We have seen the concept of radio spectrum. But the question that might arise in your mind is, hey, when my phone sends a wireless signal, does it use the entire radio spectrum? Well, the answer to that question is not really, it cannot and it doesn't even need to. When your phone transmits or receives a signal, it only uses a small subset or a sliver of the frequency spectrum. And that limited range of frequencies over which a wireless device, exchanges wireless or radio signals, it's called its frequency channel. And the word of the frequency channel currently being used by a wireless device, it's called the channel bandwidth. So these are the two terminologies that you may have heard about and used in different contexts. For example, we have tv channels. Why are they called tv channels? Well, that is because in the most classical implementation, different networks or different stations of a tv service, were broadcast on different frequencies or different channels hence the name tv channels. And we use the term bandwidth in different contexts as well. For example, I have bandwidth for a new project or I don't have bandwidth for a new project. And that bandwidth essentially is your range. And in this context, is, it is the range of frequencies over which that wireless device transmits a wireless signal. And as always, there is a simpler way to understand the concept of a radio channel. It can be compared with a sort of a data pipe or a water pipe. If you have a narrower water pipe, you will get limited water supply. Whereas if you have wider water pipe, you will get an abundant amount of water supply. Just like that, there are roughly two classifications of a wireless channel, narrowband and wideband. In narrowband channel, as the definition says, occupies just a small amount of radio spectrum, on the order of a few kilohertz or a few thousand hertz. Whereas a wideband channel, occupies a large amount of radio spectrum relatively higher than the narrowband channel, on the order of few megahertz or millions of hertz. And as the logic might go, a narrow band channel, given everything else equal, will help you send limited amount of data. And a wideband channel on the other hand, can help you send a large amount of data. Now, having understood the concept of narrow band and wideband channel, one question that might immediately prop up in your mind is, hey, why can't every channel be wideband channel? Wouldn't it be nice for every device to get a wideband channel? But unfortunately that is not the reality. So what prevents every channel from being a wideband channel? Two of the factors are something that we can understand right now, we saw a few slides earlier that this frequency spectrum is limited by definition. So that tells you that not every channel can be wideband because in that case you will quickly run out of the available spectrum and other upcoming services won't have any spectrum to work with. So availability is the primary reason why every channel can't be wideband. A corollary of availability is because spectrum is a scarce resource. It begins to get more and more expensive as you begin to buy more and more spectrum. So if you had to buy a wideband channel for every user, your system will be extremely expensive because spectrum is inherently a scarce and expensive commodity. So availability and costs are a couple of reasons for which not every channel can be a wideband channel. But beyond that, there is one more reason, for which not every user can get a wideband channel, and that is precisely what we are going to look at next.