# Physical Network Topologies

Just as a topographical map is a type of map that shows the shape of the terrain, the physical topology of a network is also a type of map. It defines the specific characteristics of a network, such as where all the workstations and other devices are located and the precise arrangement of all the physical media such as cables. On the other hand, the logical topologies we covered earlier delineate exactly how data moves through the network. Now, even though these two topologies are usually a lot alike, a particular network can actually have physical and logical topologies that are very different. Basically, what you want to remember is that a network’s physical topology gives you the lay of the land and the logical topology shows how a digital signal or data navigates through that layout.

Here’s a list of the topologies you’re most likely to run into these days:

## bus network topology

This type of topology is the most basic one of the bunch, and it really does sort of resemble a bus, but more like one that’s been in a wreck! Anyway, the bus network topology consists of two distinct and terminated ends, with each of its computers connecting to one unbroken cable running its entire length. Back in the day, we used to attach computers to that main cable with wire taps, but this didn’t work all that well so we began using drop cables in their place. If we were dealing with 10Base2 Ethernet, we would slip a “T” into the main cable anywhere we wanted to connect a device to it instead of using drop cables.

Even though all the computers on this kind of network see all the data flowing through the cable, only the one computer, which the data is specifically addressed to, actually gets the data. Some of the benefits of using a bus topology are that it’s easy to install and it’s not very expensive, partly because it doesn’t require as much cable as the other types of physical topologies. But it also has some drawbacks: For instance, it’s hard to troubleshoot, change, or move, and it really doesn’t offer much in the way of fault tolerance because everything is connected to that single cable. This means that any fault in the cable would basically bring the whole network down!

## Star network Topology

A star network topology’s computers are connected to a central point with their own individual cables or wireless connections. You’ll often fi nd that central spot inhabited by a device like a hub, a switch, or an access point.

Star topology offers a lot of advantages over bus topology, making it more widely used even though it obviously requires more physical media. One of its best features is that because each computer or network segment is connected to the central device individually, if the cable fails, it only brings down the machine or network segment related to the point of failure. This makes the network much more fault tolerant as well as a lot easier to troubleshoot. Another great thing about a star topology is that it’s a lot more scalable—all you have to do if you want to add to it is run a new cable and connect to the machine at the core of the star.

Although it is called a star topology, it also looks a lot like a bike wheel with spokes connecting to the hub in the middle of the wheel and extending outward to connect to the rim. And just as with that bike wheel, it’s the hub device at the center of a star topology network that can give you the most grief if something goes wrong with it. If that central hub happens to fail, down comes the whole network, so it’s a very good thing hubs don’t fail often!

Just as it is with pretty much everything, a star topology has its pros and cons. But the good news far outweighs the bad, which is why people often opt for star topology. And here’s a list of benefits you gain by going with it:

• New stations can be added or moved easily and quickly.
• A single cable failure won’t bring down the entire network.
• It’s relatively easy to troubleshoot.

And here are the disadvantages to using a star topology:

• The total installation cost can be higher because of the larger number of cables, even though prices are becoming more competitive.
• it has a single point of failure—the hub or other central device.

There are two more sophisticated implementations of a star topology. The first is called a point-to-point link, where you have not only the device in the center of the spoke acting as a hub but also the device on the other end, which extends the network.

## Ring network Topology

In the Ring network Topology , each computer is directly connected to other computers within the same network. in the above Looking at the above Figure, you can see that the network’s data flows from computer to computer back to the source, with the network’s primary cable forming a ring. The problem is, the ring topology has a lot in common with the bus topology because if you want to add to the network, you have no choice but to break the cable ring, which is likely to bring down the entire network!

This is one big reason that ring topology isn’t very popular—you just won’t run into it a lot as I did in the 1980s and early 1990s. It’s also pricey because you need several cables to connect each computer, it’s really hard to reconfigure, and as you’ve probably guessed, it’s not fault tolerant.

## Mesh network Topology

In this type of topology, you’ll find that there’s a path from every machine to every other one in the network. That’s a lot of connections—in fact, the mesh topology wins the prize for “most physical connections per device”! You won’t find it used in LANs very often, if ever, these days, but you will find a modified version of it known as a hybrid mesh used in a restrained manner on WANs, including the Internet.

Often, hybrid mesh topology networks will have quite a few connections between certain places to create redundancy (backup). And other types of topologies can sometimes be found in the mix, too, which is another reason it’s dubbed hybrid. Just remember that it isn’t a full-on mesh topology if there isn’t a connection between all devices in the network. And understand that it’s fairly complicated.

## Point-to-Point Topology

As its name implies, in a point-to-point topology you have a direct connection between two routers or switches, giving you one communication path. The routers in a point-to-point topology can be linked by a serial cable, making it a physical network, or if they’re located far apart and connected only via a circuit within a Frame Relay or MPLS network, it’s a logical network instead.

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