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802.5 - Token Ring

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• Regulating traffic on a shared medium
  • Another way to share a broadcast medium:

    regulated transmissions... The use (= transmission) of the LAN is regulated by well-defined rules. Because nodes do not content (compete) for access, this type protocol are called "contention-free".

  • Right to use (= transmit):

    The right to use is conveyed in a special messages from one node to another The right is passed in a round robin manner (for fairness reasons). The special message is called a token

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• Types of contention-free MAC protocols
  • There are 2 types of contention-free MAC protocols:

    Token ring (IEEE 802.5) Token bus (IEEE 802.4)

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• IEEE802.5 - Token ring architecture
  • Architecture:

    Computers are organized into a physical ring: Messages are circulated through the ring network:

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• The IEEE 802.5 Token Ring protocol

  • Preliminaries:

    The "token" is nothing more than a bit in the message header When this special bit is set (= 1), the message is a token. The node that has received a token message is called the token holder The token holder is permitted to transmit only one message A transmitted messages will circulated through the entire ring and returned back to the sender

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  • Reliability protocol in IEEE 802.5 token ring:

    The header of the message also has an ACK bit Initially, the ACK bit is reset (0) When the intended destination received the message, it sets (= 1) the ACK bit When the message returns to the sender, it can detect whether the message was received

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  • The Token ring protocol:

    Graphically: When a node has a messages to send, it waits for the token. The node removes the token from the ring - this way, other nodes in the network will not start a transmission. (Easy: the node simply do not forward the token further) When the node receives the token, it transmits the messages. The nodes waits for the message to return back !! After the messages returns, the node "issues" a new token on the ring.

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• Strength and Weakness of the Token ring protocol
  • Weaknesses:

    Poor performance in lightly loaded situation: When most nodes are idle, the token will circulate by many nodes "in vain" Addition and deletion of nodes (computers) from the token ring is very labor intensive Cable breakage will cause the entire network to fail... (Because the ring is severed....) NOTE: Locating the section of the cable that had the disruption is not easy...

  • Strengths:

    Good performance in highly loaded situation Built-in ACK scheme (Since the packet has to come back to the sender before the sender can release the token, the message frame might as well take on the role of an ACK frame...)

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• The Star-Ring configuration
  • The star-shaped ring topology was proposed to overcome the cable failure problem:

    The actual shape of the hub or concentrator is a box:

    The connection forms a physical ring !!!

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  • Suppose a section of the Token Ring was disrupted:

    Locating the cable disruption is now relatively easy:

    Just find the 2 end points that does not for a closed loop:

    Fixing the token ring is also relatively easy:

    Omit the disconnected segment:

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