Token Ring Neil Tang 9/22/2008
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CS440 Computer Networks 1
Token RingToken Ring
Neil TangNeil Tang9/22/20089/22/2008
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CS440 Computer Networks 2
OutlineOutline
Basic Architecture
Physical Properties
MAC Protocol
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CS440 Computer Networks 3
Token RingToken Ring
Basic architecture: multiple hosts share a ring.
MAC protocol: token passing
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CS440 Computer Networks 4
Basic ArchitectureBasic Architecture
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CS440 Computer Networks 5
Basic ArchitectureBasic Architecture
Host
From previoushost
To nexthost
Relay
(a)
Host
Host Host
From previoushost
To nexthost
Relay
(b)
Host
Host
Host
Host
From previousMSAU
To nextMSAU
MSAU
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CS440 Computer Networks 6
Physical PropertiesPhysical 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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CS440 Computer Networks 7
Frame Format Frame Format
Body CRCSrcaddr
Variable48
Destaddr
48 32
Enddelimiter
8
Framestatus
8
Framecontrol
8
Accesscontrol
8
Startdelimiter
8
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CS440 Computer Networks 8
MAC AlgorithmMAC 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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CS440 Computer Networks 9
MAC AlgorithmMAC 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 vol
ume 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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CS440 Computer Networks 10
MAC AlgorithmMAC 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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CS440 Computer Networks 11
MAC ProtocolMAC 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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CS440 Computer Networks 12
MAC ProtocolMAC Protocol
Token Release: early VS. delayed
Token
Fram
eToken Frame
(a) (b)
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CS440 Computer Networks 13
MaintenanceMaintenance
A Monitor Station
Monitor the token: After a period of (NumSationTHT+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.