So our next principle will be covered in the course finding material and it's perhaps the most relateable principle to your daily life is that the services that run on top of the network that we use every day are very expensive and we have to pay for them. And we'll look at why some of them are as expensive as they are. And we'll place a particular emphasis on cellular mole data applications, which will become very prevalent over the past decade or so. So in the U.S. if you have provider like at&t, or Verizon, up until about a few years ago you could obtain what's called an unlimited data plan. Basically what that meant was that if this is your cell phone, and this is the network, then at the beginning or at the end of the month, depending upon how you look at it, you pay some fixed rate to the network operator. Say, $30 per month. And then for the rest of the month. You could consume as much data as you wanted and it wouldn't cos thou anything additional, so if you have data hungry applications you'll start to consume more and more and more data, and that certainly leaves you very happy, but the problem is that as applications became more data intensive... For instance, video, where we require a lot more data for the things that we do, then that places increasing strain on the network [SOUND] since everyone is consuming so much data without having to pay anything additional. And when the network operator incurs congestion costs for this, as we call it. They have to make up that cost somehow. And that comes in the form of increased prices. So maybe rather than charging $30 per month, they might move up and say, well we'll charge $40 per month. Now you may still be okay with that. Because you're consuming so much data. And there's probably a lot more people who are also consuming a lot of data as well. But the problem is that people who aren't consuming as much are not going to be very happy in that they now have to pay $40.00 per month for their unlimited data plans. So we say that these people are subsidizing lifestyles of people on this side, so to speak. So, the way that that's really mitigated is, instead of this, as we call, flat rate pricing. We switched to usage based pricing, where if this is your cellphone and this is the network, every time you take some data you have to pay the corresponding cost. so, every piece of data you consume will be charged at the end of the month on your monthly bill. So, rather than paying a fixed amount per month, you pay a fixed amount per gigabyte that you consume. So, it's proportional to the amount that you actually consume. So, if you still want to consume a lot, that's okay, but you're going to have to pay the corresponding cost. And for people who don't want to consume as much, then they don't have to pay as much. And you probably think that that sounds more intuitive, but there's many reasons why that switch didn't happen until now, and we'll look at that as well. If you look at these data plans, like for instance Verizon as of Spring 2012, it's actually a combination of flat-rate and usage-based pricing. So, there's different tiers of plans. For instance, you can pay $30 for the first 2 gigabytes you consume as a flat-rate and then after that you have to pay $10 per gigabyte. Or you can pay, choose to pay $50 for the first five gigabytes you consume, and then after that you have to pay $10 per gigabyte. Or up here you can pay the most as a flat rate of $80 and you can pay that $80 up till ten gigabytes without having to incur the usage base cost and then do $10 per gigabyte after that. And where that switch happens to user space depends upon how much you're willing to pay for the initial flat rate. And this 10 dollars per gigabyte seems to be pretty common amongst carriers as we will see. Our next principle is network of networks. And this material will be covered In the course itself and what this is really saying is that the internet is a network that's composed of many different sub-networks and there's many ways to see why that's the case but perhaps the most intuitive way is just to look at the way that the internet is structured. So, each provider, which are called an Internet Service Provider or an ISP for short owns a different portion of the internet. And issued ISPs falls to the different tier. So, the tier 1 ISPs are the ones that have really large global footprints. And the tier 2s are one step below those, and tier 3s are the ones that may serve campus networks, like college networks, or other residential areas directly. Now there may be customers at any level or any tier but the ones that we may relate most directly to would be down, below the tier three's. And so really within any of these tiers there is a whole sub network that's going on and then the internet sort of glues together. All these different tiers, and so that's why we call it a network of networks. And so when we look at the internet, there's many different questions we have to ask in terms of how it works and all the functions that it performs. And we could actually spend an entire course doing nothing but looking at the internet. But we'll only have time to look at just the main ideas here. And the first one brings us back to our original principle of sharing, how sharing is hard. And so we're going to look at any portion of the Internet. Here I've shown just over a wire over cable. There's many different devices that are going to have to use that wire at different times, and the question is how can they all use the wire all the same time without causing interference or collisions? And the Internet has a different way of dealing with this than cellular or wifi networks, which we will look at in the first chapter. And so we'll see how the Internet goes about. Directing the traffic from the source to the destination. But how does it actually go about getting from one point to the other? And that's actually very similar to the idea of the postal system. And the way the postal system works with directing an envelope from the beginning, or from your house to wherever the recipient of the address is. So if you look at any envelope, you know that on the envelope it's always going to have some source and destination address, which is in the form of a house or a business address. And that has many different components of it like a zip code, or a specific house address, and 55 Daisy Avenue. Now on the internet every device is assigned what's called an IP Address. And we'll look at how the IP address is similar to A mailing address. And after we do that, we'll look at what happens when you, maybe sitting on your phone, wants to send a message over the internet to some computer that's sitting across the network. And how these intermittent devices, which are called routers, use the IP address of the destination, to determine, how to get, that message from you, to where it needs to go.
