So to start our journey on understanding how we share the network medium, let's go back to a time before cell phones were invented. Back to the days of the telegraph. So in the 1800s, there was someone by the name of Samuel F.B. Morse who started experimenting with this device called the telegraph, where you can have a transmitter, someone who is transmitting the message [SOUND], send a telegraph message over a wire which we'll draw looking like this as a wire at the other end to a received, who would then receive this telegraph message. And the telegraph was in the form of this dash, dot, dash, dot, dot, dash type of messages. And then the receiver could look in to code that message and see those dash, dots, dash, dots and so on and then get the telegraph. And then they could send something back to the transmitter and have them understand it. And this device as we said was called the telegraph. So, in the later 1800's, like maybe mid 1800's, 1860 to 1870's, Alexander Gram Bell came along and he took the concept of the telegraph and he brought it one step further. What Bell said was that he could have multiple transmitters [NOISE]. And Multiple receivers on both ends of this wire. Same wire. And to get each of them send messages at the same time over this wire, without having them interfere with each other, so that this person could receive his telegraph message from this transmitter And, this person could receive his telegraph message, from this transmitter, even though they were going over the same, network medium. There was a way for them to figure out, what their message was, and what came from them. And this device was called, the Multiple, Telegraph. So, how is that possible? I mean how is it possible that we could have more than one people on the same wire? Which then we can extend that to the idea of cell phones. How can we have more than one people using the air at the same time? So, let's try to take the idea here. And let's try to see if we can have two different transmitters on this side. And two different receivers on this side of the wire. Let's try to brainstorm different ways that we can allow people to use the wire without interfering with one another. Well, we have to divide them and allow for what's called multiple access, where we can have multiple conversations occurring on this wire at the same time, or multiple sessions accessing the wire. So, one way clue that we could do this is by defining them in time. So, we can have them take turns. And we could basically say that one group could use the wire at one time, then we could allow the next group to go. Then, the first group could go again, and the next group would go again. So that while one person's signal is on the wire, the other person is not, so we divide them in time. That's clearly the most intuitive way to do it. But let's try to dig a little deeper because this would get very very Inefficient as we had more and more people trying to use the same wire. You have to wait a very long time before you could have your turn again. Because someone would have to go and we have to keep scheduling where this person would go first. Then this person would go second. This person would go third, fourth, fifth and so on. And so, you Have to wait until your turn came again before you could speak over the wire. So, in understanding the second way that we can divide the resources, think for a minute about musical instruments. So think of the difference between a flute and a baritone. So a flute has a very high pitch. We can call, high pitch. While it may be very hard to distinguish between two different flutes, or two different baritones. Certainly you can tell that there's a flute, and there is a baritone if we had those two different instruments, and you could tell which one was doing which. So, the idea of pitch can be extended to here. And that if we could have everyone speak in a different pitch, and we had the receivers know which pitch they were going to be listening for, then we could separate them over pitch. So, this person could talk in one pitch to this person, this person could talk in a second pitch to this person over here. This person could talk in a third pitch. To this person and so on. And then we call that pitch. So now we can have them talking at the same time. So they are on the wire at the same time but just separated by different pitches rather than being separated in different times. You know, while it's theoretically possible to distinguish us on different pitches It would be pretty difficult over the same wire because you could obviously hear the other conversations pitches. So along those same lines then what if we could have everyone maybe speak in a different language. So we could have one person speak Spanish, one person speak English, one person speak Italian and so on. So, then, every person, rather than listening for a different pitch, they could listen for a different language. So then we can all speak in our natural tones of voice, and we can all use the network at the same time. So we're not separating over time or pitch anymore, we're separating over language instead. But clearly again we'll probably have an interference issue, because It's hard to not hear someone else's language. And it's hard to perfectly decode another person's language when you're still hearing a bunch of other people talking. And so these are all different ways of having multiple access, and enabling multiple access. And in fact, they are analogous to different ways that we will look at multiple access, and would have been. Done in terms of cellular technology. And one more term that I should bring up is the idea of a link. So, when we talk about a link, we mean a transmitter and a receiver. Like, across this wire right here. This is one link. One transmitter, one receiver. So, a transmitter receiver pair defines a link.
