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Rivier College
CS575: Advanced LANs
Chapter 8: Token Ring LANs, FDDI and MANs
Chapter 8
Token Ring LANs, FDDI & MANs
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Part 1: IEEE 802.5 Token Ring Medium Access Control
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Chapter 8
MAC Protocol
MAC Frame
Token Ring Prority
Token Maintenance
Early Token Release
Dedicated Token Ring
IEEE 802.5 Physical Layer
Token Ring LANs, FDDI & MANs
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Scope of LAN Protocols
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Token Ring Operation
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IEEE 802.5 Frame Format
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IEEE 802.5 MAC Frame Control Bits
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Token Ring Priority
Eight levels of priority supported by TWO (3-bit) FIELDS in each data frame and token:
a priority field and a reservation field
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Pf = priority of frame to be transmitted by station
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Ps = service priority: priority of current token
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Pr = value of Ps as contained in the last token received by this station
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Rs = reservation value in current token
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Rr = highest reservation value in the frames received by this station during the last
token rotation
Two stacks are maintained by each station, one for reservation and one for priorities:
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Sx = stack used to store new values of token priority
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Sr = stack used to store old values of token priority
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Token Ring Priority Algorithm
ALGORITHM:
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A Station wishing to transmit must wait for a token with Ps < Pf .
2.
While waiting, a Station may reserve a future token at Pf . If a data frame
goes by, and if Rs < Pf , then the Station may set the reservation field of the
frame to its priority (Rs  Pf ). If a token frame goes by, and if (Rs < Pf
AND Pf < Ps), then the Station sets (Rs  Pf ). This has the effect of
preempting and lower-priority reservation.
3.
When a Station seized a token, it sets the token bit to 1 to start a data
frame, sets the reservation field of the data frame to 0, and leaves the
priority field unchanged (the same as that of the incoming token frame).
4.
Following transmission of one or more data frames, a Station issues a new
token with the priority and reservation fields set as indicated in Table 8.2.
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Table 8.2 Actions Performed by the Token Holder to
Implement the Priority Scheme [VALE92]
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IEEE 802.5 Token Ring Priority Scheme
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Dedicated Token Ring
0 New Access Control Technique: Dedicated Token Ring (DTR), 1998
0 A Ring can be configured in a star topology by use of a hub, or
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concentrator.
The token-passing algorithm can be used so that the ring capacity is
still shared and access control is determined by the token.
It is also possible to have the central hub function as a switch with
the connection between each station as a full-duplex point-to-point
link.
The DTR specification defines the use of stations and concentrators
in the switched mode.
The DTR concentrator acts as a frame-level relay rather than a bitlevel repeater: each link from concentrator to station is a dedicated
full-duplex link with immediate access possible (token is not used).
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Dedicated Token Ring Configuration
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IEEE 802.5 Physical Layer Medium Alternatives
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Part 2: FDDI & MANs
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FDDI Technology
MAC Frame
MAC Protocol
Capacity Allocation
FDDI Physical Layer Specification
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Fiber Distributed Data Interface (FDDI) Standards
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ANSI ASC X3T9.5 Standard Committee
ISO 9314 Standard Series
FDDI serves both LAN and MAN
FDDI – a token ring scheme, similar to the IEEE 802.5 specification
FDDI was designed to accommodate better the rate of 100 Mpbs
Some differences are at the MAC layer and at the physical layer.
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Comparison of FDDI and IEEE 802.5 Token Ring
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FDDI Frame Formats
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FDDI Frame Control Fields
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FDDI MAC Protocol
0 The same as IEEE 802.5 Token Ring MAC Protocol
0 First Difference: in FDDI, a Station waiting for a token seizes the
token by aborting (failing to repeat) the token transmission as soon
as the token frame is recognized. The station begins transmitting
one or more data frames. No flipping a bit to convert a token [high
data rate].
0 Second Difference: a Station that has been transmitting data frames
releases a new token as soon as it completes data frame
transmission, even if it has not begun to receive its own
transmission.
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FDDI Token Ring Operation
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FDDI Physical Layer Medium Alternatives
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