The Use of Existing Electrical Powerlines for
High Speed Communications to the Home
Dr. Michael Propp
Introduction
With impending deregulation, electric utilities are investigating how they can compete through better quality, pricing, and service offerings. The deregulated utility business will become three separate businesses: power generation, transmission, and local distribution. Local distribution will be a very competitive arena, where many utilities will seek to gain competitive advantage by offering new services in addition to electric power and building brand recognition. One approach that utilities can pursue takes advantage of their vast, in-place wiring infrastructure. With the proper technology, the infrastructure can also serve as a high-speed communications medium over the "the first hundred feet" to the residence. Through the use of powerline communications, electrical utilities can supply both electric power and a pipe for high-speed, reliable communications traffic, including Internet access.
Historical Perspective
Until the recent past, the AC powerline has been precluded for use as a communications medium for applications other than low speed meter reading or load control. Powerline communications connectivity at useful data rates (above a few hundred bits per second) is confined to the secondary of a low voltage distribution transformer. The network topology is therefore such that a concentrator installed for each distribution transformer would serve to connect to a wide area network, via fiber, fixed wireless, or the Public Switched Telephone Network (PSTN), for example. In the United States, a distribution transformer supplies power to an average of four to six residences. In contrast, Europe typically has several hundred residences per transformer. With the concentrator cost distributed only over the average four to six meters per distribution transformer in the United States, there is no compelling case for automated meter reading in the United States. In Europe, with several hundred meters per transformer, utilities are moving to adopt automated meter reading over the powerline.
Automated Meter Reading (AMR) and Energy Management in Spain
An initial example of the use of this infrastructure is Iberdrola’s automated meter reading (AMR) system in northern Spain. The system monitors real-time consumption, time-of-day energy rates, overall system demand, invoice period comparisons, and peak consumption times. The system consists of in-home customer display units, metering devices, and the "electrical powerline communications medium."
This application involved networking the entire village of Zarauz in the Basque region of northern Spain using a 19.2 kilobits (kbps) per second throughput powerline communications technology. The system was a collaboration between Ikusi (San Sebastian, Spain) and Iberdrola (Bilbao, Spain), Spain’s largest utility. Ikusi, a Spanish systems integrator and communications equipment manufacturer, retrofitted approximately 10,000 meters for automated meter reading using Adaptive Networks’ (Brighton, MA) 19.2 kbps powerline communications products, together with 50 concentrator units located at transformer centers and approximately 1,500 customer display units installed in individual residences. The meters and customer display units were networked through the powerline to the concentrators, that in turn used telephone lines, radio and fiber to communicate with a central control and monitoring center.
The customer display unit provides the customer with information on elapsed consumption and cost, the current time-of-day rate, alarms, and messages sent by the utility, and allows load control for energy management (Figure 1). The customer display unit brings information directly to the consumer, enabling the consumer to make informed decisions about usage of electric power. A pulse generator in each meter is connected to a local concentrator in those buildings where the meters may be in one area, which in turn communicate the data over the powerline (Figure 2).
Powerline Communications Applications in the United States
In the United States, the business case for a utility’s use of powerline communications can become attractive through the introduction of services that require higher data rates than those offered by traditional powerline communications technologies. With the ability to communicate over the powerline at higher data rates, for example, exceeding 100 kbps throughput, meter reading and load control can become one service of many enabled over the existing infrastructure (Figure 3). With deregulation of both the telecommunications and electrical utility industries, utilities can then enhance basic service with a broad array of energy and telecommunications offerings and features. As an example, for the four to six homes serviced by the pole or pad transformer, a utility could potentially offer the following bundle of services:
Automated electric, gas, and water meter reading
Energy management (load shedding, demand side management)
Burglar, fire, carbon monoxide, and natural gas security services
Medical alert
Asset tracking
Internet access
Local telephone access
With the offering of security and medical alert services, the added communications infrastructure costs can then be justified in contrast to solely implementing meter reading. With the powerline communications network able to support data rates high enough for internet access and telephony, an even greater opportunity is presented to the utility for revenue generation.
With the advent of deregulation, the utility can use the real-time information from the meter to compete for residential customers. Just as a long distance telephone service provider may compete for customers, a utility may offer a customer a savings plan whereby the customer can use the communications link to the meter to make informed decisions regarding electricity usage. Security services are further afield from a utility’s core business, but witness such offerings by UtiliCorp United (Kansas City, MO), Western Resources Inc. (Topeka, KS), and Duke Power (Charlotte, NC). With Internet access the utility would wander even further astray. However, there may be a convergence among some aggressive utilities and telecommunications service providers, for example, Boston Edison’s (Boston, MA) announced agreement with RCN, and participation in personal communications services (PCS) ventures by utilities such as Duke Power and PECO Energy (Philadelphia, PA). For these scenarios to exist, however, a two-way, reliable, secure, high-speed powerline communications network must be utilized that presents a low cost per network node.
The Technical Solution
The AC powerline has long been recognized as a possible communications medium, although substantial difficulties have been encountered receiving data corrupted by powerline noise and attenuation. However, by understanding these obstacles, effective powerline communications is possible (Ladas, 1996, Ladas, 1995, Propp, 1995, Gershon, Propp, Propp, 1991).
A hierarchical design is central to a robust and reliable powerline communications technology. Each level of the design is optimized specifically to overcome the inhospitable characteristics of the powerline environment. Noise and frequency-dependent signal attenuation are found on almost every powerline. Unlike dedicated wiring, not having well-designed error control coding will result in bit errors occurring at unacceptably high rates. Actual error-free data throughput is always a fraction of the raw data rate.
A powerline specific approach is necessary for the physical through media access control (MAC) sublayers of the Open Systems Interconnection (OSI) reference model for network communications. Often, the OSI reference model is implemented as an abbreviated three-layer communications architecture (Figure 4) which is easily implemented using a highly-integrated chip set, with one chip implementing the physical layer and the other the data-link and higher layers (Figure 5). The key features necessary to provide immunity from powerline attenuation and noise are spread spectrum wideband modulation, fast synchronization, adaptive equalization, error control coding, and powerline-optimized network protocols. A low-cost chip set can incorporate these powerline-specific features allowing easy incorporation of a powerline communications capability into devices.
The Powerline Physical Layer
Part 15 of the FCC rules allows powerline communication outside the AM frequency band (outside 535 to 1705 kHz). In the past, efforts to use powerlines for communication made use of modems to modulate a carrier frequency of between 50 and 500 kHz using frequency shift keying (FSK) or amplitude shift keying (ASK), digital versions of FM or AM, respectively. Such powerline communication modems require constant tuning or they become inoperable when electrical devices are plugged into or unplugged from the electrical network.
In general, a spread spectrum system will exhibit improved noise immunity over narrowband systems on the powerline. However, traditional spread spectrum approaches (direct sequence, frequency hopping, chirp) do not solve the difficulty of signal synchronization on the powerline in the presence of constantly-changing noise and frequency-dependent attenuation.
However, a unique physical layer spread spectrum technology can provide very rapid synchronization. Rapid synchronization is an important component of a fast, practical, and reliable powerline communications system. In the protocol, this allows data to be transmitted in short frames.
Another important component of reliable and robust spread spectrum powerline communications systems is a method of rapid equalization of the received signal to compensate for the frequency-dependent noise and attenuation.
The Reliable Low Level Link Protocol
Several key features of a data link layer are required for reliable operation of large, multi-node networks on the powerline:
Decomposition of larger packets to powerline frames to create reliable communications.
Rigorous error correction and detection.
Effective adaptive equalization.
Reliable transfer of control.
Only a certain amount of contiguous information can be sent before it is almost a certainty that a transmission will be corrupted. This suggests a requirement for transmissions of short frames on the powerline. To further ensure the integrity of any frame of data, it is necessary to use both error correcting and detecting codes -- forward error correction to minimize the number of retransmissions, and error detection to know if there is a need for a retransmission on a frame basis. Each frame should be acknowledged by the receiver before the transmission proceeds to the next frame. To implement this low-level link protocol, the higher-level packet is broken up into such short frames.
Another benefit of the low-level link protocol is the effectiveness of adaptive equalization. Powerline conditions can change on the order of a few milliseconds, and the receiver must be able to adapt to these changing conditions. Using a low-level link protocol built upon short frames, the receiver can adapt on a frame basis and, because acknowledgments are required, no information is lost.
The Token Passing Media Access Method of the Powerline
Reliable transfer of control is accomplished by media-access control (MAC) algorithms. The MAC algorithms in use on dedicated wire are generally based on either a carrier sense or token passing. However, results for other media are not transferable to the powerline.
Token passing is uniquely suited to the powerline medium. On the powerline, there is insufficient communications reliability to distinguish between noise and signal. Since there is only one token holder at any point in time, a reliable three-way handshake is used to transfer the token between nodes. This ensures an orderly transfer of control without loss of the token.
Because the powerline characteristics are different for each node, a node will not necessarily hear every transmission on the powerline. In token passing, nodes cannot transmit unless they hold the token. Therefore, there is no possibility of nodes starting to transmit in the midst of another node's transmission.
A Likely Scenario for the Future
There are multiple applications of powerline communications technology in use today, including point-of-sale (POS) networks, vending machine monitoring, and an ISO (International Organization for Standardization) standard for communicating aboard refrigerated container ships, to name a few. These applications illustrate the merits of using the AC powerline as a high speed communications medium.
With the availability of powerline communications speeds similar to those of Ethernet, the technology will soon become available in products for personal computer (PC) networking within the residence. Powerline communications can allow utilities to offer high speed Internet access over "the first one hundred feet" to and from the residence without the potentially huge infrastructure investments faced by other service providers. Given the process by which utilities develop new business areas, the initial deployment of high speed powerline communications for the residence will most likely be for communications within the residence.
The availability of such products in the residence or small office could then provide utilities with a portion of their customers enabled for powerline communications and receptive to new service offerings, such as Internet access (Metcalfe, 1997), facilitating the decision to use high speed powerline communications for such services. As electric utilities begin to explore this avenue for enhanced services, a far greater value will be found in the powerline than simply delivering energy.
References
Auerbach, Jon and Ackerman, Jerry, "In a first, Edison deal ties services", The Boston Globe, September 30, 1996.
Gershon, Ron, Propp, David and Propp, Michael, "A Token Passing Network For Powerline Communications", IEEE Transactions on Consumer Electronics, Vol. 37, No. 2, May 1991.
Goodenough, Frank, "Chip Set Puts 100 Kbits/s Of Data On Noisy Powerlines", Electronic Design, March 18, 1996, pages 177-184.
Krepchin, Ira, "UtiliCorp United, Adaptive Networks Form PLC Team", Technologies for Energy Management, Vol. 4, No. 9, September 1996.
Ladas, Chris, "Using AC Powerlines To Network PCs In The SOHO Market", TIPCIC Proceedings, December 1996, page 175.
Ladas, Chris, "Designing High-Speed Powerline Communication Systems", Communication Systems Design, February 1996, pages 23-32.
Ladas, Chris, "Powerline Communications Another Wireless Alternative", Wireless Design & Development, November 1995, pages 17-19.
Metcalfe, Bob, "Cheap, reliable, powerful ‘net connections may be as close as an electrical socket", InfoWorld, February 10, 1997, page 44.
Propp, Michael, "Use The AC Power Line As A Wireless Medium", Microwaves & RF, May 1995.
"Con Edison of New York’s Secondary Underground Network Distribution Automation System", DA/DSM ‘96 Distributech Conference, January 17, 1996.
Telecommunication, Code of Federal Regulations, 47 CRF 15.107
The following web sites provide background information on companies mentioned in this chapter:
Adaptive Networks, Inc. http://www.adaptivenetworks.com/
Boston Edison: http://www.bedison.com/
Duke Power: http://www.dukepower.com/
Iberdrola: http://www.iberdrola.es/
Ikusi: http://www.ikusi.es/
PECO Energy: http://www.peco.com/
RCN: http://www.rcn.net/
UtiliCorp United: http://www.utilicorp.com/
Western Resources: http://www.wstnres.com/