12.5 AC Power Wiring-Based Home Networks
| Local communication using the power wiring within a premises has seen widespread use in homes and business since 1978 for low-speed signals to control lights, HVAC (heating, ventilation , air conditioning), and appliances. New systems are now addressing power line LANs at much higher data rates. Premises LANs using inside power wires are distinct from WANs using the public power line grid to communicate data between homes and a public data network. Whereas power line communications with a premises have proved to be reliable (at least a lower bit rates), the feasibility of WANs using outside power lines to communicate more than a few kb/s are in doubt due to electromagnetic interference (EMI), and network topology problems. There have been many bold announcements about plans for power-line WANs with data rates up to 2 Mb/s, but many of the projects have been quietly shut down. More information is available at www.plcforum.org. Power-line LANs permit information appliances to be anywhere there is a power outlet. In many houses , telephone jacks are found in only a few places, whereas power outlets are found in every room including the garage, and sometimes even outside the house. However, power-line LANs do not provide as much mobility as wireless LANs. The X10 power line control system consists of a control station that plugs in to any power outlet, and remote modules that plugged in series with the power cord to each controlled appliance. The control station can control up to 256 stations by sending addressed binary commands at about 1 kb/s. The appliance modules can turn the power on, off, or perform a light dimming function. X10 can also poll remote modules for their status; this is used for security alarm systems. At the time of this printing, X10 is priced under U.S. $15 per appliance module. The manufacturer of X10 equipment claims that more than 100 million X10 units have been shipped. Sending data faster than a few kilobits per second must overcome a challenging transmission environment. Inside power wiring has poor balance at high frequencies; this presents the dual challenge of dealing with high levels of radio frequency ingress noise and restricting radio frequency emissions so as to not interfere with radio systems. Power lines have high levels of conducted noise caused by motors, light dimmers, and switching-type power supplies . The line impedance changes greatly when appliances are turned on or off. Furthermore, inside power wiring has many branches that cause signal reflections (e.g., bridged taps). CEBus (consumer electronics bus) is specified in an open standard (EIA 600) and uses spread spectrum transmission over home power wiring at approximately 10 kb/s. CEBus specifications also address networking via coaxial cable, wireless, CAT5 wire, and infrared. CEBus is primarily targeted at interconnecting home entertainment audio/video systems, and control of lights and appliances. The CEBus MAC is similar to CSMA/CD, and common application language (CAL) is specified to define messages between nodes. Further information may be found at www.cebus.org. CEBus is supported by HomePlug Powerline Alliance, and further information may be found at www.intellon.com. The LonWorks protocol was developed by Echelon Corporation and adopted by the IEEE and EIA (electronics industry alliance) as standard EIA 709.1. LonWorks is widely used in business and commercial building for control of lights, security, and HVAC. Twenty kb/s is transmitted with a 16-bit CRC check for reliability. The LonWorks MAC uses the LonTalk protocol, and is based on CSMA/CD. Intellon Inc, Enikia Inc., and Itran Communications Inc. have a variety of home power-line networking schemes based on OFDM (orthogonal frequency division multiplexing, similar to DMT), claiming to achieve data rates of 1.5 Mb/s or higher. These companies and about seventy-five others have joined together to form the HomePlug Powerline Alliance (www.homeplug.com) to develop an industry specification for low-cost, interoperable, high-speed Ethernet-based networking via power wiring inside homes. A specification is available to members of the HomePlug association, and products built to this specification came to the market during 2002, priced at about $100/node and able to support data rates of up to 11 Mb/s. Initial field experience shows that rates near 11 Mb/s are feasible for most homes. However, a data rate of about 1 Mb/s may be more suitable for successful operation in virtually all homes. It is not yet known if these systems can achieve performance, reliability, and cost that is competitive with other home networking solutions. The HomePlug system specifies OFDM for the physical layer (PHY) and CSMA/CA for the MAC. The frequency band used is approximately 4 MHz to 21 MHz. The noise in the band is generally lower than for frequencies below 4 MHz. Both point-to-point and broadcast modes are supported. The HomePlug specification provides for optional 56-bit DES encryption for security. To function correctly, HomePlug equipment must be connected to the same AC wiring phase; many homes have two wiring phases and it is not obvious which plugs are on the same phase. Surge protectors and power back-up units (UPS) must not be in-line with the HomePlug equipment. Further information is available at www.homeplug.com. Power-line based LANs must deal with many of the same issues as other LAN and DSL technologies. To be competitive, data rates far above one megabit per second should be achieved even in large homes. Radio frequency emissions must not interfere with radio systems. Special care must be taken to avoid excessive latency while providing the necessary robustness against noise. Also, adequate security must be provided as eavesdropping could be possible, especially in multiple dwelling units. |
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