GOOD2USE Knowledge Network PC Powerline Technology Overview

GOOD2USE Knowledge Network PC Powerline Technology Overview

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Power-line communications systems operate by adding a modulated carrier signal to the wiring system. Different types of power-line communications use different frequency bands.
The power distribution system was originally intended for transmission of AC power at typical frequencies of 50 or 60 Hz, consequently power wire circuits only have a limited ability to carry higher frequencies. The propagation problem is a limiting factor for each type of power-line communications.

There are two competing power-line technologies. The original technology is called Passport, from Intelogis. A new technology called PowerPacket, developed by Intellon, has been chosen by the HomePlug Alliance (a trade association with a useful blog here) as the standard for power-line networking.

There are some disadvantages to connecting through power-lines when using the older Intelogis technology:

The connection is rather slow -- 50 Kbps to 350 Kbps.
The performance can be impacted by home power usage.
It can limit the features of your printer.
It only works with Windows-based computers.
It uses large wall devices to access an electrical outlet.
It requires that all data be encrypted for a secure network.
Older wiring can affect performance.

PowerPacket technology eliminates many of these concerns, citing the following advantages:

It is very fast, rated at 14 Mbps.
It "avoids" disruptions in the power-line, maintaining the network's connections and speeds.
It does not limit the features of your printer.
It can be compatible with other operating systems (depending on driver availability).
It may have the necessary circuitry embedded within the device, necessitating only a standard power cord to access an outlet.
It works independent of line voltage and frequency of current.
It includes encryption.
In tests, it showed no signal degradation due to older wiring.

The older power-line technology used by Intelogis relies on frequency-shift keying (FSK) to send data back and forth over the electrical wires in your home. FSK uses two frequencies, one for 1s and the other for 0s, to send digital information between the computers on the network. The frequencies used are in a narrow band just above the level where most line noise occurs.

Although this method works, it is somewhat fragile. Anything that impinges on either frequency can disrupt the data flow, causing the transmitting computer to have to resend the data. This can affect the performance of the network. For example, when using more electricity in the house, such as running the washer and dryer, the network can slow down.
Intelogis includes line-conditioning power strips with its network kit and encourages their use inserted between the wall outlet and omputer equipment to help reduce the amount of electrical-line noise.

Intellon's PowerPacket technology, which serves as the basis for the HomePlug Powerline Alliance standard, uses an enhanced form of orthogonal frequency-division multiplexing (OFDM) with forward error-correction, similar to the technology found in DSL modems.

OFDM is a variation of the frequency-division multiplexing (FDM) used in phone-line networking. FDM puts computer data on separate frequencies from the voice signals being carried by the phone line, separating the extra signal space on a typical phone line into distinct data channels by splitting it into uniform chunks of bandwidth.
In the case of OFDM, the available range of frequencies on the electrical subsystem (4.3 MHz to 20.9 MHz) is split into 84 separate carriers. OFDM sends packets of data simultaneously along several of the carrier frequencies, allowing for increased speed and reliability.
If noise or a surge in power usage disrupts one of the frequencies, the PowerPacket chip will sense it and switch that data to another carrier. This rate-adaptive design allows PowerPacket to maintain an Ethernet-class connection throughout the power-line network without losing any data.

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