chapter 8 by zhangyun

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									        CHAPTER 8
TOKEN RING LANS AND MANS
  8.1 IEEE 802.5 TOKEN RING MEDIUM
           ACCESS CONTROL
MAC Protocol
• token: a short frame circulating around the ring
• a station wishing to transmit seizes the token by
  changing one bit of the token as it passes by
• the seized token becomes a start-of-frame sequence
  and rest of a frame structure is transmitted right after
  that
• the transmitted frame comes back to the sender and
  is purged by the sender


Chapter 8            Networking for Manufacturing        8-2
• the transmitting station inserts a new token on the
  ring
      - if the station finished its transmission
      - and if the leading edge of the transmitted frame has returned
• bit length of the ring and frame length
      - if bit length of the ring < the frame length, the first condition
        implies the second
      - the second condition keeps only one frame on the ring to
        simplify any error recovery
• advantages
      - round robin under heavy traffic
      - fair access
      - can provide priority and guaranteed capacity service
• disadvantages
      - token maintenance
Chapter 8                  Networking for Manufacturing                 8-3
Chapter 8   Networking for Manufacturing   8-4
MAC Frame




Chapter 8   Networking for Manufacturing   8-5
• single priority operation
      - priority and reservations bits not used
      - a free token with its token bit equal to 0
      - a station wishing to transmit sets the token bit to 1
      - transmit one or more frames
      - until there is no message to transmit or
      - until its token holding timer expires
      - new token after the AC field of the last transmitted frame
        returns and its transmission is finished
      - receiving station sets the A bit to 1 when it detects its
        address
      - also sets the C bit to 1 when it copies the frame



Chapter 8                Networking for Manufacturing                8-6
Token Ring Priority
• variables
      - Pf: priority of a frame to send
      - Ps: priority of the token just received or about to send
        (service priority)
      - Pr: value of Ps in the token received most recently
      - Rs: reservation value of the token and the frame
      - Rr: highest reservation value of the frames




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• up to frame transmission
      - wait for token; while waiting, set the reservation field of a
        passing data frame (R s) to Pf if Rs < Pf
      - or set the reservation field of the token (R s) to Pf if Rs < Pf
        < Ps
      - claim the token if Pf > Ps and store the value of Ps in Pr
      - with its reservation field equal to 0, send frames that are at
        or above the Pr until
            * the station has finished the transmission of those frames
            * or until the transmission of another frame could not be finished
              before THT expires




Chapter 8                    Networking for Manufacturing                   8-8
• upon token release
      - if Pr >= max(Pf, Rr), then send the new token with Ps = Pr
        and Rs = max(Pf, Rr) (Table 8.2(1))
      - if Pr < max(Pf, Rr) then send the new token with Ps = max(Pf,
        Rr) and Rs = 0 (becoming a stacking station), store Pr in Sr
        (push) and Ps in Sx (push) (Table 8.2(2))
• stacks to remember the new and the old values of
  token priority
      - Sx : new (raised) priority
      - Sr : old priority
• the stacking station claims every token with Ps = Sx



Chapter 8                 Networking for Manufacturing             8-9
• stacking station with frames higher than token priority:
  if Ps = Sx and it has frames at or above Ps, then store
  Ps in Pr and send those frames as many as it can until
  no more frames ready or THT expired, after that
      - if Pr < max(Pf, Rr), then send the new token with Ps = max(Pf,
        Rr) and Rs = 0, remove the value stored at the bottom of Sx
        (pop), and store Ps in Sx (push) (Table 8.2(3))
      - if Pr >= max(Pf, Rr) > Sr, then Ps = max(Pf, Rr), Rs = 0, pop
        Sx, and push Sx with the new Ps (Table 8.2(4))
      - if Pr >= Sr >= max(Pf, Rr), then Ps = Sr, Rs = 0, pop Sx, and
        pop Sr (Table 8.2(5))




Chapter 8                Networking for Manufacturing             8-10
• stacking station with no frames higher than token
  priority
      - if it has no frame at or above Ps and Ps = Sx and Rs > Sr,
        then Ps = Rr, Rs = 0, pop Sx, and push Sx with the new Ps
        (Table 8.2(4))
      - if it has no frame at or above Ps and Ps = Sx and Rs <= Sr,
        then Ps = Sr, Rs = Rr, pop Sx, and pop Sr (Table 8.2(5))




Chapter 8                 Networking for Manufacturing                8-11
Chapter 8   Networking for Manufacturing   8-12
Chapter 8   Networking for Manufacturing   8-13
              Token Ring Priority Example

• 2nd message only after 1st transmission & token
  release


                                                      A         1, 3

            2, 2   B

                                                          token
                                                          Ps=0, Rs=0

                                                      C          3, 1



Chapter 8              Networking for Manufacturing                     8-14
Chapter 8   Networking for Manufacturing   8-15
Chapter 8   Networking for Manufacturing   8-16
Token Maintenance
• one station designated as an active monitor
• periodical transmission of an active-monitor-present
  control frame by the monitor station
• detection of a lost token condition
• removal of an orphaned frame
• failure of priority mechanism
• upstream neighbor by a standby-monitor-present
  frame


Chapter 8           Networking for Manufacturing         8-17
Early Token Release
• as an option
      -     when the frame length is less than the ring bit length
      -     releasing token as soon as frame transmission is finished
      -     without waiting for the frame header to return
      -     token priority set to the reservation of the most recently
            received frame
• partially disabling the priority mechanism
      - increased delay for high priority frames under heavy traffic
      - still compatible with standard stations




Chapter 8                    Networking for Manufacturing                8-18
Dedicated Token Ring
• switching device offering full-duplex connection
      - Fig. 8.4 on p. 221




Chapter 8                Networking for Manufacturing   8-19
            8.2 IEEE 802.5 Physical Layer

• Table 8.3 on p. 222




Chapter 8           Networking for Manufacturing   8-20
                      8.X TOKEN BUS

IEEE 802.4 Medium Access Control
MAC Protocol:
• logical ring
      - an ordered sequence of the stations on the bus
• token
      - a control frame containing a special bit pattern
      - to regulate the access to the network
      - containing a destination address
• network operations
      - alternating data transfer and token transfer


Chapter 8                 Networking for Manufacturing     8-21
• logical ring formation
      - active stations on the logical ring
      - each station aware of its predecessor (PS, previous station)
        and successor (NS, next station)




Chapter 8                Networking for Manufacturing             8-22
Chapter 8   Networking for Manufacturing   8-23
• minimum functions
      -     addition to ring
      -     deletion from ring
      -     ring initialization
      -     token recovery

• addition to ring
      -     each node to periodically grant an opportunity
      -     a token holding node to issue a solicit-successor frame
      -     wait for one response window
      -     no response
      -     one response




Chapter 8                     Networking for Manufacturing            8-24
      - multiple responses: garbled response
            * address-based contention
            * token holding node transmits a resolve-contention frame and
              waits for four response windows
            * demanding stations can respond in one of these windows
              based on the first 2 bits of its address
      - invalid response
      - station with the lowest address in the ring
            * solicit-successor-2 frame
            * two response windows




Chapter 8                   Networking for Manufacturing                8-25
• deletion of a station
      - a set-successor frame to its predecessor containing the
        address of its successor
      - passes the token to its successor
      - during the next token circulation, the successor station
        updates its predecessor to the source address of the
        received token

• ring initialization
      - a node with the time-out issues a claim-token frame
      - the claim-token frame padded by 0, 2, 4, or 6 slots based on
        the first 2 bits of the address
      - after transmission, a claiming station listens to the medium
      - if it hears nothing, continue with the rest of its address
      - if a station uses up the last pair of bits and hears nothing
        after its transmission, it assumes that it has won the process
        and begins to solicit successors to build a logical ring

Chapter 8                Networking for Manufacturing              8-26
• token recovery
      -     no response from the successor
      -     token pass one more time
      -     after two failures, a who-follows frame
      -     after two failures, a solicit-successor frame
      -     assuming some major faults, send any data ready and listen




Chapter 8                   Networking for Manufacturing           8-27
                    Token Bus Priority

• priority mechanism
      - classes of service as an option
      - priority classes 6, 4, 2, and 0
      - allocation of the network capacity to the higher priority
        frames
      - lower priority frames allowed only when there is enough
        capacity
      - Token Holding Time (THT)
      - Token Rotation Time i (TRTi) for class i (i = 4, 2, 0)




Chapter 8                Networking for Manufacturing               8-28
Chapter 8   Networking for Manufacturing   8-29
Chapter 8   Networking for Manufacturing   8-30
• throughput analysis
      - equal load distribution for 4 classes
      - TRT4 > TRT2 > TRT0
      - ignoring overhead under steady state




Chapter 8                Networking for Manufacturing   8-31
Chapter 8   Networking for Manufacturing   8-32
• two major techniques for MAC




Chapter 8         Networking for Manufacturing   8-33
• selection factors
      - performance requirement
      - cost
      - baseband or broadband




Chapter 8              Networking for Manufacturing   8-34
MAC Frame:




Chapter 8    Networking for Manufacturing   8-35
IEEE 802.4 Physical Layer Specifications




Chapter 8     Networking for Manufacturing   8-36

								
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