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Lecture 9: Ethernet and Token Ring Networks

Lecture 9: Ethernet and Token Ring Networks. Ethernet review. Network topology with two stars connected by a point-to-point link. Ethernet Frame. MAX=1500 bytes Min=46 bytes Type—demux key to identify application CRC-32. Addresses. Media Access Control (MAC) address

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Lecture 9: Ethernet and Token Ring Networks

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  1. Lecture 9: Ethernet and Token Ring Networks

  2. Ethernet review Network topology with two stars connected by a point-to-point link

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

  4. 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

  5. Receiver Algorithm • Adaptor • Receives all frames on the ethernet • Only 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”

  6. 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 • When collision is detected, receiver transmits 32 bit jamming sequence and shuts down-purpose of jamming sequence is to cause a collision at the sender • Frame must be long enough for this collision to occur—512 bits in classic ethernet

  7. 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

  8. Token Ring • Shared medium similar to ethernet • Distributed algorithm controls transmission • All nodes see all frames Token=small (24 bit) circulating frame To send: Remove token and send frame Sender: removes message when it comes around and reinserts token

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

  10. Token Ring Media Access Control MAC • Network adaptor contains-receiver, transmitter, buffer >1 bit • Need enough storage within the ring to store a 24 bit token • If you have less than 24 stations—one has to provide the storage—this will be the monitor

  11. MAC cont. • To seize the ring-change 1 bit in the second byte-becomes preamble to the data packet. • A host copies those packets addressed to it and retransmits it around the ring • Sender removes the packet as it comes around—may still be sending information

  12. Key Concept • Token Holding Time (THT)>transmission time of a packet • Token Rotation Time (TRT)< ActiveNodes x THT+RingLatency Note: TRT is load dependent

  13. Early release versus delayed release THT=Frame transmission time + Ring Latency THT=Frame transmission time

  14. Reliable Transmission • A-bit set to 1 by destination host when message is detected • C-bit set to 1 if successfully copied

  15. Priority • Priority (3-bits)—8 levels • Token priority, packet priority • To seize the ring: Packet priority > Token priority • Reservations: Use reservation bits (3-bits) on a circulating packet to lock out low priority users. • Increase rb’s to your packet priority. • Current token holder sets token priority to the reservation • All users with packet priority > reservation may seize the ring. • New user must reset token priority to the original value

  16. Token Ring Maintenance • Monitor roles • Ensure adequate delay • Check for token circulation— Max TRT=Number of users*THT+RingLatency • Removes orphaned and corrupted frames • Detects dead stations-beacon frame

  17. Death of a Monitor • Monitor sends control message periodically • Hosts watch for this signal-if timeout, they try to become the monitor by sending a claim token • If claim token returns-sender is the monitor • If another claim token returns first-arbitration determines the new monitor

  18. Token ring frame • Manchester encoding • Illegal Manchester codes are used as delimiters • Frame status bits-A & C

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