When it comes to networks, Maximum PC readers fall into two camps: Those who already have one (and want to expand it or extend its range), and those who wish they had one. Whichever category you fall into, power-line networking is finally becoming a viable alternative to Cat5 and Wi-Fi.
Power-line networking takes advantage of the unused bandwidth inside the copper wiring used to distribute electrical power throughout the home. Power-line adapters convert data into a carrier-signal format, so it can be transmitted from one device to another.
Since your house already has electrical outlets, there’s no need to pull any new wires or drill any new holes. And those existing power cables are endowed with potential bandwidth that’s much greater than what today’s 802.11n Wi-Fi networks are capable of delivering. (And when you think about it, most Wi-Fi devices still depend on wires—even if only to power them.)
In an ideal world, all you’d need to do to assemble a network is plug your PC, DSL/cable modem, router, and peripherals into a wall jack and BAM! You’d have an instant network. It’s not quite that easy, but the fact that it works at all is remarkable.
|New power-line networking technology should soon enable consumers to build robust data and A/V networks using their existing AC power lines.|
In the United States, home electrical systems operate on alternating current at a frequency of 60Hz. Power-line networking uses much higher frequencies—ranging from 2MHz to 30MHz—to carry data. Intelogis was the first company to offer power-line networking technology to consumers, but its Passport system proved to be slow and extremely sensitive to noise caused by appliances operating on the same power lines.
The problem was Passport’s reliance on frequency-shift keying (FSK) to encode data carried on the network. FSK uses just two frequencies to encode the data in a binary system: one frequency for the 1s and a second for the 0s. If a large appliance or high-current device (such as a hair dryer) caused an electrical surge to step on either of those frequencies, the stream of binary data would be interrupted. The device transmitting the data would then have to resend the packets, causing significant congestion on the network.
Two consortiums are competing to establish a de facto power-line networking standard: the HomePlug Powerline Association (whose members include Intel, Motorola, and Texas Instruments) and the Universal Powerline Association (or UPA, organized primarily by communications chipset company DS2). A third group, the Consumer Electronics Powerline Communication Alliance, was established to promote the coexistence of power-line networking products that use different technologies. (CEPCA’s membership roster includes nearly every major consumer-electronics manufacturer, including Sony, Toshiba, and Philips.)
Although products based on DS2 chipsets have proven extremely effective (and the company recently demonstrated a 400Mb/s product), a merged specification promulgated by the HomePlug Powerline Association and Panasonic won the most recent round of voting by the IEEE P1901 work group. Analysts blame the slow growth of the power-line networking market on the lack of interoperability between power-line networking products; if HomePlug gets the IEEE’s seal of approval, DS2 will have a difficult time bucking the trend.
The HomePlug Powerline Alliance’s baseline specification is derived from InTellon’s PowerPacket system, which takes a very different approach to power-line networking than Intelogis’s now-defunct Passport. Rather than streaming data encoded to just two frequencies, PowerPacket uses a spectrum ranging from 4.3MHz to 20.9MHz. These bands are organized into 84 “lanes” of traffic using orthogonal frequency-division multiplexing (OFDM) with forward error correction. OFDM uses a large number of closely spaced subcarriers, each of which is modulated using quadrature amplitude modulation (QAM). QAM conveys data by manipulating the amplitude of two carrier waves.
Since PowerPacket can send data over so many traffic lanes, the transmitting device will send redundant data on more than one subcarrier. If a power spike or excessive noise interrupts one lane of traffic, the data encoded in the other frequencies should still get through. The transmitter also adds redundant data to its messages, which is where forward error correction comes into play. The transmitting device sends a known preamble at the start of each data packet. The receiving device then compares that preamble to the actual data received. If there’s a difference, the receiver can either try to correct the error or use one of the redundant streams. Garbled data doesn’t need to be retransmitted unless both these methods are unsuccessful.
The use of frequencies outside the range of AC power explains why power-line network devices and surge suppressors don’t play well together: The latter interpret the data-carrying frequencies generated by the former as electrical spikes that must be tamped down.
HomePlug 1.0 is limited to throughput of 14Mb/s, which means it’s inadequate for streaming high-definition audio and video. The improved Physical Layer (PHY) and Media Access Control (MAC) technologies in the newer HomePlug AV and similar nonstandard specification, however, enable a consumer to build a 200Mb/s network using the existing power lines in their home.
The MAC layer uses Time Division Multiple Access (TDMA) and Carrier Sense Multiple Access (CSMA) with AC line-cycle synchronization. TDMA enables several devices connected to the same network to transmit over the network and share its capacity; specifically, it allows these devices to share the same frequency channel by dividing the signal into different time slots. Each device then takes turns transmitting in rapid succession. The use of TDMA enables HomePlug AV to deliver Quality of Service (QoS) guarantees so that each network application gets the bandwidth and transmit/receive time that it needs.
CSMA is a protocol under which a device about to transmit first listens to the channel it’s about to use to determine if it is in use. If the device senses that the channel is busy, it will defer its transmission; if the channel is idle, the device sends a message to all the other devices on the network not to use that channel. It then sends its data packet, waits for an acknowledgement that the packet was received, and releases the channel.
AC line-cycle synchronization enables a HomePlug AV network to identify and work around noise in power lines. General noise tends to fluctuate, but the impulse noise injected into the power lines by appliances such as refrigerators and air conditioners tends to be synchronous and of limited duration. HomePlug AV adapters use multiple time slots that are synchronized with the AC cycle, and they analyze line noise before loading bits into the carrier waves. This allows a HomePlug AV network to minimize the impact of power-line noise: If line conditions are optimal, each carrier wave can be loaded with different data to yield the highest possible bit rate; if line conditions are at their worst, every carrier wave can be loaded with the same data to ensure that the data arrives at its destination.
Where the original HomePlug spec used only QAM (which has eight unique analog phase/amplitude symbols), HomePlug AV utilizes a variety of modulation techniques based on line conditions: Binary Phase Shift Keying, Quadrature Phase Shift Keying, and 16-, 64-, 256-, or 1024-QAM. As its name implies, 1024-QAM has 1,024 unique analog phase/amplitude symbols and can represent 10 digital bits.
HomePlug AV equipment is compatible with HomePlug 1.0 equipment, but neither of these standards is interoperable with gear based on DS2’s technology (or the relatively ancient Passport hardware, for that matter). You can, on the other hand, mix power-line networking technology with wired and wireless Ethernet products.