Carrier Current LANs

Carrier Current LANs
A quasi-wireless LAN technique, called carrier currents, is the use of power lines as a
medium for the transport of data. Even though this type of LAN is not really wireless, it’s
worth mentioning because it doesn’t require the installation of network cabling. In the next year or so, you should begin seeing products that implement this approach. This technique is
very similar to using an analog modem to communicate over telephone wires. Designers of the
telephone system did not plan to accommodate computer communications, but people use
modems every day to communicate their data. The telephone system is capable of supporting
analog signals with the range of 0KHz to 4KHz. A telephone modem converts the computer’s
digital waveform to an analog signal within this range and transmits to the computer you
choose. The modem at the distant end receives the “telephone signal” and converts the data
back into a digital signal that is understood by the computer.
Power line circuits within your home and office provide enough bandwidth to support 1Mbps
to 2Mbps data signals. Utility companies and others designed these circuits to carry 60Hz
alternating current, typically at 110 volts. It is possible, then, to have a power line modem that
interfaces a computer to the power circuitry (see Figure 2.9). The interface acts much like a
telephone modem and converts the digital data within your computer to an analog signal for
transmission through the electrical wires.
The 110 volt alternating current in the circuit does not affect the signal (or vice versa) because
the signals are at different frequencies. The interface has filters that will block the lower 60Hz
frequency from being received The advantages of this technique are ease of installation and low-cost products. A disadvantage
of the power line approach is that the presence of electrical transformers, designed to electrically
couple signals at 60Hz, will block higher frequency data signals. Most homes and smaller
facilities will not have this problem because usually only one side of the transformer is available;
however, larger buildings, especially industrial centers, will have multiple electrical wire
legs connected by transformers. The presence of transformers, therefore, will limit connectivity
among sites