Physical Star, Logical Bus

To understand this difference, think of bus and star topologies. As we learned earlier in this lesson, signals travel exactly the same way in both of these. However, for reasons described in the next lesson, hardly anyone is wiring new bus networks. New broadcast networks almost always use the more flexible star topology. Despite this fact, a broadcast network is usually diagrammed as a bus topology.

Therefore, this physical topology:

is equivalent to this logical topology:

As you can see, the logical diagram does not show the hub that connects the group of computers. However, the drawing of the bus makes it easy to see that all of the devices directly share signals among themselves. Thus, when you see a bus diagram in this book or any other network text, remember that you are probably looking at a hub-centered star network.

Physical Star, Logical Ring

Ring topologies provide another good example of the difference between physical and logical diagrams. As we just learned, stations on a local area ring network are usually not connected directly to each other. Instead, they are all plugged to a central MAU. Thus, the physical diagram of a ring network looks almost identical to that of a broadcast network:

It is very easy to confuse these two networks. However, if we use logical diagrams, the difference between them is clear:

Thus, when you see a LAN represented as a ring, remember that the devices are probably all connected to a central MAU.

   Activities

See the Activities and Extended Activities section in Unit 2 Lesson 2 in your textbook Introduction to Networking to test what you have learned so far.

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