Network+ Certification Tutorial:

Bus Topology Pros And Cons

By Chris Bryant, CCIE #12933

Of the different topologies you need to know for the CompTIA Network+ certification exam, the bus topology is probably the rarest. Bus topologies have built-in problems that don't come close to making up for their perceived advantages. Before we take a look at some bus topology examples and diagrams, let's make sure we have the definition of a bus topology straight!

A bus topology is a shared transmission medium. There will be a single physical network segment connection multiple hosts. When one host transmits data, the other hosts must not do so or a collision will result, resulting in all data involved becoming unusable. All hosts on the bus will see data placed there by other hosts, but only the intended destination will take the data "off the wire" - that is, accept it. In the following diagram, if Host A sends data destined for Host E, all the hosts will see the data, but only Host E will actually receive it.

Since the physical bus has to end at some point, terminators are placed on each end of the bus to indicate this ending of the physical segment.

Bus topologies have one advantage over other topologies such as star and ring - bus topologies use less cable than the others. If your first reaction to that is "big deal", you're just about right. Bus topologies have several disadvantages that far outweigh the cable savings.

• As mentioned, only one host can transmit at a time.

• Bus topologies don't react well to more and more hosts being added. The most hosts we have on a bus, the more collisions we have. That results in more retransmissions, which results in more work for all the hosts on this segment. This lack of scalability is a major drawback in today's ever-expanding networks.

• Since a bus topology is crippled by a break in the segment, it's said to have a single point of failure. We don't want all our hosts to be put out of commission by a single network problem.

For instance, what if we have nine hosts on a physical bus....

... that suddenly breaks?

Then we have nine hosts that can't talk to each other. Which isn't good. Bus topologies have no fault tolerance - that is, if something goes wrong, every host on the topology is affected.

It's also said that bus topologies have one other advantage over other topologies in that they're easier to assemble than star and ring topologies. They may be easier to assemble, but easiest rarely means best in networking.