So, Packet Switching is, one of the fundamental concepts, behind the internet. Distributed Hierarchy, we're going to look at now, that's the second fundamental concept. And then the third fundamental concept is the idea of Layering, which we'll look at in the next section. That'll really be the focus of going in specifically on different layers of the Internet. so, the internet is really large, geographically, as you can tell. And it has not only just a large number of users, but it's also expanded to every portion of the world, basically. So different ISPs, as we said, own different portions of the Internet. And that's their commercial venture ISPs, profit seeking ISPs, we talked about them also in the lecture on pricing. We talked about different pricing models for mobile data, in that case. But each ISP owns a different portion of the internet. As this is how it looks. it really becomes a three-tiered structure, and this is how the hierarchy is built. The spatial hierarchy of the Internet, right, all the way down at the bottom. And notice how it's similar again to the NCF's structure where they go from campus to residential to to the highest tier which forms, as we said, the backbone of the Internet. So, at the bottom down, we have, all of our tier 3's. then, we have tier 2's in the middle and tier 1's up at the top. So, let's, let's look at each of these, and see kind of what they are. So, on top is the tier 1's. those are like AT&T and Verizon, level 3 communications, and you've heard of them. Those are all, tier 1 ISP's. Those are the big shots. they, they always have global footprints, so, they, they extend, you know, outside of different countries, and they, they can go all around the world. they formed, they have full mesh peering with one another. Basically means that any tier 1 ISP can get to any other tier 1 ISP in terms of sending traffic between them. so they're all connected, and they form the internet backbone, it's called. so, this is really what we, when we refer to the internet. Backbone are really talking about all of the huge ISPs connected together to one node, than rather at the core. And the, each of the, each of the tier 1's peer with one another, they have their peer relationships. Which means that they actually pass they pass traffic to one another without having to pay each. Right, so tier-1 ISPs don't have to pay, anyone to send their traffic anywhere. Now once we go down and we get onto the tier 2 ends, then tier 2's one step below tier one clearly. And it's hard, typically it's very hard to draw the line between 1 and tier 1 or tier 2, tier 3 ISP's, they're blurred a little bit. Just, these are just the main ideas that you would use to characterize one or the other, the main one with the tier one is that you're not paying anyone, right? So you don't have to pay any tier 1 ISP's, clearly not for your tier 2 ISP's, and you're part of that backbone. Tier 2 ISP is like Vodafone, Comcast, or different tier 2's. They may also peer with one another. They're not going to peer with the tier ones. They form what is called a customer-provider relationship with tier 1, so if you're at this tier two right here, you have some traffic and need to get to another tier two or a tier one, you have to send traffic upstream to a tier one. And then you have to pay the tier-1 for sending your traffic up to the tier-1. And so that's that, so in order to reach the internet backbone you automatically have to pay because you have to enter into mode tier-1. Now tier-3 is all the way down at the bottom, or below tier-2s. Those are like campus networks, like if you're on an academic campus, they'd like, corporations will have tier-3 ISPs. residential areas have tier-3 ISPs. And tier-3s are focused with taking traffic to and from from their customers, right. So, you may have, you have your customers in, either inside your tier-3 ISPs which you're sending your traffic to. And you're always going to be paying someone to do that. Right? So, you perform the customer-provider relationship with tier two ISPs, Tier threes pay money to tier twos, and tier twos pay money to tier ones in order to send the traffic. Right? So tier threes have to go all the way up the ladder to get to the backbone of the network. So, this idea, this is the idea of the distributive hierarchy, and this really should give the idea of a network of networks, which is really the main recurring theme here. The Internet is really a network composed of many different sub-networks, and in this case, we're saying the sub-networks are really different geographical ISPs.
