 This shows some of the common units used in data communications and networking. The units are used to measure some quantity that we're interested in. For example, when we're measuring time, we use seconds, and the lowercase s is the symbol used for seconds. Length is measured in meters. An angle is measured in radians, or even in degrees. Frequency is measured in hertz, where one hertz is equivalent to one time per second, or one cycle per second. With data transmission, we often care about transmitting signals at some strength, and the strength is often measured using power, so the units used is a watt. With data communications, the main purpose is to get information from one point to another. To measure the amount of information, we use bits, using a lowercase b as the symbol in many cases. Of course, there are many other units as well. Another example is a decibel, or dB, which in fact not an individual unit, but is a ratio of two different quantities, usually two different power levels. One power level in watts divided by another power level in watts. We'll see in a different lesson a more detailed explanation of decibels. Of course, there are other units, and we can combine these basic units to derive further units. For example, we know that we have time measured in seconds, length, distance measured in meters. Some speed we can usually measure in meters per second, for example. So we get the unit because it's the number of meters we travel in one second, the units being meters per second. So this is one derived from the basic units. We're not limited to just two different units. We get, for example, acceleration, which is meters per second per second. In data communications, we're often transmitting information across some communications network or link, so often we care about the data rate, the rate at which that information is delivered. And we get the unit of bits per second. And in fact, we can have more complex things like spectral efficiency, which is how efficient we use some range of frequencies to achieve some data rate. So spectral efficiency is usually measured in bits per second per hertz. The key point is that when you're performing calculations, then giving answers is to make sure you specify the correct units. Without the units, your answer is usually considered wrong. An example of how we can use the units in calculations. Let's say, for example, we have a system where we transmitted 1,000 bits of information across a communications link in a time period of two seconds. So what's our data rate in this case? Well, we can calculate where we have bits per second, and we see how the units are derived. We have 1,000 bits of information divided by two seconds, which is 1,000 divided by two and B, the bits, divided by seconds, S, which gives 500 bits per second. So you can see when you perform the calculations, when you use different units, when we perform the calculations, we can derive the final unit for the answer. In this case, we have a quantity measured in bits and another in seconds, and the final answer, bits per second. So make sure again that you use the correct units and you always give units when you give answers.