BROADBAND OVER POWER LINES

Alternating Current and Radio Frequencies

As you can see, getting electricity from the power plant to a home is basically a three step process. To add Internet data to this process will complicate matters more, but not by too much because everything I have described so far about the power grid already exists. At the most elementary level, then, the only additional pieces of equipment we need for PLC is some form of encoding and decoding hardware: the modem/inductive coupler.

Broadly speaking, the PLC modem, or coupler, packages the Internet data for transmission over the power lines by wrapping around the line, without directly connecting to the line. To really understand how the coupler works, however, we need to understand electricity, specifically alternating current and radio frequencies, and its characteristics.

The giant generators at power plant produce a type of electricity called alternating current (AC). Most electronic devices nowadays run on alternating current, as opposed to direct current (DC). What makes AC special is that it can oscillate, that is, its magnitude and direction vary cyclically in a sine wave pattern (see Figure 2), whereas DC cannot [9]. It is important to emphasize here that AC is not necessarily electricity. Audio and radio signals are also forms of alternating current.

Figure 2. A sine pattern of single-phase residential-use AC [13].

Encoding Internet data with the BPL coupler

Now we can discuss what role the BPL encoder/coupler plays. Recall that Internet service providers typically contract with phone companies to get access to the fiber-optic backbone (in some cases, wireless access is also a possibility), so-called last-mile technologies. A BPL provider must also do this. The BPL coupler serves as the vital link between the fiber-optic cables and power lines. Specifically it encodes or modulates the data into a radio frequency (RF) energy not used by the electricity. In other words, the coupler must encode the data into another wave pattern that will not be canceled out by the electric current's wave pattern [4, 8]. Specifically, frequency is the measurement of the number of times that a repeated event occurs per unit time. The most common standard of measurement is the Hertz, which is equal to the number of times that an event repeats itself in a second. Thus, 1 Hz means that an event repeats itself once per second [9].

Theoretically, the fiber-optic backbone network can be connected at the power plant, but in practice, however, this is not viable. As it turns out, power plants produce what is known as three-phase alternating current:

Figure 3. Three-phase alternating current, typical of high voltage lines.

Without even having to go into detail, you can see by comparing Fig. 3 to Fig. 2 that it is several times more difficult to graft an additional frequency, namely the Internet data RF, that would not interfere with any these three frequencies. Not surprisingly, BPL companies have concluded that high voltage lines are too disruptive—or noisy—and too prone to spikes to transmit data reliably, and thus the companies bypass the high volta ge section completely.

Instead, the solution has been to hook the fiber-optic backbone to medium voltage power lines since by this point the current's voltage is much lower and more consistent (see Figure 4). The frequency of electricity in homes is most commonly either 50 or 60 Hz, so 50 or 60 cycles per second, which is relatively low [9]. BPL, on the other hand, occupies a much higher frequency band, between 1.6 MHz and 80 MHz [4]. Thus, because the electrical current and radio frequency vibrate at different frequencies, the two do not interfere with each other.

Figure 4. Backbone connection and power/BPL distribution grid. HV, MV and LV denote high, medium, and low voltages. Notice a backbone is connected at the HV to MV transformer [4].