Hello. Thanks again for joining us in Network's Illustrated principals without calculus. In this lecture, we're going to look at a device that you hold in your hands probably every day, your cell phone. Specifically, we're going to look at the types of decisions that it has to make in order to decide what power that it should be transmitting at any given time. As we will see, the principle that sharing is hard, is very applicable. Have you ever stopped to think, how many cellphones are there, either in the world, or in a region, like a country? So, suppose we had six people? Would we have, on average, less cellphones than people? So, we have maybe four cellphones for every six people, meaning that every person has on average less than one cellphone. Would we have the same number, meaning that every person's going to have on average one cellphone? Or would we have more? Like, eight cellphones for every six people, meaning that every person has on average more than one cellphone. A decade or two ago there were many, many more people than there were cellphones, but the number of cellphones has risen very dramatically in that time. So, these numbers are becoming closer and closer to one another. At the end of 2012, there were about 7 billion people in the world, and there were about 6 billion mobile subscriptions. So, we're not actually looking at the number of manufactured devices, or the number of cellphones that have ever been made, but we're looking at those that are currently in use or currently subscribe to a mobile plan. And so, if we looked at the number of cellphones relative to the number of people, we would have six over seven. Meaning that for every six phones there are on an average for seven people. So, this is approaching one, meaning that in the world they're soon going to be on an average one cellphone per person. So, you may have guessed that number would have been around one, maybe even a little less than one. But what you probably didn't know is it's drastically different depends upon which country you are look at. So, in this graph over here, we're showing the penetration rate which is this number that we showed down here of taking the number of mobile subscriptions and dividing it by the number of people in the population. And this is as of December 2012. So, if you look at the countries on the right-hand side, over here, you see that the penetration rates for these countries are less than 100%. Or, it's just really just this decimal number in percentage form.. And what that means is that there are more peoples than cellphones, that might make sense because some of these countries have very large populations or a dense populations. But on the left hand side of this arrow, these countries have ratios that are over 100%, for instance, United States has recently gone up to 103%. And that keeps going up more and more which means that there are actually more cellphones then there are people. And if we come all the way over here to Russia that is at 160% which means that there is between one in two cellphones per person and it's closer to the two. So, there's closer to two cell phones per person than there is one cell phone per person in Russia. And just to sort of write this out, the penetration rate is equal to the number of mobile devices. Or the number of mobile subscriptions, to be exact, divided by the number of people in whatever region we're talking about. So, with all of the six billion cellphones there are in the world, how is it possible that we could all transmit on the same network medium? Meaning, the air, without interfering with each others conversations? Certainly, you've been next to someone before who's also been on their cell phone and has been talking to someone that you're not talking too, but you can't hear their conversation and you can't hear what's going on. So, how is it possible that we can all transmit at the same times and not cause a new problems with one another? Well, the modern cellular system, we can write the cellular word now ,is the results of decades of technological innovations in which engineers have experimented with different ways of allowing us to share this network medium. Or really share the air in this case, because the network medium is cellular is the air. And so, they came up with many different methods of allowing us to do that as efficiently as possible which is why you can't hear your friend's conversation at the same time as yours.
