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Module 3: Stay on Top of Your Topologies

4. 3.4 Bus Topology

A bus topology was one of the early topologies used in the Ethernet world, where we had a coaxial cable, and that coaxial cable could run from room to room to room.That's actually the first type of network that I worked on, and the network I was on was a 10-base-T network. There was also a 10-base-five network that had a thicker coaxial cable. But with a bus topology, all of the network devices tapped in to this single coaxial cable that was the bus. And on an Ethernet bus, we can only have one packet at any one time. That means that we cannot have two devices that are transmitting at the same time. How do we avoid that? Well, Ethernet uses something called the CSMA/CD carrier. because of multiple access and collision detection What's going to happen is before a device communicates on the network, it's going to listen for a brief period of time to see if anybody else is talking on the network. And if they're not—if the coast is clear, in other words—then they'll transmit. The challenge comes up when two different devices are listening to the same period of silence. They both simultaneously conclude that nobody's talking, so it must be safe to talk. And when that happens, let's say that the two bottom devices on screen listen to the same period of silence. They simultaneously transmit. What happens? Well, those packets collide, and that collision is detected on a bus by a spike in the voltage that the computer's network interface card consents to, and it knows something bad happened in my packet; I've got to retransmit. But what's to keep it from happening again? Well, what happens with Csmacd is that the PCs that transmitted and had a collision are going to set a random back-off timer. And by adding that element of randomness, hopefully they're now going to transmit at different times. Let's say the PC on the bottom left selected a random back-off timer of ten milliseconds, while the bottom PC on the right selected a random back-off timer of 20 milliseconds. That means that the bottom left PC transmits before the bottom right PC transmits. And the result is, hopefully, no collision this time. Now this is the way that Ethernet began, with a bus tapping into a queue cable. Later, Ethernet adopted a star topology where there was a hub in the middle. And that brings up the topic of a physical topology versus a logical topology. For example, here we see how an Ethernet hub logically operates. Even though everything is plugged into a centralised hub, logically it still acts like an Ethernet bus. However, physically, yeah, it looks like a star. We have an Ethernet hub in the middle, and all our devices start out from that centralised Ethernet hub. And that's a look at bus topology.

5. (N10-007 ONLY) 3.5 Point-to-Point Topology

A point-to-point topology is a topology that we might see in a wide-area network or WAN where we're interconnecting geographically separated sites. Consider R1 and R2 on screen. Let's imagine that they're at different sites in our company, E, and if we have a direct connection between them, that's a point-to-point connection. As the name suggests, we're just going from one point to another point. We only have two devices on this wire, and I'd like you to know some characteristics of a point-to-point topology. First, as we've already mentioned, it interconnects two and only two devices, and we're probably going to be running some sort of protocol on that link, specifically a layer two or a data link protocol. A really common one is the PPP protocol. That's the point-to-point protocol. And as we see here, we could have a physical point-to-point connection. Perhaps we have a digital circuit, such as an AT or an E, connecting those routers. Or it could logically be a point-to-point connection. How is that possible? Well, imagine you've got two sites that communicate with one another through the Internet. You've got a massive internet cloud in the middle that's routing through who knows how many routers. But what you can do is logically form a point-to-point connection. You can set up what's called a VPN, a virtual private network, which gives you a logical tunnel connecting your two routers at your two sites, making it look like they are directly connected to one another. But physically, yeah, they're going through the Internet, going through lots and lots of routers. And that's a look at a point-to-point topology.

6. (N10-007 ONLY) 3.6 Point-to-Multipoint Topology

A point-to-multipoint topology, as the name suggests, is going to go from one point to more than one point. In this example, we'll look at a wide area network with location A reaching out to locations B, but there is only one physical connection, or point, between router R1 and the service provider. But beyond that single connection, we can have some logical connections. In this example, we're using an older frame relay network as an example. And with frame relay, we can create virtual circuits. And those virtual circuits are identified by something called a Delci, a DLCI, a datalink connection identifier, and that can keep packets destined for location B separate from packets destined for location C. And again, this is a weighing example. This is not the only time we see a point-to-multipoint topology. You might also see this in wireless networks where you have one wireless antenna on an access point reaching multiple other wireless access points. And as one other example of a point-to-multipoint topology, you might have an ATM cloud, an asynchronous transfer mode cloud like frame relay clouds, and an ATM cloud can have virtual circuits going through their cloud where a single connection, a point, can contain virtual circuits going to multiple points. Hence the name "point to multipoint you."

7. (N10-007 ONLY) 3.7 Hybrid Topology

A hybrid network topology is a collection of more than one topology type. For example, here we have a point-to-point connection between locations D and A. But notice that location A is going out through a single physical connection to locations B and C. That means the location A is going out to the service provider's frame of a cloud. That's a point-to-multi-point topology. But again, from A to B, that's a point-to-point topology. And this is just one example. You can have hybrid topologies in a local area network as well. For example, consider a data center. In a data center, redundancy is a very important thing, and you might have a bunch of switches that are fully meshed together for performance reasons or for redundancy reasons. So that's a full mesh topology. But you might have a server connecting to one of those switches with a single cable. That would be an example of a point-to-point topology. That's it. A hybrid topology. Anytime you're looking at a network and you can identify more than one topology type making up the overall network,

8. 3.8 Client-Server Network

One reason we might want to interconnect computing devices together in a network is so that they can share resources. And the traditional way of doing this was via a client-server network. Here we have a server, and we've got clients PCs 1, 2, and 3 that want to use resources on that server. For example, PC One might want to access the hard drive on the server. PC Three might want to print to that printer connected to the server. And PC Two may also want to connect to that hard drive. And here are some characteristics of a client-server network I'd like you to know about. Another name you might hear for a client-server network You might hear it called a client-server architecture. And here you'll notice all of our clients' PCs One, Two, and Three. They're accessing a common server. It's the server that has the resources. And the server is going to share those resources, such as printer resources and file resources, with the clients. And there are different operating systems out there that you might be running on a server. For example, you might be running a version of Microsoft Windows Server. And it can provide, for example, file services, print services, and other services that are available as well. Those are just a couple of examples. And we can have servers running Linux. There's a Mac OS Server version that you might put on an Apple Mac computer. And this client-server network technology has been around for a while. A really popular option back in the 90s was Noel's Network, and Microsoft came out with Microsoft Land Manager. That was the predecessor to Microsoft Windows NT, which acted as a server, and then later Microsoft Windows Server. When I used to work at a university, we had a bunch of UNIX-based servers. But I want you to know that a client-served server network is where we have clients accessing resources made available by a server that is serving up those resources.

9. 3.9 Peer-to-Peer Network

When hard drive space and quality printers were not as widely available as they are today, it was common to have a high-powered server that would serve up those printer and hard drive resources. However, as hard drives and printers became more available, we started to see more and more peer-to-peer networks. An early example of this was Microsoft Windows Three one one.It was called Windows for Workgroups. And in that version and in many operating systems since then, we've been able to have our clients, our PCs, serve up their resources. Let's say that PC Four has a collection of files that everyone in the office needs. We don't necessarily have to put that on a server. We could just let PC Four share those out with PCs 1, 2, and 3. We might have a high-quality printer attached to PC One. There's no need to buy printers for the other PCs. They could all just share that high-quality printer over the network that high quality printer.And that's an example of a peer-to-peer network. Now, here are a few characteristics I'd like you to know about peer-to-peer networks. You might hear it called a peer-to-peer architecture. And in this architecture, we've got clients that can share their own resources. We're not necessarily relying on a server. We still might have a server in the mix, but we're not dependent on that server. However, a peer-to-peer network is flexible, but it's not going to be as powerful as using a network operating system or an OS such as Microsoft Windows Server. That's going to have a lot of features and capabilities that we would not have with a peer-to-peer network. But a peer-to-peer network is a highly flexible, easy-to-install resource sharing solution.

10. 3.10 Local Area Network (LAN)

An L-A-N is a local area network. That's a network that's located in a local location. In other words, it's not widespread like a wide-area network. Perhaps it's in an office in a building. It could be on a floor of a building, or it could be on a single plot of land. But the devices are grouped fairly locally. You might have routers and switches and different devices connected, but a couple of characteristics I want you to know about a local area network Number one, it's a fairly high speed. You're typically connecting via Ethernet, which is likely to be a gigabit Ethernet connection or a ten gig Ethernet connection. And again, another characteristic that distinguishes the land from other types of network topologies It's centrally located, probably not spanning more than one building.

11. 3.11 Wide Area Network (WAN)

A wide area network, or WAN, is going to interconnect geographically dispersed sites. For example, let's say that we've got a headquarters location (HQ) and a branch office (BR one). Well, if we want to communicate between those two different offices, we could do so. So, over a Wan connection, you might see a couple of different symbols representing a Wan connection. You might see what looks a bit like a lightning bolt. And if it's a solid lightning bolt, it means the connection is always active. However, if it's dashed, that means it's an on-demand circuit. It's going to be brought up as needed. For example, back in the days of the old dial-up modems, that was an example of an on-demand circuit. You wanted to call into America Online or something like that. You would dial up the American online number, and that connection would be brought up on demand. That could be represented by the dashed lightning bolt. And traditionally, with a Wan network, the speeds, or in other words, the throughput, are less than what we would experience on a local area network, within an office, on a local area network. Today, we have gigabit, ten gigabit, and Wan speeds are typically lower than that. However, that is rapidly changing. When I first got into networking, the first WAN circuit I worked on was a 56 kilowatt per second WAN circuit. It was a dedicated AT line, but it only ran at 56,000 bits per second. Later we upgraded to a T; one circuit at the university where I worked ran at 1.54 megabits per second. still not very quick. We later upgraded to a ten-meg circuit, and things just kept growing from there. But today it's easier to get a higher-speed wing connection. I mean, even coming into my home, I'vegot gig fibre from at and T, andI've got gigabit speeds upload and download. And the connection from a site to another site is going to go through a service provider. So if you have a leased line, you're paying one or more service providers to interconnect your two sites. Something that we're starting to see more of today as a replacement for a land line, though, is a VPN, or virtual private network. With that, we can actually go through the Internet. Now, the internet is considered to be an untrusted network. There are security concerns with sending sensitive corporate data over the Internet. However, if we use a VPN, that can encrypt traffic such that if anybody were to eavesdrop on it, they wouldn't be able to read it because it's all scrambled up. But these are the characteristics I want you to know about a wide-area network. Traditionally, the speeds have been slower than on land, although that's changing. They interconnect geographically dispersed sites. So we're not talking about offices within a building. We're not talking about buildings within a campus. And we're not talking about locations around the city. We're talking about greater geographical dispersion than that between states or countries, as a couple of examples. And each side is going to connect to a service provider. And if we're using a virtual private network, that service provider might be your Internet service provider.

12. 3.12 Metropolitan Area Network (MAN)

A metropolitan area network or a man is going to interconnect sites in a metropolitan area. For example, let's imagine that in Chicago we have a company with offices located in the John Hancock Center, the Willis Tower, the Chicago Board of Trade, and the Tribune Tower. And we want to have high-speed connectivity between those different sites. Well, we could get a leased line, perhaps, and pay a lot of money for that, or we might have the opportunity to tap into a metropolitan area network. Let's imagine that there is a service provider in Chicago that has connectivity to all these different sites. And logically, this works as a ring topology where we send traffic from one location to the next, to the next to the next, and so on. There are a few characteristics I want you to know, though, about a metropolitan area network. It is somewhat limited in availability because it's only available in large metropolitan areas. However, if your company is fortunate enough to have multiple locations that can connect to the Internet, you can typically enjoy very high speeds. Common metropolitan area network speeds are on the order of ten gigabits per second or 40 gigabits per second, and that's growing from there. And the connectivity between sites and a man is going to be accomplished using fibre optic cabling. And if this service provider supports many different customers, you might wonder, how do we keep one customer's packets separate from another customer's package? It seems like they're using the same cable, the same fiber-optic cable. Well, within fibre optics, we can use different wavelengths of light to separate our customers. Here's a term I want you to know. It's a lambda. In your physics class in college, you might remember the term lambda referring to a wavelength of light. It's the Greek symbol of lambda. That is the term commonly used in metropolitan area networks. We say that different customers have their own lambda; they've got their own wavelength; or, think of it metaphorically, they have their own colour of light. So the customer using a red light is going to be separated from the customer using a blue light. And another benefit we get from this metropolitan area network is redundancy, because logically we're interconnected using this ring topology. So imagine we had a failure in this logical ring between the Tribune Tower and the John Hancock Center. Do we still have connectivity from every site to every other site? Absolutely. We can still get from anywhere to anywhere after we have that single failure in the ring because of the redundant topology of a metropolitan area network. You.

13. 3.13 Campus Area Network (CAN)

A network that interconnects buildings within a campus environment is called a campus area network. And oftentimes, when we use the word campus, we think of a university campus. However, a campus can be an office campus as well. For example, I went out to Cisco's headquarters in San Jose, California, and they have a campus containing dozens and dozens of buildings. It's bigger than any university I've ever seen. And they can have a campuswide network interconnecting those nearby buildings. And this is often done through conduit systems laid underground, where we could have fibre optic cabling running between buildings. We might run back to a central site, like we have pictured here on screen. And the speed can be very high because we're typically using fiber. We might be running at ten gigabits per second, 40 gigabits per second, or even higher than that. But again, this is going to interconnect nearby buildings. We're not connecting buildings scattered across a metropolitan area. This is within a campus. We're talking about nearby buildings. And you might look at this and say, Well, that centralised device seems like a single point of failure. If it goes down, then the building can no longer talk to anyone else. That's true. However, in this campus area network through the conduit system, we can run additional fiber, and it's very easy and recommended to add redundancy. So with a few redundant links, we can tolerate the failure of one or sometimes more than one link. Let's say that that link between Building A and Building B goes down. Can we still get from Building A to Building B? Absolutely. We, in fact, have multiple paths to get there. In this case, we went through that centralised device. But those are some characteristics I want you to know about a campus area network. It's typically running over fibre at very high speeds, on the order of ten or 40 gigabits per second or higher. It's going to interconnect nearby buildings within a campus, not just university campuses but also office campuses. and we can readily add redundancy. So we don't have that single point of failure.

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