1. Token Ring Neil Tang 9/22/2008
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2. Outline Basic Architecture Physical Properties MAC Protocol
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3. Token Ring Basic architecture: multiple hosts share a ring.
MAC protocol: token passing
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4. Basic Architecture
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5. Basic Architecture
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6. Physical Properties
Data flow: It goes along a particular direction around the ring, with each node receiving frames from its upstream neighbor and then forwarding them to its downstream neighbor.
Encoding: Manchester
Error detection: CRC
Max number of hosts: 250
Data rate: 4Mbps/16Mbps
Cables: Twisted pair cable
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7. Frame Format CS440 Computer Networks Body CRC Src addr Variable 48
Dest 32
End
delimiter 8
Frame
status
control
Access
Start
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8. MAC Algorithm Basic Idea: Token Passing
A token (a special bit sequence) circulates around the ring.
Every node forwards it.
A node with data to send takes the token off the ring and inserts its frame into the ring.
Each node along the way forwards the frame.
The destination node save a copy and continue to forward it.
The sender removes the frame when it gets back.
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9. MAC Algorithm Token Holding Time (THT):
THT: The maximum duration within which a node is allowed to hold the token.
Default value: 10ms
Tradeoff between throughput and fairness: allow one with a large volume of data to send as mush as it could VS. give everyone a chance to send.
Token Rotation Time (TRT): TRT = i (Actual THT of Node i) + RingLatency (ring propagation delay)
TRT  NumNodes  THT + RingLatency
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10. MAC Algorithm Priority:
Frames and token have priorities. Only the frame with priority higher than or equal to that of the token can be transmitted.
The token’s priority changes over time. When a station X having frames with a higher priority n to send sees a frame with lower priority m passing by, X will set the frame’s reservation bits to n. When the station currently has the token gets the frame back, it elevates the priority of the token to n and release it. After X transmits its frames, it changes the priority of the token back to m and releases it.
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11. MAC Protocol Reliable Transmission:
2 bits in “Frame Status”: A bit and C bit.
Initial value: A=C=0.
The destination node: sets A=1 when receiving the frame and sets C=1 after copying it to its adaptor.
The sender: If A=0, the destination is down. If A=1 but C=0, the destination cannot accept the frame (error detected, lack of buffer), retransmit it later.
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12. MAC Protocol
Token Release: early VS. delayed
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13. Maintenance A Monitor Station
Monitor the token: After a period of (NumSationTHT+RingLatency), if no token comes back, it creates a new token.
Monitor the corrupted or orphan (dead sender) frames: It sets “monitor” bit to 1 at the first time the frame passes and get rid of it at the second time.
A monitor station will periodically announce its presence by a special control frame. If a station fails to see it for a certain period of time, it transmits a “claim token” frame to announce its intent to become a new monitor. Tie can be broken by “highest address wins” rule if multiple stations compete for this at the same time.
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