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802.5 Token-Ring Networks Differential Manchester Encoding No Transition at Start Transition at Start Transition in Middle 10 No Transition in Middle JK
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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 Transitionat Start
Transitionat Start
Transitionin Middle 1 0
No Transitionin Middle J K
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
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
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
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]
Addresses are the same as in 802.348-bits long
Destination Address [6 octets]
Source Address [6 octets]
Full 802.5 Frame
Earlier Fields
Information field can be up to 17,997 octets for 16 Mbps Token Ring Networks
Frame Check Sequence identical to 802.3 FCS field
Routing Information for Source Route Bridges [2-6 octets]
Information
Frame Check Sequence [4 octets]
802.5 Full Frame
Earlier Fields
End Frame Delimiter as in Token Frame
Frame Status for Reporting Specific ErrorsA(ddress): If receiving station recognizes its address, changes from 0 to 1C(opy): if receiving station copies the frame successfully, changes 0 to 1
End Frame Delimiter (JK1JK1IE) [1 octet]
Frame Status (ACrrACrr)
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
Backup Ring(Norm ally Unused)
M ain Ring(Norm ally Carries A ll Traffic)
Access Unit Access Unit
Access UnitAccess Unit
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
Break
Wrapped RingAccess Unit Access Unit
Access UnitAccess Unit
