Star Topology

In a star topology, each network node is connected to a central device called a hub or a switch. Star topologies are also commonly used with LANs.

If a cable in a star network breaks, only the node connected to that cable is isolated from the network. The other nodes can continue to operate without interruption - unless, of course, the node that is isolated because of the break happens to be the file server.

You should be aware of the somewhat technical distinction between a hub and a switch. Simply put, a hub does not know anything about the computers that are connected to each of its ports. So when a computer connected to the hub sends a packet to a computer that is connected to another port, the hub sends a duplicate copy of the packet to all its ports. In contrast, a switch knows which computer is connected to each of its ports. As a result, when a switch receives a packet intended for a particular computer, it sends the packet only to the port that the recipient is connected to.

Strictly speaking, only networks that use switches have a true star topology. If the network uses a hub, the network topology has the physical appearance of a star, but is actually a bus. That is because when a hub is used, each computer on the network sees all the packets sent over the network, just like in a bus topology. In a true star topology, as when a switch is used, each computer sees only those packets that were sent specifically to it, as well as broadcast packets that were specifically sent to all computers on the network.

Expanding stars

Physicists tell us that the universe is expanding, and network administrators know they are right. A simple bus or star topology is suitable only for small networks, with a dozen or so computers, but small networks inevitably become large networks as more computers are added. For larger networks, it is common to create more complicated topologies that combine stars and buses.

For example, a bus can be used to connect several stars. In this case, two or more hubs or switches are connected to each other using a bus. Each of these hubs or switches is then the center of a star that connects two or more computers to the network. This type of arrangement is commonly used in buildings that have two or more distinct workgroups. The bus that connects the switches is sometimes called a backbone.

Another way to expand a star topology is to use a technique called daisychaining. When you use daisy-chaining, a hub or switch is connected to another hub or switch as if it were one of the nodes on the star. Then, this second hub or switch serves as the center of a second star.