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X10 is regarded as the grand-daddy of Smart Home protocols as it was first developed in 1975 by a small company in Scotland, UK. It was the first general purpose home automation networking protocol and remains in use even today, with millions of devices using X10 still in use worldwide. It quickly became popular because it used to be one of the few home automation that could be easily retro-fitted into existing homes without installing additional cabling, and because of it's entry-level quality and pricing.
By using Power Line technology your home's existing power lines are used to create the network that connects devices together. Wireless devices are also available where the X10 protocol is transmitted and received over RF.
X10 sends bursts of 120 kHz(RF) signal, representing digital information, onto the powerline at the zero crossings of the 50/60 Hz AC voltage waveform, with only one digital bit being transmitted at each zero crossing. Special measures need to be taken to help X10 signals pass from one phase to another, or across split phase systems, when this is required. This usually involves using coupling capacitors or an active X10 repeater, or X10 signal amplifiers and a coupling capacitor.
It's a one-way technology (signals are not expected to, nor required to be acknowledged). Therefore, each signal is sent twice (A-1:On, A-1:On). Signals take about ½ to ¾ of a second to reach the responder – leading to clearly visible delays between pressing a button and the “light going on”.
X10 is more sensitive to attenuation and line noise than other, more modern powerline technologies. Home owners may experience load-related unreliable X10 communications, where the reliability decreases and increases as particular 240V loads such as ovens or hair dryers are switched on or off. The operation of televisions or wireless equipment may also result in spurious X10 on/off signals. For that reason filters, couplers and repeaters are commonplace requirements.
X10 is beginning to show its age nowadays and has a number of limitations compared to newer protocols such as Z-Wave and Insteon. Power Line as a transport medium can be prone to interference which can cause problems for X10 and the ability for the signals to pass "across circuits" can sometimes be limited - so a light switch downstairs may not be able to control a light fitting upstairs if they are on different electrical circuits.
In our opinion the lack of speed, limited functionality and unreliable transmission are all reasons to avoid X10 in a modern Smart Home. By looking for products that use protocols such as Z-Wave, LightwaveRF or ZigBee you'll be able to build a solid, fast and stable system that will rarely let you down.
That said, X10 devices can be cheaper than other technologies so if low cost is your primary aim then X10 may be a good start on your Home Automation journey. Once you've cut your teeth on X10 you can expand your system to other technologies. By choosing a Home Automation controller such as HomeSeer you can likely retain your X10 devices too, making the transition to something like Z-Wave less of a financial impact.