Communications Technology

Though most digital control systems rely on hardwired connections between devices, there are alternative technologies that can be used to simply installation (particularly in retrofit applications), provide a communication link with remote locations, and increase data transmission rates. These include:

• Power Line Carrier (PLC). PLC systems use the existing electrical wiring system as a means of achieving inter-device communication. This is accomplished when a device generates a high frequency communication signal that is superimposed over the standard sixty hertz power sine wave. Other devices that are connected to the same electrical system can analyze the spectral content of the incoming power, and extract the superimposed "message" from the sixty hertz carrier wave. PLC systems can provide reduced installation cost because of the reducing wiring requirements, but may be less reliable and slower than hard-wired systems.

• Wireless Technology. For locations that are located a significant distance away from a building, there is the ability to send and receive control messages using wireless technology. Some systems are based on

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