National Institute of Standards and Technology (NIST), are investigating a variety of connectivity options, including power line technologies, to transmit data and control commands both within and between buildings and grids. This type of data is suited to PLC as it generally comprises many small packets from a large number of sources, rather than the large video streams increasingly being viewed by consumers.

Network ready?

Both power line communications and home media networking are beset with a large number of standards, many of which are not interoperable. Existing PLC implementations do not offer the speed of most consumer alternatives, let alone meet the requirements of large educational establishments. However, they can provide low cost, point-to-point connections in situations where aggregate data does not exceed the demands of a single HD video stream and where technical limitations might make the alternatives unattractive. (Such limitations might include radiofrequency interference, the cost of running new cables or aesthetic concerns regarding surface cabling.)

There may be potential for some form of BPL to deliver 'last mile' internet connectivity. This may be of particular relevance in rural areas where telephone lines are too long for ADSL connections. However, the most likely application for power line networking appears to be to provide the physical channel to connect smart devices to meters and to form the local backbone for developing smart energy grids.

Comments [2]

Note: All comments will be reviewed by Becta before being published.

Posted by Neil Adam 22 April, 2010

Reports suggest that IEEE P190 will be finally ratified this autumn (2010). See Powerline technology to be standardized http://news.cnet.com/8301-17938_105-20003066-1.html

Posted by Neil Adam 15 March, 2010

Netgear has launched what it clls "Two Innovative, Ultra High-Performance Powerline Products", promising "revolutionary speeds of up to 500 Mbps" based on IEEE P1901. See http://www.netgear.com/About/PressReleases/en-US/2010/20100301a.aspx

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