Network topology

Network topology refers to the way computers are connected. A topology is the configuration in which the network is laid out, specifically how the computers are connected to one another and to a central point. There are four main network topology configurations and a number of variations of each. Larger networks may include several different topologies; these are often called hybrids. Network topologies make it possible for computers to communicate with one another, whether that communication takes place across a room or across the world.

Background

Electronic computers were first available in the 1940s. Computer languages and computer chips followed in the 1950s, and the first personal computers became available in the 1970s. In the earliest days, the only way to transfer information from one computer to another was to put it on a storage device such as a tape drive or floppy disk, which was then physically carried and inserted into another computer. A Xerox employee named Robert Taylor is considered the earliest pioneer of computer networking. Taylor is also credited with inventing Ethernet, or a form of local area network (LAN). Computer networks provide a way to share information between computers.

Computer networks include computers, cabling, a network hub, a network switch, or a network router that can serve as a central control point for the information in the system, and peripherals such as printers and scanners. These components are laid out according to one of four computer topologies, or a variation of one of these. The word topology comes from the Greek topos, which means “place,” and logos, which means “study.”

Overview

Computer topologies generally fit into one of four major categories: bus, star, ring, or mesh. The characteristics of each determine whether it will be appropriate for use in a particular setting. When a particular application requires two or more topologies, a hybrid of multiple topologies might be used.

The bus topology was commonly used in early coaxial Ethernet systems; many modern Ethernet LANs use a star or extended-star layout with switches. Ethernet is used to set up the architecture of a majority of LAN systems; therefore, the bus topology has become very common. In a bus system, sometimes called a linear bus topology, all the computers and peripherals are connected to a single cable, which serves as a sort of spinal cord for the system. A typical system will include a file server to back up the systems, some form of printer or other peripheral device, and a number of individual computers. Each end of the cable is finished with a device called a terminator, which prevents signals from bouncing back to the connected computers.

Besides the fact that it is part of Ethernet systems, there are several advantages to bus topologies. Connecting each computer directly to the main line means that these networks use less cable than some other topologies, making them less expensive to set up. It is also relatively easy to add a computer to the network.

The same single cable that makes the bus system easier and less expensive to set up is also one of its weaknesses. If the cable fails, all computers on the system will become inoperable. The single cable also makes it difficult to identify exactly where a problem is if the system does fail. The two terminators that the bus system requires also add to the cost of setting it up. Finally, the linear configuration means that this system is not a good solution for a large building; it can only work effectively with a few dozen computers.

In contrast to the linear bus system, with each component connected to one line, a star topology features an individual cable that goes from each computer or peripheral directly to a central component. This central point, called a hub node, controls the flow of data through the system and can be a network hub, a switch, or a router; the way each computer radiates out from this central point gives the system the name “star.” Information shared between computers goes through this hub node, whether it is traveling to another computer within the network or off to another computer through the Internet.

This central hub makes it easy to control security on the data on a star topology system. The layout makes it easy to add components up to the limits of the central hub node. While the failure of the hub will result in an outage of all the computers on the system, the failure of a single computer or other component will not affect the rest of the system and is easy to detect and repair. However, the additional cable used to connect each system and the cost of the hub add to the cost of installation for the system.

The star topology is also limited in size depending on how many components can be added to the central hub. However, it is possible to get around this by using one of the input areas on the hub to add a line to another hub that can be the central point for additional computers. This configuration is called an expanded star or a tree topology.

In a ring topology, each computer on the system is paired with two others. All information shared from one of these computers travels around the ring—this can be clockwise or counterclockwise—to the other computers. A special device known as a Token Ring (IEEE 802.5) is often used to connect the components in this topology, which is commonly used in offices and schools. However, failure of one computer or the cable connecting them will cause all the computers in the ring to fail as well.

Some networks are built with routes, or multiple paths, that data can take in traveling from computer to computer. Systems like this, where each computer is connected directly or indirectly to all the others, are called mesh topologies. The Internet is a global network of networks, and many parts of it use multiple paths for routing.

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