Introduction to Data Communications
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45. Token Ring

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Token Ring is a token passing bus arbitration topology for the Physical and Data Link Layers. It is a logical ring and a physical star topology.

Token Ring uses a token passing scheme for bus arbitration. A special packet is passed around the ring called a token. When a node requires access to the ring, the node claims the token and then passes its information packet around the ring. All nodes read the destination address and if it is not addressed for them, the information packet is then passed on to the next node. When the destination node reads the packet, it marks it as read and passes it on to the next node. When the information packet completely circulates the ring and arrives back at the source node, the source node releases the token back on to the ring.

Token Rings are not usually drawn as the above drawing indicates: a separate line between each node. They are usually represented as understood that separate paths exist between nodes and are drawn as in the figure to the right.

45a. IBM Token Ring

Token Ring was originally developed by IBM for their PC LAN networks. It started out in 1969 as the Newhall Network, named after the originator of the token ring concept. IBM's Token Ring is the basis for the IEEE 802.5 standard Token Ring. They are very similar and have minor differences which we will cover.

Introduction to Data Communications
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