802 5 TokenRing Networks Differential Manchester Encoding For

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 • • No transmission at start of bit period No transition in middle – For a K • • Transition at start of bit period No transition in middle

802. 5 Token-Ring Networks • Differential Manchester Encoding No Transition at Start Transition in Middle 1 0 No Transition in Middle J K

Token Frame in 802. 5 Start Frame Delimiter (JK 000) [1 octet] Access Control (PPPTMRRR) (T=1 for token frame) [1 octet] End Frame Delimiter (JK 1 IE) [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

Token Frame in 802. 5 Start Frame Delimiter (JK 000) [1 octet] Access Control (PPPTMRRR) (T=1 for token frame) [1 octet] End Frame Delimiter (JK 1 IE) [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

Full 802. 5 Frame Start Frame Delimiter (JK 000) [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

Full 802. 5 Frame Start Frame Delimiter (JK 000) [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

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

802. 5 Full Frame Earlier Fields End Frame Delimiter (JK 1 IE) [1 octet] Frame Status (ACrr) End Frame Delimiter as in Token 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

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

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

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 Wrapped Ring Access Unit Break