The Internet Power Line Adapter - University of Queensland

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Home Automation, Power Lines and the Internet Quenten Alick 2.4.2 CEBus Transmission Protocol The CEBus standard does not define exactly how each medium should represent data. It defines that the transmission medium should be capable of taking on two different states, superior and inferior. The inferior state is defined as the idle state of the medium. To begin making a transmission, the medium starts in the superior state. It then alternates states for each successive symbol that is transmitted. The length that each state is transmitted for indicates which symbol is being transmitted. There are four different symbols, logical one, logical zero, end-of- field and end-of-packet. The transmission lengths and symbols are summarized in figure 2-4. Symbol Name Transmission Length Logical One 100μs Logical Zero 200μs End-of-Field 300μs End-of-Packet 400μs Figure 2-4 List of CEBus symbols & Transmission times In the case of power line CEBus products, the superior state is defined as a “chirp” of noise which is transmitted at the zero crossing point of the 50Hz AC power signal. The “chirp” is defined as a signal that changes frequency from 200kHz to 400kHz, then 100kHz to 200kHz within the space of 100μs. The spread-spectrum chirp is shown in figure 2-5. In order to create the different length states for the different symbols, this 100μs chirp is repeated to create the - 12 -
Home Automation, Power Lines and the Internet Quenten Alick required length symbol. Finally, the frequency sweep means that the “chirp” is spread-spectrum, which gives the “chirp” better noise resistance. Figure 2-5 CEBus Power line Chirp All data in the CEBus protocol is transmitted in packets with a header and a “payload” or message. The header contains information on the type of service being provided as well as source and destination addresses with the total length being around nine bytes. The packet message can be up to 32 bytes long and can contain any data that the recipient understands. 2.4.3 CEBus Summary The CEBus standard has several advantages. Firstly it allows for a large number of appliances within a home. While sixty-four thousand per house code may be a little excessive, it is many more than the sixteen that the X-10 standard offers. Secondly, the open and versatile nature of the CEBus standard that allows different vendors to create a device and be reasonably certain that it will be able to communicate with the device from another vendor. Thirdly, the spread- spectrum nature of the power line implementation means that the system has quite good noise immunity. Finally, the way in which the network can be set up to suit the needs of the user is extremely versatile. Both reliable and unreliable networks can be created using the CEBus standard. These factors mean that the CEBus standard was the choice made for the power line communications standard in this thesis. - 13 -
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The Internet Power Line Adapter - University of Queensland

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