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While wireless dominates home networks and Internet access today, it is not a good solution for some environments where range and obstructions are present. A good alternative is powerline communications (PLC), where the communications medium is the ac powerline. Several technologies and standards for PLC are available, led by the HomePlug standard. But a new contender has emerged in the form of G.hn, a standard from the International Telecommunications Union (ITU).
Table Of Contents
• How PLC Works
• HomePlug
• G.hn
• Related Standards
• References
How PLC Works
The most convenient communications technology is wireless simply because no cabling is needed. All you need to establish a communications link is free space. Yet wireless doesn’t fit all applications. The range between communicating nodes may be too great or obstacles like walls, ceilings, floors, and other objects may block or severely attenuate the signal. In such situations, cable such as Ethernet is the preferred medium, though running new cables is difficult and expensive.
However, existing wiring and cabling also can be used. Unshielded twisted-pair (UTP) telephone wiring exists in most homes and buildings. Cable TV coax wiring can be found in most homes and many businesses as well. Both are used for networking. That’s also true of ac powerline wiring. While not designed for communications purposes, the ac mains have proven themselves as a good communications medium.
The basic process is simply to superimpose the signal to be transmitted on the ac line. This was initially done with the original X10 system in the 1970s for controlling lights. Electric utilities widely use it for carrying control signals for the power grid. The information to be transmitted, usually digital, is modulated onto a carrier that is then capacitively coupled to the ac line voltage.
The value of this technique is that ac powerline wiring exists everywhere, meaning no new wiring. The communications medium is accessed by just plugging into any ac outlet. The modulated signals easily propagate throughout the home or building. However, there are several downsides to this approach.
First, the ac line is severely band limited. It was designed for 60-Hz voltage, not high-frequency communications. Furthermore, the attenuation over distance and through transformers is enormous, severely limiting the communications range. On top of this, ac power lines are noisy with all kinds of disturbances from lightning pickup to switching transients stemming from the turning off and on of lights, motors, or other inductive devices.
While these disadvantages are limiting, they can be overcome by using the right modulation technologies. Early systems used amplitude shift keying (ASK) and frequency shift keying (FSK) with some success. Today, orthogonal frequency division multiplexing (OFDM) is the modulation method of choice since it helps mitigate noise and the attenuation problems. It also provides spectral efficiency to permit high data rates.
OFDM is a multicarrier modulation method in which the serial digital data to be transmitted is divided up into many slower parallel data signals, each of which is modulated onto one of the many carriers. These so called subcarriers are closely adjacent but are orthogonal so they don’t interfere with one another. Modulation methods are typically binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), or some form of quadrature amplitude modulation (QAM). OFDM can accommodate high-speed data.
However, OFDM in PLC can interfere with shortwave radio communications. Amateur radio hobbyists and shortwave (SW) listeners have been complaining for years about the noise and interference OFDM produces, making the SW and ham bands nearly unusable. While such interference is typically limited to the home where PLC is used, it can extend its reach under some conditions. Notching out the offending OFDM subcarriers in the ham or SW bands can minimize this kind of interference.
HomePlug
HomePlug (HP) is the trade name of the PLC technology and standard developed by the HomePlug Alliance. The organization was formed in 2000, and it issued the first HP standard 1.0 in 2001. The Alliance released HomePlugAV in 2005. Other versions such as HomePlug Green PHY (GP) were released in 2010. HomePlugAV2 is the latest version, released in 2011. With each new release, the Alliance has further improved the standard. In the case of HomePlugAV, it improved the data rate to handle the multimedia needs of home networking (see the figure).
HomePlugAV uses the spectrum from 2 to 28 MHz to transport the OFDM signal bearing the digital data to be transmitted. Up to 1155 subcarriers can be used in an adaptive modulation scheme. The most common use is Internet access in the home, although video transport is also a major activity. The standard can achieve a data rate to 200 Mbits/s.
HomePlugAV2 extends the OFDM spectrum on the line from 30 to 86 MHz with up to 4096 QAM, allowing data speeds up to 1 Gbit/s or a net rate over 500 Mbits/s considering network overhead. Furthermore, it features a powerline version of multiple-input multiple-output (MIMO) wireless technique for further link reliability and higher data rates. MIMO uses alternate ac lines including the second phase of the 240-V line, neutral or ground for additional signal paths if available.
HomePlug Green PHY is a special simplified version of the standard designed for Smart Grid applications like smart electric meters, HVAC control, and appliance monitoring. Its maximum data rate is 3.8 Mbits/s with a total throughput exceeding 1 Mbit/s.
The HomePlug Alliance also has worked with the ZigBee Alliance to develop the Smart Energy Profile 2 (SEP2), a common applications layer that enables interoperability between applications that may run on a ZigBee wireless network or on HomePlug devices. The HPA has worked with the ZigBee Alliance and the Wi-Fi Alliance as well to form the Consortium of Smart Energy Profile 2 Interoperability (CSEP) to produce a common testing document and process for certifying SEP2 interoperability.
G.hn
G.hn is a standard of the International Telecommunications Union – Telecommunications (ITU-T) organization, promoted and certified by the HomeGrid Forum. It supports PLC but also accommodates other wired media like twisted-pair telephone wiring and cable TV coax. Its main applications are home networking, Internet access, and connecting TV sets, DVD players, set-top boxes, and other video equipment. The ITU standards designations are G.9960 and G.9961.
Like HomePlug, G.hn uses OFDM to achieve gross speeds up to 1 Gbit/s. The OFDM signals occupy the 1.8- to 80-MHz range on the ac line. OFDM carrier spacing is 24.41 kHz for powerline operation, 48.82 kHz for telephone lines, and 191.31 kHz for coax. G.hn permits MIMO if other ac power wires are available. Security is by way of AES 128 encryption.
G.hn also works with phone-line wiring or cable TV wiring. These two media are already spoken for by the HomePNA Alliance, which has a standard using the phone lines, and the Mltimedia over Coax Alliance (MoCA), which offers a coax standard. Both are already in use. The HomePNA Alliance recently merged with the HomeGrid Forum to consolidate their standards and work toward an improved marketing effort for G.hn.
Related Standards
Both G.hn and HomePlug are the primary PLC standards for home networking. They were designed for the highest speeds needed for Internet access and video transport. There are also other PLC standards designed for lower speed monitoring and control operations in Smart Grid and industrial applications. The most widely used standards in these applications are G3, PRIME, and S-FSK. G3 and PRIME also use OFDM but are designed for the spectrum below 500 kHz. Data rates are typically a maximum of several hundred kilobits/s.
IEEE 1901 and 1905.1 are more directly related to G.hn and HomePlug. The IEEE 1901 standard defines a method of broadband PLC that can achieve a data rate of 500 Mbits/s. The basic specifications for 1901 are those of HomePlugAV. All HomePlug standards such as AV, AV2, and GP are interoperable with 1901 products.
Another IEEE standard, 1905.1, defines an abstraction software layer that is a common interface for HomePlugAV as well as other home networking technologies such as Wi-Fi, Ethernet, and MoCA. It permits any combination of these home networking technologies to work together to provide whole home coverage.
When multiple networking technologies are present, the 1905.1 software provides automatic switching from wireless to wired or vice versa to avoid any link degradation that may occur, maintaining the speed and the quality of the signal. A new certification program called nVoy promises to help ensure interoperability between the different communications methods.
Summary
HomePlug technology has been around for years and dominates PLC technology. While several other PLC standards were developed, they all failed to gain market acceptance, leaving this space to HomePlug. HomePlug is a proven PLC method readily available today.
G.hn is a more recent standard, and few products have been developed, primarily due to the delays in finalizing it and the absence of chipsets supporting it. Today, the standards are final, and chips are now available from Marvell and Sigma Designs. G.hn networking products will be available this year and beyond.
Which standard will OEM set-top box and TV set makers adopt to embed in new equipment? Will G.hn really be able to compete with the well established HomePlug in the home market?
References
HomeGrid Forum, www.homegridforum.org
HomePlug Alliance, www.homeplug.org
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