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Token Ring : 802.5

Complete explaination of Token Ring

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Token Ring : 802.5

  3. 3. PROJECT 802 Project 802 defined network standards for the physical components of a network. It got started in the month February of the year 1980 , that is why the project name 802. 3
  4. 4. TOKEN PASSING PROTOCOLS  Before workstation can transmit it must possess the one and only token.  Two types:  Token ring  Token bus 4
  5. 5. TOKEN BUS : IEEE STANDARD 802.4  This standard 802.4(Dirvin and Miller and IEEE, 1986)     described a LAN called a token bus. A simple system in which stations takes turns sending a frame was introduced know as a TOKEN BUS SYSTEM. The token bus is linear tree shaped cable onto which stations are attached. Logically stations are organized into a ring with each station knowing the address of the station to its left and right. During logical ring initialization the highest number station may send the first frame. 5
  6. 6.  It passes permission to its immediate neighbor by sending the neighbor a special control frame called a TOKEN.  Only the token holder is permitted to transmit frames.  Only one station at a time holds the token therefore no collision occurs.  It uses a broadcast medium for each station to receive frames . 6
  7. 7. TOKEN RING IBM created TOKEN RINGS to compete with DIX (DEC/Intel/Xerox) standard of Ethernet to improve upon its previous token bus technology. 7
  8. 8. IEEE 802.5 TOKEN RING Proposed in 1969 and initially referred to as a Newhall ring. Token ring :: a number of stations connected by transmission links in a ring topology. Information flows in one direction along the ring from source to destination and back to source. Can both be implemented using star as well as ring topologies but basically it uses ring topology logically and star topology physically. Medium access control :: is provided by a small frame, the token, that circulates around the ring when all stations are idle. Only the station possessing the token is allowed to transmit at any given time. 8
  9. 9. IEEE 802.5 TOKEN RING  There is a point to point link between stations that form a ring.  Physical Layer Topology: Ring  Stations connected in a loop  Signals go in only one direction, station-to-station  In a token ring a special bit format called a token circulated around all the stations. 9
  10. 10. TOKEN RING OPERATION  When a station wishes to transmit, it must wait for the token to pass by and seize the token.  One approach: change one bit in token which transforms it into a normal data frame and appends frame for transmission.  Second approach: station claims token by removing it from the ring.  The data frame circles the ring and is removed by the transmitting station.  Each station interrogates passing frame. If destined for station, it copies the frame into local buffer. 10
  11. 11.  Ring interfaces have three operating modes.  Repeater  Insert  Copy Repeater: the input bits are simply copied to output with a delay of 1 bit time. Insert: the interface breaks the connection b/w the input &output entering its own data onto the ring. Copy: the interface or the station simply copies data into its local buffer as it is the destined station. As bits have propagated around the ring & they come back, they are removed from the ring by the sender. 11
  12. 12. TOKEN RING NETWORK WITH STAR TOPOLOGY A Wiring center Or the multistation attachment unit E B C D 12
  13. 13. Multistation Attachment Unit Earlier there was problem of segmentation using just by - pass relays , but that problem got easily solved using multistation attachment unit because there were many stations attached to this single unit . Also the problem of going down of stations got corrected. 13
  14. 14. Token Frame Format S AC (1 Byte) E (1 Byte) (1 Byte) Start Frame Delimiter End Frame Delimiter P Priority Bits 000 – lowest 111 - highest Access Control P P Token Bit 0 – Token frame 1 – nontoken frame T M R R R Monitor Bit 0 = “clean” frame 1 = “orphaned" frame Reservation Bits 3-bit field used for reserving a token at a prescribed priority level 14
  15. 15. IEEE 802.5 Data Frame Structure Data Frame Format 1 SD 1 1 AC FC Starting delimiter 2 or 6 Ending delimiter Frame status 4 Destination Source Information FCS Address Address J Access control Frame control 2 or 6 K 0 J M PPP T FF J A K C Z 1 K 0 Z Z A 0 Z C J, K 1 ED FS non-data symbols PPP Priority; T Token bit M Monitor bit; RRR Reservation RRR J K1 I xx 0 1 Z E x x Z FF ZZZZZZ I E frame type control bit intermediate-frame bit error-detection bit A addressrecognized bit xx undefined C frame-copied bit 15
  16. 16. Frame control FC 00000000 = duplicate address test; used if two lobes have the same address. 00000010 = beacon; used to locate breaks in the ring 00000011 = claim token; used to recover from an inactive monitor 00000100 = purge; used to reinitialize the ring. 00000101 = active monitor present;used by the active monitor to inform all lobes that it is alive. 00000110 = standby monitor present; used to notify all lobes that a standby monitor is present. Frame Status FS 00xx00xx = address not recognized; nothing copied. 10xx10xx = address recognized; nothing copied 01xx01xx = address not recognized; data copied(major error) 11xx11xx = address recognized; data copied (implies transmission was successful). 16
  17. 17.  Starting delimiter and ending delimiter mark the beginning & ending of the frame.  Access control consist of token bit, monitor bit, priority bit.  Destination address & source address fields gives the address.  Checksum field is used to detect transmission errors. 17
  18. 18. Frame status field  When a frame arrives at the interface A bit is turned on.  If the interface copies the frame to the station the C bit is turned on.  3 combinations: A=0 C=0 : Destination not present or not powered. A=1 C=0 : destination present but frame not accepted. A=1 C=1 : Destination present and frame copied. This increases reliability and acts as automatic acknowledgement. 18
  19. 19. STAR TOPOLOGY OR WIRE CENTER • Physically each station is connected to the Multistation Attachment Unit by a cable. While logically it is still a ring. • A bypass relay that are energized by the current from the station are inside the Multistation Attachment Unit. • When station goes down the station is bypassed from the rest of the ring. • Multistation Attachment Unit improves the reliability and maintainability. 19
  20. 20. MAINTANAINCE Here we use a MONITER STATION that oversees the ring. Active Monitor Station Selection takes place using MAC addresses. Active Monitor Station Observes the ring for its correct functioning. 20
  21. 21. Tasks Performed by Active Monitor Station • • • • • • Upstream Neighbour Determination Sending purge-frame in the ring Loss of token (no token circulating. Garbled frames appears. Orphan frames. Length of the ring. Monitor cannot handle breaks in the ring. 21
  22. 22. GARBLED AND ORPHAN FRAMES Both actually refers to the corrupted frames. For e.g. sometimes what happens that in case a station after sending data got corrupted. Now as it is the duty of the sending station to release the token so as there is no sending station so this frame becomes orphaned frame. And that needs to be removed by the monitor station . Similar is the case with garbled frame . Monitor Station changes monitor bit to 1 and when it again receives it , it removes it from the ring. 22
  23. 23. 802.4 Vs 802.5         802.4 does not use any monitor like 802.5. Which is more reliable ? (depends upon the situation) Token ring & token bus both have priorities but not 802.3. Adding new station to token bus was difficult as compared to the addition in token ring. In token bus there is no relation between the physical location of a station on the bus to its address whereas it was not so in token ring. Both uses broadcasting addresing technique. Basically token ring is primarily used for control of industrial and factory automation processes. Main problem : No active Monitor Station. 23
  24. 24. Applications :  Used in large Local Area Networks.  Provides security and save time as there was no packet collision. Why It Got Down:  Single point of Failure.  Very High Cost.  The era of Switching technique gave ETHERNET an edge over it. 24
  25. 25. REFERENCE  A.S. Tanenbaum, “Computer Networks”.     25
  26. 26. THANK YOU 26

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Complete explaination of Token Ring


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