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802.5 Token-Ring Networks

802.5 Token-Ring Networks. Differential Manchester Encoding For a 1 No transition at start of bit period Transition in middle For a 0 Transition at start of bit period Transition in middle. 802.5 Token-Ring Networks. Differential Manchester Encoding For a J

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802.5 Token-Ring Networks

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  1. 802.5 Token-Ring Networks • Differential Manchester Encoding • For a 1 • No transition at start of bit period • Transition in middle • For a 0 • Transition at start of bit period • Transition in middle

  2. 802.5 Token-Ring Networks • Differential Manchester Encoding • For a J • No transmission at start of bit period • No transition in middle • For a K • Transition at start of bit period • No transition in middle

  3. 802.5 Token-Ring Networks • Differential Manchester Encoding No Transition at Start Transition at Start Transition in Middle 1 0 No Transition in Middle J K

  4. Token Frame in 802.5 Start Frame Delimiter (JK0JK000) [1 octet] Access Control (PPPTMRRR) (T=1 for token frame) [1 octet] End Frame Delimiter (JK1JK1IE) [1 octet] PPP gives the priority of the token T=1 for a token, 0 for a full frame M is set to 0 by the sending station, to 1 by the monitor station; if the bit is not changed, there has been a problem, i.e. transmitting stationd didn’t remove frame after a complete run

  5. Token Frame in 802.5 Start Frame Delimiter (JK0JK000) [1 octet] Access Control (PPPTMRRR) (T=1 for token frame) [1 octet] End Frame Delimiter (JK1JK1IE) [1 octet] In the End Frame Delimiter … I=1 in a single frame or the last frame of the series; otherwise set to 0 E=0 initially; Set to 1 by any station detecting an error

  6. Full 802.5 Frame Start Frame Delimiter (JK0JK000) [1 octet] Access Control (PPPTMRRR) (T=0 for full frame) [1 octet] Frame Control (FFZZZZZZ) Type of Frame [1 octet] • FF tells type of frame • 01 if contains 802.2 LLC Frame (data) • 00 for supervisory frames • ZZZZZZ tells specific type of frame

  7. Full 802.5 Frame Start Frame Delimiter (JK0JK000) [1 octet] Access Control (PPPTMRRR) (T=0 for full frame) [1 octet] Frame Control (FFZZZZZZ) Type of Frame [1 octet] Destination Address [6 octets] Source Address [6 octets] Addresses are the same as in 802.3 48-bits long

  8. Full 802.5 Frame Earlier Fields Routing Information for Source Route Bridges [2-6 octets] Information Frame Check Sequence [4 octets] Information field can be up to 17,997 octets for 16 Mbps Token Ring Networks Frame Check Sequence identical to 802.3 FCS field

  9. 802.5 Full Frame Earlier Fields End Frame Delimiter (JK1JK1IE) [1 octet] Frame Status (ACrrACrr) • End Frame Delimiter as in Token Frame • Frame Status for Reporting Specific Errors • A(ddress): If receiving station recognizes its address, changes from 0 to 1 • C(opy): if receiving station copies the frame successfully, changes 0 to 1

  10. Error Handling in 802.5 • Lost Token • There is an active monitor station that checks for problems • If active monitor does not correct the problem, another station assumes the role of active monitor • Complex; Adds to cost

  11. Error Handling in 802.5 • Repairing Breaks in the Ring • A single break makes the ring useless • Really a dual ring; Normally, only one used

  12. Error Handling in 802.5 • If there is a break, the ring is wrapped • Still forms a closed loop • Can only be done once, so breaks should be fixed promptly Access Unit Access Unit Wrapped Ring Access Unit Access Unit Break

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