CS440 Computer Networks 1

Token RingToken Ring

Neil TangNeil Tang9/22/20089/22/2008

CS440 Computer Networks 2

OutlineOutline

Basic Architecture

Physical Properties

MAC Protocol

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Token RingToken Ring

Basic architecture: multiple hosts share a ring.

MAC protocol: token passing

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Basic ArchitectureBasic Architecture

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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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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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Frame Format Frame Format

Body CRCSrcaddr

Variable48

Destaddr

48 32

Enddelimiter

8

Framestatus

8

Framecontrol

8

Accesscontrol

8

Startdelimiter

8

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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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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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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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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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MAC ProtocolMAC Protocol

Token Release: early VS. delayed

Token

Fram

eToken Frame

(a) (b)

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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.