Shannon's equation, proposed by Claude Elwood Shannon back in 1948, computes the upper-end of the capacity of a communications link (in bits/second). This is computed as a function of the link bandwidth and the signal-to-noise ratio (or the percentage of how much useful information passes through all the random interference on the line).

To compute information capacity, Shannon's equation has been applied. If you're an engineer, you're going to be familiar with the equation shown in Figure A.

FIGURE A

Shannon can tell you just how fast your feed should be.

Looking at the above equation, C is the maximum information rate with an optimum coding technique, measured in bits/second. B is the bandwidth, measured in hertz. S is the encoded information (signal) power. N is the noise and interference power.

I've used Shannon's equation to attempt to make a reasonable and fair comparison but even so the plot shown in Figure B should be viewed as a qualitative indicator rather than a precise one. The plot is a comparison of the estimated maximum information capacity versus distance for several last-mile propagation media.

FIGURE B


Different technologies result in different bandwidths. Roll over picture for a larger image.

The table in Figure C shows the parameters used for each medium in the above plot.

FIGURE C


Here's what was used to measure maximum information rate. Roll over picture for a larger image.

I've estimated total information capacity by dividing the available spectrum into 100 segments, spreading the signal power evenly over the spectrum and performing a piece-wise integration of the information capacity using the parameters appropriate to the center of each individual segment.

Where possible, 0 dBm (1 milliwatt) is assumed for signal power. Fiber systems normally use less than this and BPL has been allowed more to better match existing practice. A spectrum width of 7 GHz is used except for conventional (HF) BPL and DSL lines.

BPL capacity has been calculated using the full spectrum studied by the OPERA (Open PLC European Research Alliance) report, which is about twice that used by the HomePlug standard. Spectrum has not been reduced to account for notching. Signal power is also about five times higher than that used for HomePlug devices. Although BPL chip sets which can operate at 200 Mbps exist, it still would not be possible for them to operate at full rate under these conditions.