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[This is a question I have realised has been bugging me since I had to learn about Token Rings, 30-odd years ago. I clearly have a wrong understanding here, but I never managed to figure it out.]

I understand the Token Ring cabling is logically ring shape, even if it is physically a star shape (e.g. see Why token ring uses a physical star?).

What I don't understand is whether the ring is a "bus" arrangement, where the wires are directly connected all the way around. (i.e. if I took a continuity tester to the incoming and outgoing wires connected to the node, they would be continuous.)

If I assume they are, like this answer suggests, and the token is just a way of preventing collisions over a shared resource, I am stuck with the question of when a token is passed to the left, how does the node know it is the one on the left?

If I assume they are not, and there is effectively N different unidirectional point-to-point connections, with all the received data not intended for a node being retransmitted to the next node, then collisions are impossible, so I don't understand why such an inefficient protocol was chosen. If Node 1 is transmitted to Node 2, the Node 3 doesn't need to wait for a token. It could send a packet to Node 4 immediately. If and when a packet comes from Node 2, it can give it priority as the next packet.

Are all the networking wires in a token ring physically connected together?

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  • [Anyone with rep wanna create a token-ring tag? There seem to be enough questions to warrant it.] Apr 11, 2023 at 7:02
  • 2
    Given NE's scope (see the help center), long obsolete Token Ring could be considered as largely off topic ("historical trivia"). I'm not sure if we should encourage more questions...
    – Zac67
    Apr 11, 2023 at 7:23
  • @Zac67: Ah, Fair. Sorry if this is off-topic. Apr 11, 2023 at 11:56
  • Odd, old, and rare... but sadly not completely gone. I won't name names, but I've seen banks and government networks that still have enclaves of TR. (sometimes they won't ["can't"] replace their mainframe systems/applications)
    – Ricky
    Apr 11, 2023 at 15:53
  • Technically, this could also be asked of fiber channel. FC hubs create a logical ring in the same manner as TR.
    – Ricky
    Apr 11, 2023 at 16:05

1 Answer 1

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That largely depends on your definition of "bus".

On the physical layer, Token Ring is a ring - each frame traverses the ring until it returns to the originator.

On the data link layer, it is a logical bus - in the sense that you send a frame out of your local interface, no matter where the destination is located (point-to-multipoint). Of course, the destination is identified by the frame's destination MAC address.

In contrast, Ethernet's physical layer initially used an electrical bus (shared wire for 10BASE5 and 10BASE2), then changed to a star/tree topology with multi-port repeaters/hubs (10BASE-T or 10BASE-FL), and then became point-to-point (fully switched Ethernet). Still, on the data link layer it continues as a logical bus - with the restriction that switches only forward frames to their indicated destination(s).

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  • While it would be before my time, if you travel far enough back into history, I'm sure there were MAU's that were literally relays creating an actual physical bus -- the same as the PSTN long long ago physically connected subscriber lines. But for many decades that's been done electronically with buffers ("store-and-forward") where a frame is fully received before being copied to the next port. (eg. today, there are no ethernet hubs; they're ALL switches.)
    – Ricky
    Apr 11, 2023 at 15:58
  • @Ricky Token Ring's scalability is severely limited by the need to pass back each frame to the originator. In theory, a switch could do that right away, but then it would ruin TR's delivery guarantee, the one feature that really set it apart from cheap Ethernet. Of course, delivery tracking is much more efficient at a higher layer - if at all.
    – Zac67
    Apr 11, 2023 at 17:09
  • TR put all of the work in the NIC, which is how a MAU could be a box of relays. There was a 100Mbps standard few ever used. And a 1000 that was only on paper. I've seen a fair bit of 16M TR, but by the time 100M ethernet was a reality, everything was already moving to ethernet, and many of the older TR systems were left to do their thing. Those are the mainframes still doing the same thing today. (clinging to the mantra: if it isn't broken, don't break it.)
    – Ricky
    Apr 11, 2023 at 19:11

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