So, Aloha is about as simple as the random access protocols get. And it's simple and it's really, really easy. But the problem is that it doesn't really have any sensing going on at all. So, each of the transmitters, each of the stations, the WiFi stations, are doing everything blindly. That, that would be like going to a stop sign and putting a blindfold on and then just randomly deciding when to go, and when not to go. So, clearly that's not a desirable thing to do. We will admit, though, that having a car collision is a lot worse that having a simple, really small data collision that can just be resent. But still the idea is that, you know, you really need to quote unquote look both ways before you go into the intersection. Just like, you really should be sensing to see if anybody else is currently transmitting so that you don't interfere with them, and then you would expect that they would do the same as you. So, we need to add sensing into the mix, really it's the key idea here. And what we mean by carrier sensing is that you're going to refrain from transmitting if you hear someone. So, carrier sensing is the term here. And you have to observe a full wait and listen period before sending. And this is, now we're describing what called Carrier Sensing Multiple Access or CSMA. Notice again, now this is the next multiple access method, it's just not a controlled access method, it's a random access method. So, we had, we had FDMA, TDMA, CDMA and now we have CSMA not CDMA. But under carrier sensing we have to refrain from transmitting if you hear someone and you had to observe a full wait and listen period before you will send anything. So, here's the idea. Suppose we have two stations, A and B, okay? Now, A starts his wait and listen period, right? And then he has to wait, it's just a prescribed amount of time, it's fixed, it's just a fixed period. And then, B starts his after A has already started his. So, A finishes first because they both have the same wait and listen period. And then A starts to transmit, because it gets through the period and he doesn't hear anybody else. Okay? If he had heard someone else, then he would have had to wait. But then A starts transmitting, and immediately then B senses that A has just started transmitting and says up, you're busy, I can't transmit now. And so I have to end. Whereas B's wait-and-listen period would have ended over here, he didn't get through the whole thing. So, B won't start to transmit. And there's the way that we do it. Then A sends his data. And then some time later, B decides that he wants to send his data. So, he goes through another wait-and-listen period. And then he also sends his data, through the network. But we can draw an analogy here to typical conversations. Right? So, the idea would be that if you're in a group of people. Right? It's common courtesy to wait until someone is done talking before you start speaking yourself. And you don't immediately start talking as soon as they stop, you have to wait just, you know, a second or two to make sure what have they done for you to continue. And that's just a a typical courtesy [UNKNOWN] conversation, so that's well, we have going at here. Whereas under an aloha scheme, if you were, everybody would just randomly being saying random things at different times and responding to each other and that's very strange, would be a very strange and rude way of going about. So, this is the more common courtesy, which leads to better throughput as we will see. So, now the question is, how do you know that your data actually got there? So, you have to know if you have to retransmit or not? And the way that this done is by sending what's called an Acknowledgement frame. So, when your data gets to the source, right. If you're a device and you send to the axis point, right. And you send some frame. If the frame gets there, everything okay. Then the access point will send back to you, and you will send an acknowledgement. So, and that's sent upon again, proper receipt, just to let you know that everything went okay. And if there's no acknowledgement, you assume that a collision happened and then you have to retransmit at some point later. So, let's consider another example here. And now we have five stations, okay. So, now suppose B is transmitting to D, so B is sending something to D first. B goes through it's wait and listen period, and he sends some data, okay. Now, B's transmission gets there because there's no one else interfering with him. But B doesn't necessarily know that. So, then D a little while later, remember D also would have to go through a wait and listen period before he transmits himself because D, even though it's an access point is still a station, so you have to wait. Then he sends this acknowledgement frame back to B. And that acknowledgement frame also doesn't have any collisions or anything. And it could happen that the acknowledgement frame does collide. Then, after that acknowledgement ends, both A and C need to send something. So, they both go through a wait and listen period, and now here's what happens. Okay. A and C both decide that they want to send. And they both go through the wait and listen period. So, they send, and they start sending at the same exact time. Okay. And then what happens is, they both sent at the same time. So, they're both colliding. But they didn't know that they were trying to send because they just started speaking at the same time. It'd be like in a conversation. After someone's done, you've probably had that happen before where then two people will start to speak at the same time. And then what happens is you have a collision and there is no acknowledgement that comes back. So, now the question is, what do you do when you don't get an acknowledgement? So, what happens when there's no acknowledgement frame back? Well, you need to back off and send again. And we're going to talk about what back off means right now.
