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Lecture 8: Ethernet and Token Ring Networks

Lecture 8: Ethernet and Token Ring Networks. Ethernet. Carrier Sense, Multiple Access and Collision Detect (CSMA/CD) LAN Ethernet Standard-DEC, Intel, Xerox 1978 Ì IEEE 802.3 10 Mbps—We are going to talk about this

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Lecture 8: Ethernet and Token Ring Networks

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  1. Lecture 8: Ethernet and Token Ring Networks

  2. Ethernet • Carrier Sense, Multiple Access and Collision Detect (CSMA/CD) LAN • Ethernet Standard-DEC, Intel, Xerox 1978 • Ì IEEE 802.3 • 10 Mbps—We are going to talk about this • 100,1000 Mbps—These are used for point-to-point links in switched networks

  3. 50 W cable • Taps at least 2.5 meters apart • Up to 500 m between hosts- • < 4 repeaters between hosts- • End-to-end length < 2500 m • Propagation delay <8.7 ms • Not more than 1024 hosts

  4. Network topology with at most 2 repeaters between hosts Repeaters have queuing delay is 4.2 ms for 10 Mbps ethernet Max queuing delay=16.9 ms/between any two hosts Max delay= 25.6 ms between any two hosts

  5. Network topology with two stars connected by a point-to-point link

  6. Newer Technologies • 10BaseT • Cat 5 copper • 100 Mbps ethernet • Switched, point-to-point • Gigabit ethernet • Switched, point-to-point technology

  7. Access Protocol • Frame Format • Addresses • Receiver Algorithm • Transmitter Algorithm

  8. Ethernet Frame MAX=1500 bytes Min=46 bytes Type—demux key to identify application CRC-32

  9. Addresses • Media Access Control (MAC) address • 48 bits/6 bytes-X1X2:X3X4:X5X6:X7X8:X9X10:X11X12 • Unique worldwide for all time • Belongs to adapter • Broadcast address • FF:FF:FF:FF:FF:FF • Multicast address • 1xxxxxxx:XX:XX:XX:XX:XX

  10. Receiver Algorithm • Adaptor • Receives all frames on the ethernet • Accepts Frames Addressed to: • Its unicast address • the broadcast address • A mult-cast address it has been programmed to accept • Accepts all frames in “promiscuous (eavesdropping) mode”

  11. Transmitter Algorithm • If adapter has a frame to send: • Line is idle-sends its frame-up to 1500 bytes • Line is busy-wait til line goes idle and send the frame • Collisions • These are detected as overvoltage AT AN ADAPTER • Sends 32 bit jamming sequence and shuts down-purpose of jamming frame is to cause a collision at the sender • So, the frame will be truncated-could be as small as 96 bits (64 bit preamble+32 bit jamming sequence) • Called a “runt” frame

  12. Worst-case collision scenario: • A transmits at time t=0 • B sends frame just before t=d(delay) • B detects collision and sends runt frame • A detects collision at time 2d (if its still sending!) • Frame must be 2d=51.2 seconds long for collision to occur at A • Frame must be at least 512 bits long at 10 Mbps

  13. Exponential backoff collision recovery • Wait k*52.1 ms where k=0,1,…,2n-1 • n=number of unsuccessful attempts to transmit • delay selected at random in each attempt

  14. The ethernet advantage • It’s simple—no routing tables to maintain • It’s flexible—just plug in your PC • It’s cheap • Cable—$ 0.12/ft • Adaptors—$ 20 • Hubs--$ 30/port • Switch--$ 50/port

  15. Token Ring • Shared medium similar to ethernet • Distributed algorithm controls transmission • All nodes see all frames Token=small circulating frame To send: Remove token and send frame Sender: removes message when it comes around and inserts token

  16. Token ring specification • IBM Token Ring • IEEE 802.5 • Fiber Distributed Data Interface (FDDI)

  17. Relays (normally closed) in the network adapter protect the ring from host failures

  18. Multi-station access unit (MSAU) create a star-like topology

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