Before we move on, let's pause to consider an analogy that will help us to understand the multiple access technologies we've been discussing. Suppose you're going to a cocktail party, and there's a host inside and the host wants to make sure that every two pairs of people that arrives can have their conversation. So, we'll just assume that you're going to the party with someone else and that you want to just be talking to that person at the party. So, one way that the host can accomplish this is by putting everybody in a separate room. That's socially introverted but you can have everyone in a separate room. And then, nobody would be able to hear anyone else's conversations. So, clearly this wouldn't actually happen at a cocktail party but just bear with me for a minute. So, you could put this group in this one room, you put the next group that arrives in the second room, and so on. So, what each of these rooms is representing is a different frequency. So, we have one pair, or one link, on one frequency, one pair, another link, on another frequency, and so on. Which is anonymous with FDMA system. So, again, each room was a different frequency channel. And we continue as every future pair arrives to put them in a separate room. But now, what happens when another pair arrives right now, or a few different pairs arrive? Well, we don't have any more rooms in the house to fit anybody else. So, we need to figure out a way to have multiple people be able to stay in the same room and still communicate. So on top of this FDMA, we can put multiple people in the same room if we allow some other sort of schedule, or some other way of sharing. So now, even that resource, this frequency resource that originally every link had one of, now we have to even divide that resource up. And one way we could do that as we have a lot of people arrive is through TDMA, where we basically, every, in, within every room, we have a scheduling happen, and everyone takes turns. So, in this room up here, for instance, one group could go first, and the next group, next group, and the next group. And then, it would repeat. So, this group, then this group, this group, and this group goes. And so then, nobody in one room is speaking at the same time. So, clearly, this is very unlikely to happen at any party that you would ever go to. But this is an example of TDMA now, over the top of FDMA. Or sometimes it's abbreviated as F/TDMA. Where now if in each room we have a time schedule and people are taking turns within that time. But the main idea is again that at any given time in any given room only one link is speaking. So now, with CDMA, we break down these walls. And we would allow everyone to speak, but say in a different language. So, again, for CDMA we need different codes, but so suppose then everybody at this party was speaking a different language. And we had every group of individuals speaking in a different language to one another. Now, if these languages would not interfere with each other at all. If they two are completely orthogonal languages, which that doesn't necessarily make any sense, but if you could design two languages that would not interfere with one another. Or you would design a set of how many people we have here that many links that many languages that were not interfere with each other. Then, that would work really well and everyone can speak at the same time just in different languages. But clearly human vocal cords were not designed to be perfect transmitters in different languages. So, the problem is that even if I'm speaking in English and you're speaking in Spanish, even if you can understand what it is that I'm saying, I'm still going to interfere with you. because you can still hear my noise. It would really be noise to you, my conversation. So, if I'm speaking one language and you're speaking another language, you can still hear me even if what, what you're hearing doesn't make any sense. So, we need a way to control the volume of our voices, say, some courtesy procedure so that each of us can say how loud we are going to be talking. So, we've come up with some consensus that will work for every group of people in this room, so that we can all have our conversations without causing too much interference to one another. And that idea will be recurring throughout the rest of this lecture.