- Power line communication
Power line communication or power line carrier (PLC), also known as power line digital subscriber line (PDSL), mains communication, power line telecom (PLT), power line networking (PLN), or broadband over power lines (BPL) are systems for carrying data on a conductor also used for electric power transmission.
Electrical power is transmitted over high voltage transmission lines, distributed over medium voltage, and used inside buildings at lower voltages. Powerline communications can be applied at each stage. Most PLC technologies limit themselves to one set of wires (for example, premises wiring), but some can cross between two levels (for example, both the distribution network and premises wiring). Typically the transformer prevents propagating the signal, which requires multiple technologies to be used to form very large networks.
Power line communications systems operate by impressing a modulated carrier signal on the wiring system. Different types of powerline communications use different frequency bands, depending on the signal transmission characteristics of the power wiring used. Since the power distribution system was originally intended for transmission of AC power at typical frequencies of 50 or 60 Hz, power wire circuits have only a limited ability to carry higher frequencies. The propagation problem is a limiting factor for each type of power line communications.
Data rates and distance limits vary widely over many power line communication standards. Low-frequency (about 100-200 kHz) carriers impressed on high-voltage transmission lines may carry one or two analog voice circuits, or telemetry and control circuits with an equivalent data rate of a few hundred bits per second; however, these circuits may be many miles long. Higher data rates generally imply shorter ranges; a local area network operating at millions of bits per second may only cover one floor of an office building, but eliminates the need for installation of dedicated network cabling.
Long haul, low frequency
Power line carrier systems have long been a favorite at many utilities because it allows them to reliably move data over an infrastructure that they control. Many technologies are capable of performing multiple applications. For example, a communication system bought initially for automatic meter reading can sometimes also be used for load control or for demand response applications.
PLC is one of the technologies used for automatic meter reading. Both one-way and two-way systems have been successfully used for decades. Interest in this application has grown substantially in recent history—not so much because there is an interest in automating a manual process, but because there is an interest in obtaining fresh data from all metered points in order to better control and operate the system. PLC is one of the technologies being used in Advanced Metering Infrastructure (AMI) systems.
In a one-way (inbound only) system, readings "bubble up" from end devices (such as meters), through the communication infrastructure, to a "master station" which publishes the readings. A one-way system might be lower-cost than a two-way system, but also is difficult to reconfigure should the operating environment change.
In a two-way system (supporting both outbound and inbound), commands can be broadcast out from the master station to end devices (meters) -- allowing for reconfiguration of the network, or to obtain readings, or to convey messages, etc. The device at the end of the network may then respond (inbound) with a message that carries the desired value. Outbound messages injected at a utility substation will propagate to all points downstream. This type of broadcast allows the communication system to simultaneously reach many thousands of devices—all of which are known to have power, and have been previously identified as candidates for load shed. PLC also may be a component of a Smart Grid.
Medium frequency (kHz)
Home control (narrowband)
Power line communications technology can use the electrical power wiring within a home for home automation: for example, remote control of lighting and appliances without installation of additional control wiring.
Typically home-control power line communication devices operate by modulating in a carrier wave of between 20 and 200 kHz into the household wiring at the transmitter. The carrier is modulated by digital signals. Each receiver in the system has an address and can be individually commanded by the signals transmitted over the household wiring and decoded at the receiver. These devices may be either plugged into regular power outlets, or permanently wired in place. Since the carrier signal may propagate to nearby homes (or apartments) on the same distribution system, these control schemes have a "house address" that designates the owner. A popular technology known as X10 has been used since the 1970s.
The "universal powerline bus", introduced in 1999, uses pulse-position modulation (PPM). The physical layer method is a very different scheme than the X10. LonTalk, part of the LonWorks home automation product line, was accepted as part of some automation standards.
Narrowband power line communications began soon after electrical power supply became widespread. Around the year 1922 the first carrier frequency systems began to operate over high-tension lines with frequencies of 15 to 500 kHz for telemetry purposes, and this continues. Consumer products such as baby alarms have been available at least since 1940.
In the 1930s, ripple carrier signalling was introduced on the medium (10-20 kV) and low voltage (240/415 V) distribution systems. For many years the search continued for a cheap bi-directional technology suitable for applications such as remote meter reading. For example, the Tokyo Electric Power Co ran experiments in the 1970s which reported successful bi-directional operation with several hundred units. Since the mid-1980s, there has been a surge of interest in using the potential of digital communications techniques and digital signal processing. The drive is to produce a reliable system which is cheap enough to be widely installed and able to compete cost effectively with wireless solutions. But the narrowband powerline communications channel presents many technical challenges, a mathematical channel model and a survey of work is available.
Applications of mains communications vary enormously, as would be expected of such a widely available medium. One natural application of narrow band power line communication is the control and telemetry of electrical equipment such as meters, switches, heaters and domestic appliances. A number of active developments are considering such applications from a systems point of view, such as demand side management. In this, domestic appliances would intelligently co-ordinate their use of resources, for example limiting peak loads.
Control and telemetry applications include both 'utility side' applications, which involves equipment belonging to the utility company (i.e. between the supply transformer substation up to the domestic meter), and 'consumer-side' applications which involves equipment in the consumer's premises. Possible utility-side applications include automatic meter reading (AMR), dynamic tariff control, load management, load profile recording, credit control, pre-payment, remote connection, fraud detection and network management,  and could be extended to include gas and water.
A project of EDF, France includes demand management, street lighting control, remote metering and billing, customer specific tariff optimisation, contract management, expense estimation and gas applications safety.
There are also many specialised niche applications which use the mains supply within the home as a convenient data link for telemetry. For example, in the UK and Europe a TV audience monitoring system uses powerline communications as a convenient data path between devices that monitor TV viewing activity in different rooms in a home and a data concentrator which is connected to a telephone modem.
The Distribution Line Carrier (DLC) System technology used a frequency range of 9 to 500 kHz with data rate up to 576 kbit/s.
A project called Real-time Energy Management via Powerlines and Internet (REMPLI) was funded from 2003 to 2006 by the European Commission. In 2009, a group of vendors formed the PoweRline Intelligent Metering Evolution (PRIME) alliance.
Transmitting radio programs
Sometimes PLC was used for transmitting radio programs over powerlines. When operated in the AM radio band, it is known as a carrier current system. Such devices were in use in Germany, where it was called Drahtfunk, and in Switzerland, where it was called Telefonrundspruch, and used telephone lines. In the Soviet Union, PLC was very common for broadcasting since the 1930s because of its low cost and accessibility. In Norway the radiation of PLC systems from powerlines was sometimes used for radio supply. These facilities were called Linjesender. In all cases the radio programme was fed by special transformers into the lines. To prevent uncontrolled propagation, filters for the carrier frequencies of the PLC systems were installed in substations and at line branches.
An example of the programs carried by "wire broadcasting" in Switzerland:
- 175 kHz Swiss Radio International
- 208 kHz RSR1 "la première" (French)
- 241 kHz "classical music"
- 274 kHz RSI1 "rete UNO" (Italian)
- 307 kHz DRS 1 (German)
- 340 kHz "easy music"
Utility companies use special coupling capacitors to connect medium-frequency radio transmitters to the power-frequency AC conductors. Frequencies used are in the range of 24 to 500 kHz, with transmitter power levels up to hundreds of watts. These signals may be impressed on one conductor, on two conductors or on all three conductors of a high-voltage AC transmission line. Several PLC channels may be coupled onto one HV line. Filtering devices are applied at substations to prevent the carrier frequency current from being bypassed through the station apparatus and to ensure that distant faults do not affect the isolated segments of the PLC system. These circuits are used for control of switchgear, and for protection of transmission lines. For example, a protective relay can use a PLC channel to trip a line if a fault is detected between its two terminals, but to leave the line in operation if the fault is elsewhere on the system.
While utility companies use microwave and now, increasingly, fiber optic cables for their primary system communication needs, the power-line carrier apparatus may still be useful as a backup channel or for very simple low-cost installations that do not warrant installing fiber optic lines.
High-frequency (≥1 MHz)
High frequency communication may (re)use large portions of the radio spectrum for communication, or may use select (narrow) band(s), depending on the technology.
Home networking (LAN)
Power line communications can also be used in a home to interconnect home computers and peripherals, and home entertainment devices that have an Ethernet port. Powerline adapter sets plug into power outlets and establish an Ethernet connection using the existing electrical wiring in the home. (Power strips with filtering may absorb the power line signal.) This allows devices to share video and data without the inconvenience of running dedicated network cables.
The most widely deployed powerline networking standard is from the HomePlug Powerline Alliance. HomePlug AV is the most current of the HomePlug specifications and was adopted by the IEEE P1901 group as a baseline technology for their standard, published 30 December 2010. HomePlug estimates that over 45 million HomePlug devices have been deployed worldwide. Other companies and organizations back different specifications for power line home networking and these include the Universal Powerline Association, the HD-PLC Alliance and the ITU-T’s G.hn specification.
Internet access service through existing power lines is often marketed as broadband over power lines (BPL), also known as power-line Internet or powerband. A computer (or any other device) would need only to plug a BPL modem into any outlet in an equipped building to have high-speed Internet access. International Broadband Electric Communications or IBEC and other companies currently offer BPL service to several electric cooperatives.
BPL may offer benefits over regular cable modem or digital subscriber line (DSL) connections: the extensive infrastructure already available appears to allow people in remote locations to access the Internet with relatively little equipment investment by the utility. Cost of running wires such as Ethernet in many buildings can be prohibitive; Relying on wireless has a number of predictable problems including security, limited maximum throughput and inability to power devices efficiently.
But variations in the physical characteristics of the electricity network and the lack of standards mean that provisioning of the service is far from being a standard, repeatable process. And, the bit rate a power line system can provide compared to cable and wireless is in question. The prospect of BPL was predicted to possibly motivate DSL and cable operators to more quickly serve rural communities.
PLC modems transmit in medium and high frequency (1.6 to 80 MHz electric carrier). The asymmetric speed in the modem is generally from 256 kbit/s to 2.7 Mbit/s. In the repeater situated in the meter room the speed is up to 45 Mbit/s and can be connected to 256 PLC modems. In the medium voltage stations, the speed from the head ends to the Internet is up to 135 Mbit/s. To connect to the Internet, utilities can use optical fiber backbone or wireless link.
Deployment of BPL has illustrated a number of fundamental challenges, the primary one being that power lines are inherently a very noisy environment. Every time a device turns on or off, it introduces a pop or click into the line. Switching power supplies often introduce noisy harmonics into the line. And unlike coaxial cable or twisted-pair, the wiring has no inherent noise rejection. The system must be designed to deal with these natural signaling disruptions and work around them. For these reasons BPL can be thought of as a compromise between wireless transmission (where likewise there is little control of the medium through which signals propagate) and wired transmission (but not requiring any new cables).
Broadband over power lines has developed faster in Europe than in the United States due to a historical difference in power system design philosophies. Power distribution uses step-down transformers to reduce the voltage for use by customers. BPL signals cannot readily pass through transformers, as their high inductance makes them act as low-pass filters, blocking high-frequency signals. So, repeaters must be attached to the transformers. In the U.S., it is common for a small transformer hung from a utility pole to service a single house or a small number of houses. In Europe, it is more common for a somewhat larger transformer to service 10 or 100 houses. This makes little difference for power distribution. But delivering BPL in a typical U.S. city requires an order of magnitude more repeaters than in a comparable European city. On the other hand, since bandwidth to the transformer is limited, this can increase the speed at which each household can connect, due to fewer people sharing the same line. One possible solution is to use BPL as the backhaul for wireless communications, for instance by hanging Wi-Fi access points or cellphone base stations on utility poles, thus allowing end-users within a certain range to connect with equipment they already have.
The second major issue is signal strength and operating frequency. The system was expected to use frequencies of 10 to 30 MHz, which has been used for many decades by amateur radio operators, as well as international shortwave broadcasters and a variety of communications systems (military, aeronautical, etc.). Power lines are unshielded and will act as antennas for the signals they carry, and they will interfere with shortwave radio communications. Modern BPL systems use OFDM modulation, which allows them to mitigate interference with radio services by removing specific frequencies used. A 2001 joint study by the American Radio Relay League (ARRL) and HomePlug Powerline Alliance showed that for modems using this technique "in general that with moderate separation of the antenna from the structure containing the HomePlug signal that interference was barely perceptible at the notched frequencies" and interference only happened when the "antenna was physically close to the power lines" (however other frequencies still suffer from interference). What the effects of large scale deployment on PLT modems in house will do to the notching has still to be defined, however in lab tests the notches appear to fill in due to intermodulation between modems.[clarification needed]
Ultra-High-frequency (≥100 MHz)
Even Higher information rate transmissions over power line use RF through microwave frequencies transmitted via a transverse mode surface wave propagation mechanism that requires only a single conductor. An implementation of this technology is marketed as E-Line. These systems claim symmetric and full duplex communication in excess of 1 Gbit/s in each direction. Multiple Wi-Fi channels with simultaneous analog television in the 2.4 and 5.3 GHz unlicensed bands have been demonstrated operating over a single medium voltage line conductor. Because the underlying propagation mode is extremely broadband (in the technical sense), it can operate anywhere in the 20 MHz - 20 GHz region. Also since it is not restricted to below 80 MHz, as is the case for high-frequency BPL, these systems can avoid the interference issues associated with use of shared spectrum with other licensed or unlicensed services.
Government promotion and regulation
On 14 October 2004, the U.S. Federal Communications Commission adopted rules to facilitate the deployment of "Access BPL", the marketing term for Internet access service over power lines. The technical rules are more liberal than those advanced by the US national amateur radio organization, the American Radio Relay League (ARRL), and other spectrum users, but include provisions that require BPL providers to investigate and correct any interference they cause. These rules may be subject to future litigation. One service was announced in 2004 for Ohio, Kentucky, and Indiana.
On 3 August 2006 FCC adopted a memorandum opinion and an order on broadband over power lines, giving the go-ahead to promote broadband service to all Americans. The order rejected calls from aviation, business, commercial, amateur radio and other sectors of spectrum users to limit or prohibit deployment until further study was completed. FCC chief Kevin Martin said that BPL "holds great promise as a ubiquitous broadband solution that would offer a viable alternative to cable, digital subscriber line, fiber, and wireless broadband solutions".
'Notching out' and dynamic adaptation to contention
New FCC rules (and the IEEE standards) require BPL systems to be capable of remotely notching out frequencies on which interference occurs, and of shutting down remotely if necessary to resolve the interference. BPL systems operating within FCC Part 15 emissions limits may still interfere with wireless radio communications and are required to resolve interference problems. A few early trials were shut down, though whether it was in response to complaints is debatable. The need to deal with signals that inevitably will propagate through thick metal wires hanging above crowded areas was always an issue in BPL standardization and the technologies to resolve it are those already used for wireless, so the issue was primarily one of thresholds and agreement on who had priority for spectrum.
In the US, simply ignoring wireless users was apparently not legal. The ARRL sued the FCC, claiming that the FCC violated the Administrative Procedure Act in creating its rules. On 25 April 2008, a US Court of Appeals agreed with the ARRL that the FCC violated the APA, especially by redacting data from the public that could have shed doubt on the FCC's decision.
- "It is one thing for the Commission to give notice and make available for comment the studies on which it relied in formulating the rule while explaining its non-reliance on certain parts", D.C. Circuit Judge Judith Rogers wrote. "It is quite another thing to provide notice and an opportunity for comment on only those parts of the studies that the Commission likes best."
US power and telecommunications companies had meanwhile started tests of the BPL technology, over the protests of the radio groups. After claims of interference by these groups, many of the trials were ended early and proclaimed successes, though the ARRL and other groups claimed otherwise.
Some of the same providers conducting those trials later began commercial roll-outs in limited neighborhoods in selected cities, with some level of user acceptance but also many documented cases of interference reported to the FCC by Amateur Radio users. Some wireless users filed a petition for reconsideration with the FCC in February 2005.
United Kingdom and Europe
In the United Kingdom and many other countries, high-voltage power lines are called "the mains" and power line communication is often called power line telecommunication (PLT). Concern of radio users about the proliferation of PLT technology was acknowledged by the European Commission who, in August 2001 issued a Harmonised Standard. A Harmonised Standard can be used by manufacturers to demonstrate compliance with the Electromagnetic Compatibility (EMC) Directive (but is not mandatory) and will act as a benchmark for enforcement authorities across Europe.
Ofcom has also investigated a number of alleged complaints of interference attributed to PLT apparatus. All complaints are from hobby radio users (radio amateurs, CB radio users and shortwave shortwave listeners) and are normally resolved. No other radio service has been affected.
The independent study found that “there will be a low likelihood of interference, providing certain technology enhancements are implemented”. Newer PLT products have adopted interference mitigation in their design.
An ‘adjournment debate’ on PLT took place on 18 May 2011, Mark Prisk (Minister for Business and Enterprise) responded on behalf of the government to a question tabled by Mark Lancaster) the adjournment debate can be viewed here, the transcript is here.
A draft European Standard (FprEN 50561-1) for PLT has been developed by CENELEC.
The Electromagnetic Compatibility Industry Association (EMCIA), formed in March 2002 for the benefit of companies involved in the supply, design, test or manufacture of EMC products, or the provision of EMC Services and is a UKTI Accredited Trade Organisation, submitted a stern report to the Parliamentary Committee overseeing broadband, stating that they "..very strongly recommend that the Committee specifically excludes the use of PowerLine Telecommunication (PLT*)..." 
Austria, Australia, New Zealand and other locations have also experienced early BPL's so-called "spectrum pollution" and raised concerns within their governing bodies. In the UK, the BBC has published the results of a number of tests (The effects of PLT on broadcast reception,PLT and Broadcasting, Co-existence of PLT and Radio Services) to detect interference from BPL installations. It has also made a video (Real Media format), showing broadcast of data and interference from in-home BPL devices.
In April 2009 the Wireless Institute of Australia reported that radio amateurs in Australia appear to be safe from the rollout of a nationwide Broadband over Powerline or BPL system. Australia's government announced that it will be building a system based on fibre optic technology for its backbone - though it would likely still rely on BPL on high-voltage lines in remote areas. This decision would appear to remove the possibility of widespread interference to radio communications from any network-wide adoption of BPL technology, but still leaves as a concern the possibility of interference from in-home use of G.hn over AC.
In June 2007, NATO Research and Technology Organisation released a report titled HF Interference, Procedures and Tools (RTO-TR-IST-050) which concluded that widespread deployment of BPL may have a "possible detrimental effect upon military HF radio communications and COMINT systems."
Power-line technology enables in-vehicle network communication of data, voice, music and video signals by digital means over direct current (DC) battery power-line. Advanced digital communication techniques tailored to overcome hostile and noisy environment are implemented in a small size silicon device. One power line can be used for multiple independent networks. The benefits would be lower cost and weight (compared to separate power and control wiring), flexible modification, and ease of installation. Potential problems in vehicle applications would include the higher cost of end devices, which must be equipped with active controls and communication, and the possibility of intereference with other radio frequency devices in the vehicle or other places.
LonWorks power line based control has been used for an HVAC system in a production model bus.
The SAE J1772 committee developing standard connectors for plug-in electric vehicles proposes to use power line communication between the vehicle, off-board charging station, and the smart grid, without requiring an additional pin; SAE and the IEEE Standards Association are sharing their draft standards related to the smart grid and vehicle electrification.
There are many ways in which the communication signal may have error introduced into it. Interference, cross chatter, some active devices, and some passive devices all introduce noise or attenuation into the signal. When error becomes significant the devices controlled by the unreliable signal may fail, become inoperative, or operate in an undesirable fashion.
- Interference: Interference from nearby systems can cause signal degradation as the modem may not be able to determine a specific frequency among many signals in the same bandwidth.
- Signal Attenuation by Active Devices: Devices such as relays, transistors, and rectifiers create noise in their respective systems, increasing the likelihood of signal degradation. Arc-fault circuit interrupter (AFCI) devices, required by some recent electrical codes for living spaces, may also attenuate the signals. 
- Signal Attenuation by Passive Devices: Transformers and DC-DC converters attenuate the input frequency signal almost completely. "Bypass" devices become necessary for the signal to be passed on to the receiving node. A bypass device may consist of three stages, a filter in series with a protection stage and coupler, placed in parallel with the passive device.
Two distinctly different sets of standards apply to powerline networking as of early 2010.
IEEE 1901, ITU G.hn home grids
Within homes, the HomePlug AV and IEEE 1901 standards specify how, globally, existing AC wires should be employed for data purposes. The IEEE 1901 includes HomePlug AV as a baseline technology, so any IEEE 1901 products are fully interoperable with HomePlug AV, HomePlug Green PHY or the forthcoming HomePlug AV2 specification (under development now and expected to be approved in Q1 2011).
Smart grids and use of BPL for telemetry and data provision by powercos
Power providers are also standardizing their internal and external communications including use of BPL technologies to provide direct links to power system components like transformers. In North America another IEEE standard group is supervising these activities.
Unlike home users, power providers are more able to consider widespread deployment of fiber optic cables immune to electromagnetic interference (and which do not generate any) and for which mature devices (switches, repeaters) are available. Accordingly there is no one single compelling reason to carry data on the existing power lines themselves as there is in homes, except in remote regions where fibre optic networks would not normally be deployed at all. Power network architectures with many transformers are more likely to be served using fibre.
Even if a home is using BPL it may not necessarily connect to the Internet using a BPL-based gateway (typically a smart meter), although this would have major advantages to both the consumer and provider. NIST and IEEE have considered whether requiring smart meters to all be fully functioning BPL gateways would not accelerate demand side management and create a uniform market into which security, home control and other providers can sell.
Several competing organizations have developed specifications, including the HomePlug Powerline Alliance, Universal Powerline Association and HD-PLC Alliance. On December 2008, the ITU-T adopted Recommendation G.hn/G.9960 as a standard for high-speed powerline, coax and phoneline communications. The National Energy Marketers Association was also involved in advocating for standards. IEEE P1901 is an IEEE working group developing the global standard for high speed powerline communications. In July 2009, the working group approved its "IEEE 1901 Draft Standard for Broadband over Power Line Networks: Medium Access Control and Physical Layer Specifications" as an IEEE draft standard for broadband over power lines defining medium access control and physical layer specifications. The IEEE 1901 Draft Standard was published by the IEEE in January 2010, the final standard approved on 30 September 2010 and published on 1 February 2011.
- Digital subscriber line
- Electric power transmission
- HomePlug Powerline Alliance
- KNX (standard)
- List of broadband over power line deployments
- List of PLC manufacturers
- Multimedia over Coax Alliance
- PLC carrier repeating station
- Power line carrier communication
- Power over Ethernet
- Residential gateway
- Single-wire transmission line
- Universal Powerline Association
- IEEE 1901
- IEC 61334
- ITU-T G.hn
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