(See Part One at http://www.smarthouse.com.au/Automation/Industry/R7X7C6F8?
page=1 )
Power-line signaling
In response to the drawbacks of cabling, home-automation technologies seek to use one type of wiring in every residence: the power line. As a networking medium, a power-line connection has two advantages. One is that they are in place and run to nearly every location where endpoint devices exist. The second is that endpoint devices need no external power source, such as a battery. Both help satisfy the low-cost and ease-of-use requirements of a consumer technology.
Power-line networking has its challenges, however. The medium is noisy, carrying a variety of voltage spikes that arise as lights and motors switch on and off, loads change, and disturbances on the power grid propagate into the home. As a result of this noise, power-line-networking technologies have either restricted their signaling bandwidths or employed sophisticated and expensive noise- and error-reduction strategies.
The X10 standard serves as an example of the first approach: restricted bandwidth. To avoid noise, X10 signaling occurs during the zero crossing of ac power. A burst of 120 cycles at 120 kHz, repeated at the next zero crossing for basic noise immunity, signals a one, and its absence signals a zero. The result is a raw data rate of 60 bps, with synchronization, framing, and addressing bits adding overhead to reduce the achievable data rate by 60%. This low data rate prevents the network from handling any but the most basic control and sensing functions and adds considerable latency when implementing a string of commands.
