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Token ring 802.5

token ring 802.5 completely covered

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Token ring 802.5

  1. 1. Computer Networks Token Rings 802.5
  2. 2. 2 Token Rings • IBM Token Ring & IEEE 802.5 • FDDI • Resilient Packet Ring IEEE 802.17… • IBM Token Rings are the most famous of this series
  3. 3. 3 About IBM Token Rings • Network consists of a set of nodes connected in a ring & data always flows in particular direction • Each node receives frames from its upstream node and transmits to its downstream node • Token rings share two features with Ethernets – Requires an algorithm that controls when each node is allowed to transmit – All nodes see all frames with destination saving a copy of it
  4. 4. 4 Tokens • Special sequence of bits that circulates around the ring – 24 bits in case of 802.5 • When node with a frame to transmit sees the token, it takes the token off the ring and instead inserts its own frame on the network – Frame is forwarded by all nodes while destination too makes a copy of it and forwards it – Packet is received again by sender which strips its frame off the ring and reinserts the token
  5. 5. 5 Physical Properties • Link failure in ring may disrupt transmission of the whole network • Electromechanical relay is used to overcome this problem • As long as the station is healthy, the relay is open • When the stations stops powering the relay, it gets closed and the station is removed from the network
  6. 6. 6 Use of Electromechanical relay From Previous Host To Next Host Host Relay
  7. 7. 7 Use of Electromechanical relay From Previous Host To Next Host Host Relay
  8. 8. 8 Multi Station Access Unit… MSAU • Several Electromechanical relays packed into a single box • It makes token rings look like a star topology • Systems are plugged in and out of MSAU • One of the main differences between 802.5 and IBM Token Rings is that the latter requires MSAU – The other difference however can be that that IBM Token Rings can have up to 260 stations per ring while 802.5 supports 250 – Physical media for IBM is twisted pair but not defined for 802.5
  9. 9. 9 Multi Station Access Unit… MSAU Previous MAUS Next MAUS
  10. 10. 10 Token Ring Media Access Control • Any station that wants to transmit data, has to seize the token from the network • Once the token is seized, transmission can be done • In 802.5, seizing process involves simply modifying 1 bit in the token, and it acts as a preamble for the subsequent data packet
  11. 11. 11 Data Transmission • Each transmitted packet contains a destination address • As the packet flows past each node on the ring, each node looks inside the packet to see if it is the intended recipient • If so, it copies the packet into its buffer as it flows through the network adapter • But it does not remove the packet form the network • The sending station has the responsibility of removing the packet from the ring • Other stations don’t harm the packet
  12. 12. 12 Token Holding Time • Setting THT to infinity will be dangerous because other devices wont have fair access to the ring • Setting THT to one message will be silly – If node has more packets to send, after every THT it would let the token circulate around the whole ring before seizing it again • Hence it is set to 10ms – A device should take care of the fact that before transmission of a packet, whether it got sufficient THT left to transmit this packet
  13. 13. 13 Token Rotation Time • Time required by token to traverse the ring • TRT < ActiveNodes * THT + RingLatency • Active Nodes are the number of nodes that have data to transmit and Ring Latency is the time a packet takes to traverse the ring when no node has data to send… Ideal
  14. 14. 14 Reliable Delivery • A & C bits contained in frame status byte • Both set to 0 by transmitter • As the destination station sees the packet, it set the A-bit to 1 • When the destination station copies the packet into its buffer successfully, the C bit is set to 1 • At Sender again – A=1 & C=1 means Successful Delivery – A=1 & C=0 means Host is alive but frame could not be copied
  15. 15. 15 Priority • Priority allows a particular host to access the token • Token contains a 3-bit priority field • Token has certain priority n at any time • Each host that wants to transmit a packet, has to assign a priority to the data packet • The host that holds the token, before releasing the token sets its priority to the priority of the data packet it received
  16. 16. 16 Token Ring Maintenance • Monitor Station • A monitor periodically announces its presence by special control messages • If these periodic messages stop arriving, the monitor will be assumed to have failed • And a new monitor is elected – Each station that wants to be monitor sends a claim frame – If the token circulates back to the sender, it will be the monitor – If more than one claim frames, break the tie by some algorithm (say the highest address wins)
  17. 17. 17 Monitor • Monitor is to take care of the health of the token ring network – Clocking… In case of Token – Setting bits… In Case of Data Frame • Reinserts a new token if token is not seen by monitor after the interval – (NumStations * THT) + RingLatency • Checks for orphaned & Corrupted frames o M-bit is set to 0 by transmitter and set to 1 as it passes the monitor o If a token passes the monitor with the m-bit already set to 1, it is removed from the network
  18. 18. 18 Frame Format • Take it as a Reading Assignment – Included in Exam 
  19. 19. 19 Recommended Text • Computer Networks: A System Approach by Lary Peterson and Bruce Davie

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  • Zarii21

    Jan. 7, 2016

token ring 802.5 completely covered


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