Network Topologies Table of Contents Network Design and Architectures ................................................................................................. 2 Network Topology........................................................................................................................... 3 Bus ................................................................................................................................................... 7 Ring ................................................................................................................................................. 9 Bus ................................................................................................................................................. 11 Ring ............................................................................................................................................... 12 Dual-Ring ....................................................................................................................................... 13 Star ................................................................................................................................................ 14 Extended Star ................................................................................................................................ 16 Mesh ............................................................................................................................................. 18 Wireless ......................................................................................................................................... 20 Notices .......................................................................................................................................... 22 Page 1 of 22 Network Design and Architectures Network Design and Architectures 4 **004 So first we'll start with network design and architecture. This is an availability concept: How do we get these things in place, and what is reasonable? When you're attacking availability, what we usually talk about in the case of network design and architectures is, is the physical cable plant or devices protected? So you have to think about the tools that you would use at the physical layer to protect all of this cabling. Don't think that you can shove some wires up into the ceiling and everything will be okay. Don't think that you can put one lock on the Page 2 of 22 server room door and everything will be okay. You're going to have to apply defense-in-depth in order to protect these assets. It's not just one control. Network Topology Network Topology Network topology defines a network’s layout • Affects security, scalability, resiliency to failure, and complexity Many different kinds of topologies exist, including hybrids Usually defined by physical parameters • Distance between buildings • Operating environment • Cost of cabling 5 **005 So let's start with topology. This is the network's layout. And we have to separate the physical layout from the logical layout. And also, by the way, in certain instances, the physical layout looks like one thing, but is entirely different. There are limitations to our physical wiring plant and the distance from one node to another or from one hub Page 3 of 22 or switch or router to another, and that's based upon basic physics. Sound can only travel down-- I mean, light can only travel down a particular piece of fiber before there's some sort of degradation of signal. Not because light has these properties, but because our ability to manufacture accurately-- well, it's not perfect. So light's perfect, but we're not. When we talk about copper cabling, when we're sending an electrical signal down the line, at this point we do have some physical limitations based on the concept of resistance, and we don't want to mess around with too much resistance, so what we do is we say, "Okay, we're only going to push across this piece of copper for this distance at this resistance," and then after that what we'll say is, "That's too long. Start over again. Add something into the mix." Which is nice, because we've got plenty of networking technologies that will actually help us to get further distances. We'll talk about the network technologies after the network topology. So we have to know what the distance between our buildings is. That means that we work with our physical plant people. When we're going from building to building, we don't want to do things like hang the piece of copper out the window of one building and shoot it over to the other building, and leave that arc there-- which is also called a circuit-- for lightning to come by and strike. Page 4 of 22 About 15 years ago, when I was working with a car dealership, they decided to put up a second building. And that's exactly what they did. They wanted to make the service department be able to communicate with the showroom floor department, and they actually ran a piece of conduit between two buildings. The buildings were three-story buildings, so they said, "Oh, well we won't have any problem with trucks rolling through here." It was three stories, and then they literally had a piece of conduit going from one building to another. I said, "You can't do it. You're going to be susceptible to lightning strikes." They go, "It's such a little tiny bit of copper." I said, "No, but you've turned this into a giant circuit, so now both of those buildings now become places to actually zap and then run between them." And what would happen-- and what happened to them-- was they put up these piece of conduit, and sure enough they got lightning strikes, and it would knock out their network over and over and over again. By the way, when you do lightning strikes, on that you could also damage the equipment. So we've got to think about where this is going to be run. Now, most of us don't have that problem. Most of us run within the physical context of a building and the building designers are smart enough to give us removable ceiling tiles and all the other good stuff that goes along with that. Page 5 of 22 When you're dealing with cable installation, know the concepts, but know the people that you hire to actually do the physical wiring in the cable plant. They will know these constraints for you. So consult them before you set this stuff up. If you're inheriting somebody else's stuff, you need to make sure that the copper run is actually accurate and the correct length, and you should get somebody to come in and test that. You can use all sorts of testing tools. One of the things that we use is a time domain reflectometer. That's a fancy way of saying copper-- you can send a signal down the copper and it will tell you exactly how long it is because of the repeat back. Page 6 of 22 Bus Bus Logical layout Physical layout is usually a star Considerations • One node transmits, all receive (broadcast) • Only a single node can transmit at a time • Cable failure breaks the entire network • Addressed by link-layer encryption or different topology 6 **006 So first let's start with the simplest. And I used to, about five or six years ago, say, "Well, nobody uses a bus anymore-- a straight line layout. But what happens is you find out that the physical layout is usually a star, which means it goes from me to you-- that's still one continuous line-- and then it goes from me to you-- and that's the line again-- and from me to you-- so that's one piece of zigzagged infrastructure that comes back to a central location. Well, why would it come back to me? I don't know what the answer is, but it may be that it can be physically laid out as a star. Page 7 of 22 Where you see the bus topology still in play today in a lot of locations is with your local cable companies. They run that one big piece of copper all the way down the telephone poles, and then hang a node off the end. Your cable box that sits inside of your house is one of these nodes that we see here. Now, we go back in history before that, and we talk about thin net and thick net. We actually used coax cable of different ohm ratings that would actually set up just like this for our computers. So, the end point for your house could theoretically be a computer. And in some cases, for telecom, it actually is, when it does things like splitting off the phone, and if you've got Voice Over IP. But that's still a bus architecture. It's one node that transmits, and all receive. Now, the cable companies have put things in place to protect themselves from the fact that neighbors want to snoop on neighbors. That's an entirely different setup that's not our normal network that we pay attention to. But the problem is-- and the good thing is-- is that when one node transmits on a bus, all the other nodes hear all of the traffic. Makes it easy to communicate. But if we pile too many nodes onto that network and everybody's listening to everybody, what'll happen then? We'll get contention. We'll get packets crashing into each other. And therefore we want to limit the number of nodes on a particular bus when we're doing it from a networking standpoint. Page 8 of 22 Now, when we try to do encryption here, we don't do it at the physical layer, we do it at a layer up, which will be discussed in the protocols. Ring Ring Same physical and logical layout Commonly implemented as Token Ring network Considerations • One node transmits, all receive (broadcast) • Each node transmits in turn • Failure in a node affects the entire ring 7 **007 What about a ring? I won't say, "Token Ring is dead." I won't say that anymore. But what I'll say is it's less used today. The great thing about ring topologies is that they don't have to actually be physically laid out like a ring, but they can be. In older technologies, instead of a hub for a ring, what they use is Page 9 of 22 something called a MAU, where the wire is actually run back to the centralized location, and then connected through a series of loops. The beautiful thing about ring is, is that each node on that particular network retransmits the signal, and so they're doing kind of double duty at that point.