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Initially, I thought that the reason token ring uses a physical star topology is to make up for the shortcomings of a physical ring. That is: In a ring, when a failure occurs at any point, the system goes down.

However, I have read the following about daisy chains in Wikipedia (emphasis by me):

By connecting the computers at each end, a ring topology can be formed. An advantage of the ring is that the number of transmitters and receivers can be cut in half, since a message will eventually loop all of the way around. When a node sends a message, the message is processed by each computer in the ring. If the ring breaks at a particular link then the transmission can be sent via the reverse path thereby ensuring that all nodes are always connected in the case of a single failure.

So it says that a message can be transmitted in both directions, which ensures the system not to go down in case of a failure at a point.

My question is, if such thing is possible, why this isn't used in token ring as well? If it was used in token ring, then token ring wouldn't need a physical star topology. It could be a physical ring as well.

So is there an error in this Wikipedia article or am I missing something?

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Token ring may appear to be a physical star, but it really is a ring from the perspective of the cabling. A physical token ring cable has an outbound cable, and an inbound cable inside it. Breaking one of those internal cables will break the ring, and the MAU will bypass the cable, thus preserving the ring.

The token ring star can be thought of as a circle (ring) with various points pushed tightly toward the center of the circle. It is still a physical ring, just distorted:

enter image description here

The token travels around the ring, out to a workstation on the internal outbound cable leg, and back in on the inbound cable leg. If the workstation or one of the cable legs fails, the MAU heals the break, and the faulty link is isolated from the ring, preserving the ring.

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  • Thanks for the image edit. I was confused and on my way to writing a comment for clarification :) – Utku Nov 8 '15 at 17:07
  • I grabbed the wrong image. I have multiple versions. – Ron Maupin Nov 8 '15 at 17:09
  • But how does the MAU bypasses the broken section? From this image, I don't see any way to bypass it. – Utku Nov 8 '15 at 17:12
  • The MAU has relays at each cable connection point. If the electrical connectivity of an attached cable is broken (broken leg, shutdown workstation, etc.), the relay trips and closes the the cable off from the ring. – Ron Maupin Nov 8 '15 at 17:15
  • @Utku, I found the picture showing the ring healing. – Ron Maupin Nov 8 '15 at 17:21

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