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Chapter 1 – Token Rings

Chapter 1 – Token Rings. Monday, January 22, 2001. Token Rings. Originally proposed by Olaf Soderblum in 1969 IBM purchased the rights and released its product in 1984 IEEE 802.5 standard finalized in 1985 Operates at 4-16 Mbps. Features of Token Ring Network. Highly reliable network

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Chapter 1 – Token Rings

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  1. Chapter 1 – Token Rings Monday, January 22, 2001

  2. Token Rings • Originally proposed by Olaf Soderblum in 1969 • IBM purchased the rights and released its product in 1984 • IEEE 802.5 standard finalized in 1985 • Operates at 4-16 Mbps

  3. Features of Token Ring Network • Highly reliable network • More complex than Ethernet • 802.3 standard is physically a star but is logically a ring • Uses a “free” token that is passed around the ring in one direction • If a node receives the “free” token, it can change the token from “free” to “busy” attach data and send it onto the ring • Each station receives the data and sends the data and token to the next node

  4. Token Ring – Cont. • The receiving station reads the data and sends it to the upper level protocols • The token is marked to indicate that the data has been received (2 bits are switched) • The token with data is then retransmitted and finally gets back to the sender • This serves as an acknowledgement that the data was received • This process is a “collision avoidance” system

  5. Fault Management Techniques • No circulating token • Persistent busy token • To address these, one station is designated as the active token monitor • See handout for details

  6. Advantages of Token Ring • Operates under heavy load with significantly superior performance to Ethernet • Known upper bound to the amount of time any station must wait before transmitting • Built-in diagnostic and recovery mechanisms (beaconing, auto-reconfiguration)

  7. Disadvantages of Token Ring • Token ring cards and equipment are more expensive than Ethernet • Can be very difficult to troubleshoot • Requires considerable expertise

  8. Operation of Token Ring – See Handout • Eight levels of priority • Priority field • Reservation fields

  9. Token Ring (802.5) Frame Format • 802.5 Header • 802.2 Header (Logical Link Control) • Data • 802.5 Trailer • See p. 16 for specific fields

  10. 802.5 Header • Start Delimiter – 1 byte – provides synchronization for the receiver • Question –Why is only one byte used here instead of the 8 bytes used by Ethernet II? • Access Control – one byte – • 3 bits for priority of token • 3 bits for reservation level. These set the priority of the token once the token is released • 1 bit to indicate the ring monitor station has been passed. What does ring monitor do if it sees this bit set? • 1 bit to indicate whether the frame that follows is a token or a frame

  11. Frame Control • One byte • Indicates whether the frame that follows is a management frame or an LLC frame (2 bits) • Indicates the type of management frame

  12. Destination Address • 6 bytes long • Unicast address • Universal broadcast address (all 1s) • Token ring broadcast (C0-00-FF-FF-FF-FF) • Multicast address

  13. Source Address • 6 bytes • Indicates the sending node’s address

  14. Payload • No minimum frame size • Maximum size depends upon bit rate and token holding time

  15. Frame Check Sequence • 32-bit CRC • Access and Frame Control bits not included in this process, so these bits can be changed without recalculation of CRC bits • These bits are checked as it passes each node in the ring • If the FCS fails at any node, the Error bit is set and the receiving node doesn’t copy the frame

  16. FDDI • Network developed by ANSI • Uses fiber optics • Token ring that operates at 100Mbps • Designed to operate over long distances • Usually acts as a high-speed backbone • Allows for several frames on ring • Station can release token as soon as it is through transmitting frames • Uses two counter-rotating rings

  17. Homework • Begin reading Chapter 2 • Be ready for possible quiz – Chapter 1

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