Token Ring IEEE purging

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					        Token Ring - IEEE 802.5

What the IEEE standard covers
"Physical layer standard (gives link layer format)"

Essentially an IBM standard 'given' to the industry"

Differences between 802.5 and 802.3
"Guaranteed response
Controlled delays"
          Token Ring History
• Presented by IBM in 1982 to IEEE 802
• First prototype developed in 1983 in
  Geneva, Switzerland.
• Cabling System was announced in 1984.
• Officially announced in 1985.
• Standardized by IEEE in 1985.
 Token Ring Technology Summary
• Access method by which network attachments gain access to the cable
  plant by acquiring a special frame called the token. {Token is a special 24-bit
  pattern that continuously circulates the ring.}
• Token Ring is a broadcast medium. {To receive data, a destination station performs
  an address match.}
• The destination station merely copies the frame as it repeats it back to the
• When the frame arrives back to the source station, it strips the frame from
  the ring and then releases the token (4 megabit operation only).
       • The token is allowed to be released prior to frame reception on 16-
         megabit rings.
• Token Ring originally ran at 4 Mbps. Upgraded in 1989 to 16 Mbps
• Maximum frame size for 4 Mbps is 4472.
   –This is based only on the fact a station cannot hold the
    token longer than 0.010 milliseconds.
• Maximum frame size for 16 Mbps is 17,800.
                    TRN Features
  "data rate of 4 or 16Mbps"

Traffic usually (always in 802.5) unidirectional
 "one frame on the net at a time..."
RAR (802.5) vs RAT (FDDI) for Token Passing

Recovery from lost token


Frame Structure
Controller Attachment to a MAU
        The IBM 8228 MAU

         Shielded or UTP cable
              Lobe cables
                                           Hermaphroditic or RJ-45
        Cable Connectors                     connectors on MAU

                        DB-9 connector

Token Ring controller
                            Media filter
                           for UTP only

                          RJ-11 or RJ-45
      Media filter
    can be on-board
     Multiple MAU Connection

                      Ring out   MAU   Ring in

                      Ring out   MAU   Ring in
Type 6 patch cables

                      Ring out   MAU   Ring in
                        MAU Operation
                                       Lobe cables


                             Closed                             Closed   Closed

                                      MAU top view
                                                                                  Ring out
Ring in

                   MAU bus

                                      All stations are active
  MAU Operation (Inactive Station)
                                Lobe cables


                       Closed                  Closed      Closed

                                MAU top view
                                                                           Ring out
Ring in

             MAU bus

                                                        Inactive station
           Token Ring Cable Types
• Type 1
  –A shielded data grade cable with two solid wire twisted pairs.
  –Available in indoor and outdoor versions.

• Type 2
  –A Type 1 indoor cable with four solid twisted pairs of 24 AWG wire.
  –Contains four voice grade wires along with four data grade wires.

• Type 3
  –Unused existing telephone wire or EIA category 3 wire (4 Mbps
  –Category 4 is needed for 16 Mbps (speed of the Token Ring) operation.
  –Must use a special media filter.

• Type 5
  –100/140 micron fiber cable used for fiber optic repeater links.

• Type 6
  – Often used for patch cables.
     • Patch cables can be used for MAU-to-MAU connection or from a wall outlet
            Type 3 Media Filter
• Type 3 cable requires a device known as a media filter.

• Its purpose is to filter out any unwanted signals.

• It is a small rectangular device that is usually part of the
  UTP cable itself.

• It can be a separate device that attaches to the UTP cable at
  the end of the cable that attaches to the controller card.

• It can be used on 16- or 4-mb Token Rings.

• It is only used with Type 3 (UTP) cable.
                802.5 Framing
• IEEE 802.5 uses special characters, but
  does not use bit stuffing!

      “1” bit                   “0” bit

          Token Ring Frames
      Physical header                                  no preset size     Physical trailer

SD   AC     FC      DA    SA           Information             Data     FCS     ED     FS

                           MAC or LLC Frame

                             Token frame

                           SD     AC    ED

                         1 byte 1 byte 1 byte

                             Abort frame

                           SD           ED

                         1 byte        1 byte
Token Ring Frame Field Definitions
                                                                                  no preset size

     SD          AC          FC       DA        SA        Information                 Data     FCS         ED      FS

     1 byte    1 byte       1 byte   6 bytes   6 bytes   <= 18 bytes                               4 bytes 1 byte 1 byte

                                                                  DSAP      SSAP         Control

                                                                 1 byte    1 byte      1 or 2 bytes
 SD - Starting Delimiter
 AC - Access Control
 FC - Frame Control
 DA - Destination Address
 SA - Source Address
 FCS - Frame Control Sequence
 ED - Ending Delimiter
 FS - Frame Status
      The SD and the AC

        Bit 0    Bit 7
SD        JK0JK000
                         PPP - priority bits
 AC      PPPTMRRR        T - Token bit
                         M - Monitor bit
                         RRR - Reservation bits
The FC, ED, and FS Fields
          Bit 0   Bit 7
                           FF - indicates a MAC or
                           LLC frame.
  FC        FFrrZZZZ

                           ZZZZ - indicates the type
                           of MAC frame.

                          I - Intermediate bit
  ED       JK1JK1IE

                          E - Error bit

                          A - Address recognized bits
  FS       ACr rACrr

                          C - Frame copied bits
             Bit Order Transmission
                 for Token Ring
• Bit 0 is the first bit transmitted.
  –Bit 0 is the left most bit of the byte.
     • Unlike Ethernet, the bits in the bytes are not reversed as
       they are transmitted.

• Example:
  –40-00-12 are the first three bytes of a MAC
     • Translated to binary:
     • As transmitted on a Token Ring:
  Token Passing Policies (Defn)
• Multiple Token
  – RAT (FDDI): free token is appended to tail of
    last packet
• Single Token
  – ?: Token is released upon receipt of leading
    edge of own packet
• Single Packet
  – RAR (802.5):Token is released upon receipt of
    trailing edge of own packet
 Token Passing Policies (Usage)
• Multiple Token
  – Allows multiple packets on the segment at one
    time. Good when packet length is less than
    ring latency
• Single Token
  – More efficient than RAR; when packet length
    is about the same as ring latency
• Single Packet
  – Least efficient, but allows controlling station
    knowledge of (un)successful transfer before the
   Token Passing Policies (Perf.)
• Multiple Token
   – Always the best performer, but more complex
• Single Token
   – Closer to RAR than RAT
• Single Packet
   – ‘Worst’ performance

KEY POINT: Ratio of ring latency to packet length, a, is
  real determiner of performance. For a << 1, RAR is OK.
Controller Operation - Phases 0 and 1
 • Five-phase initialization
    –Phase 0 - Lobe test
       • The controller transmits frames between the controller card and the
         cable attached between the controller card and the MAU.
       • The controller tests to ensure that the lobe cable can successfully
         transmit and receive frames.
    –Phase 1 - Monitor Check
       • Station inserts into the ring (flips the relay in the MAU) and looks
         for special frames that are transmitted by the monitors.
       • Sets a timer to wait for these frames.
       • If the station does not receive any of the frames, the controller
          –it is the first ring station on the network,
          –there is not an Active Monitor present, or
       Controller Initialization -
          Phases 2, 3, and 4
• Phase 2 - Duplicate address check.
  –Checks to ensure that it can successfully transmit
   and receive a frame and to detect other stations
   that might have the same MAC address.
     • The controller transmits a frame to itself.
     • If the frame returns with the address recognized bit set, it
       notifies one of the monitors and removes itself from the
• Phase 3 - Participation in neighbor notification.
  –The station transmits a special frame that will
   identify itself to its downstream neighbor.
                Claim Token Process
• A ring cannot operate without a token circulating on the ring.
   –There is only one token per ring.
• The token-claiming process allows one station to insert the token onto the
   –This station will be elected as the AM.
      • It will purge the ring (ability to transmit a frame to itself).
      • After purging the ring, it will insert a new token on the ring.

• The Token-Claim process can be started when the AM
   –detects a loss of signal,
   –a timer expires and it has not yet received its AM frame
    back, or the AM
   –cannot receive enough of its own Purge Ring MAC frames.
Details of the Claim Token Process
• If there is no token on the ring, all activity will cease on the ring.
   –The Active Monitor should be able to recover by purging the
    ring and issuing a new Token.
   –If the Active Monitor cannot recover, the token-claim
    process will begin.

• Any station will insert its master clock, a 24-bit delay, and start to transmit
  Token-Claim frames.
   –These frames are received by all stations on the ring.
   –The station will follow these frames with idle (clock)
   –After transmitting the Token Claim frames, the station starts
    a timer.
Claim Token Process Example
                        Detected condition
           1                                                 4
      Token Claim              C                     B transmits its own
        frames                                       Token Claim frames

    Not             D                            B          B has higher
participating                                              priority than A

          2                                              3
     Repeat frame                                A transmits its own
                                A                Token Claim frames

                           Higher priority
                           than C. Does
                           not repeat C’s          6                                      5
                                          Stops transmitting                         Continues
                                        its own Claim frames                        transmitting
                                           and repeats B’s                            its own

                                                           D                         B

                                               Repeats                       A                8
                                              B's Token                            Stops transmitting its own
                                             Claim frame                         and repeats B’s claim frames
       Token Ring Transmit Mode
• A station that needs to transmit receives the SD of approaching frame. This
  station quits transmitting idles (clock signals).

• Checks for priority.
   –If the priority in the frame is greater than the station's
    priority, then
      • the station sets reservation bits and awaits new token.

• If the priority in the frame is less than or equal to the station’s priority then
   –the station changes the T bit in the AC field from a 0 to a 1,
   –appends its information to the rest of the frame and transmits
    the frame.
   –If the end of its transmission is reached and it has not
    received its current transmission back, the station
      • transmits idle characters and awaits current transmission.
          Token Ring Copy Mode
• The destination Token Ring controller recognizes its address in
  the destination field of a received frame and copies the frame
  into its buffer.

• If at any time an error is detected, the copy phase ends and the
  controller sets the A and E bits and repeats the frame back to
  the ring.

• If no errors are found, the destination sets the A and C bits and
  repeats the frame back to the ring.

• The destination station enters Normal Repeat mode.

• The frame travels on the ring until it reaches the originator and
          Normal Repeat Mode
• A station in normal repeat mode checks
  current frames and token for signalling errors.
 –If any errors are found the station sets the E bit and
  repeats the frame back to the ring.

• A station in this mode also checks every frame
  for its address.
 –A duplicate address could be found.
 –If a duplicate address is found, the station will
  transmit a soft error MAC frame to one of the
          The Active Monitor (AM)
• Functional address is C00000000001.

• It must be present in order for the ring to function properly.

• The AM is the kingpin of the ring.

• The AM:
   –tracks lost tokens and ensures that only one token exists on
    a single ring.
   –monitors frames and priority tokens that circulate the ring
    more than once.
   –initiates neighbor notification,
   –provides a latency buffer to recover the clock signal and so
            Token Recovery
• Monitor Station
  – 1 station becomes responsible for monitoring
    the token for token loss or token busy
• Time Outs
  – Token time out (‘Beaconing’)
  – No monitor (Claim frames (highest addr wins)
            Options for Token Ring
• For 16 megabit rings, early token release allows a ring station to release the
  token before receiving its original frame back.

   –It is based on the ring length
      • A station will not release the token when it is still transmitting its
        frame and it has started to receive its frame back.
   –Allows greater use of Token Ring bandwidth.
• Token Ring operates at 4 and 16 Mbps.
   –4 and 16 Mbps controllers are not allowed on the same ring.
      • Ring will beacon when this condition occurs.

   –To have 4 and 16 Mbps ring interoperate, you must use a
    data forwarding device such as a bridge or a router.
• IBM is currently experimental with a new Token Ring controller which

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