How far will light travel in the blink of an eye? Normal eyeblink duration is typically 150 to 200 milliseconds (ms), so for this example let’s call it 175 ms. In one second, that beam of light will go around the Earth nearly 7-1/2 times! For this example, I’m ignoring terrain obstructions, reflection, atmospheric refraction, the atmosphere’s slightly different velocity of propagation compared to a vacuum, and the fact that light prefers to travel in a straight line.
How fast? Let’s imagine a beam of light that could somehow travel unimpeded around the Earth’s equator. In a vacuum, light or any electromagnetic signal, for that matter zips along at 299,792,458 meters per second, or 186,282.397 miles per second. One measure that we deal with, often indirectly, is the speed of light. Nice mathematical shorthand! Speed of light Can you imagine dealing with numbers like these on a daily basis? The decibel allows us to express these same levels as 0 dBmV, +20 dBmV and +48 dBmV respectively, using the formula dBmV = 20log (signal level in mV/1mV). 0.001 volt isn’t too cumbersome as far as the number of zeros to the right of the decimal point, but how about the same signal level expressed as power, or 0.00000001333 watt (13.33 nanowatts)? A line extender’s typical perchannel input level is 0.010 volt (10 millivolts) or 0.000001333 watt (1.33 microwatt), and its per-channel output is 0.251189 volt (251.189 mV), or 0.000841276 watt (841.276 µW). Think about the minimum per-channel input to a subscriber terminal, as defined in Part 76 of the FCC’s Rules: 1 millivolt (75 ohms), or 0.001 volt. Too many zeroes Have you ever wondered why we use the decibel to express signal levels?Have you ever wondered why we use the decibel to express signal levels? Why not just use the signal’s actual voltage or power? One reason is the very small numbers that make up typical cable network signal levels. Our 1,000,000 Hz frequency becomes 1 megahertz (MHz), and 1,000,000 bits per second is 1 megabit per second (Mbps).
It’s generally inconvenient to deal with large numbers like that, which is where the International System of Units (SI) – specifically SI prefixes () – can help. From that perspective, a million is a pretty big number.
If you were to count to 1,000,000, how long would it take? If we assume the average person needs second per number, and you count nonstop (no sleeping, eating, drinking, or …) to 1,000,000, it will take a million seconds. It also might represent data rate – say, 1,000,000 bits per second. When discussing an RF signal, 1,000,000 might be the signal’s frequency, as in 1,000,000 Hertz (Hz). In reality, it’s hard to grasp just how big or small numbers can be. Fortunately, there are several ways to simplify how we describe those numbers, which I’ll get to shortly. Ours is a world of numbers, and in cable we tend to use a lot of really big and really small numbers.