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Lecture 10: Token Ring and FDDI Networks

Lecture 10: Token Ring and FDDI Networks. Homework for Chapter 2: All problems, particularly time lines for sliding window protocol Exam 2: October 16, 2000. Token Ring Review. Shared medium similar to ethernet Distributed algorithm controls transmission All nodes see all frames.

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Lecture 10: Token Ring and FDDI Networks

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  1. Lecture 10: Token Ring and FDDI Networks • Homework for Chapter 2: All problems, particularly time lines for sliding window protocol • Exam 2: October 16, 2000

  2. Token Ring Review • 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

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

  4. Fiber Distributed Data Interface (FDDI)

  5. Single Attachment Stations (SAS) attached to a concentrator (DAS)—Star-like topology

  6. FDDI physical properties • 100 Mbps—bit time is only 10 ns • Network adaptor buffers-9-80 bits-retransmit when half full • Queuing delay=50 ns for 10 bit buffers • Up to 500 stations • 2 km between stations (propagation delay=10 ms) • 100 km ring circumference--200 km total fiber length • 4B/5B encoding • Packet size=18,200 bytes

  7. FDDI physical properties (cont.) • Complete Ring: • Ring Latency=24 (10 ns)+500*50 ns+0.5 ms=0.53 ms • Packet transmission time=1.45 ms • Every node should see a valid transmission (token or data frame) every 1.98 ms (or less). This is called the idle time • Broken ring: • Ring Latency =24 (10ns)+1000*50ns+1.0ms=1.05 ms • Idle time=2.5 ms

  8. Key Parameters • Target token rotation time (TTRT)-upper bound to TRT as seen from any node • So, TTRT is a measure of the maximum possible delay as seen by each station • TTRT is negotiated—the station with the most sensitive application sets TTRT (more later) • Measured TRT-every station measures current value of TRT

  9. Media Access Algorithm(Timed Token Algorithm) • If Measured TRT > TTRT, the token is delayed—station is not permitted to send • Measured TRT < TTRT, station can send TTRT-Measured TRT=THT • Ideally, this algorithm provides feedback needed to keep the ring from being overloaded

  10. Access Problems • Station are allowed to start packets that cause THT> TTRT-Measured TRT • If you are downstream from such a node, you may never be able to send your packets • Two traffic categories • Synchronous—e.g. time sensitive-Total synchronous traffic limited TTRT per ring rotation • Asynchronous-governed by Timed Token Algorithm

  11. Worst Case Performance • Token Rotation Time=2TTRT • Does not happen on successive rotations

  12. FDDI Token Maintenance • Recall idle time of a broken ring is 2.5 ms • Stations send claim if idle longer than 2.5 ms • Claim contains a bid for TTRT that is the maximum that is acceptable to by sending station • Bid is passed on by nodes with higher TTRT-they reset their TTRT to the bid value • Bid is replaced by nodes with lower TTRT requirements • Eventually, TTRT is set to the largest value that satisfies all stations.

  13. FDDI frame format

  14. Wireless LAN - 802.11 • LAN - Local Area Network. A network based on any physical network technology that is designed to span distances of up to a few thousand meters. • IEEE 802 LAN-Shared mediumpeer-to-peer communications network that broadcasts information to all stations to communicate

  15. Wireless LAN - 802.11 • OSI Layer 1 - Physical • Encoding/Decoding • Preamble generation/removal • Bit transmission/reception • OSI Layer 2 - Data Link • Medium Access Control/Arbitration • Addressing • Protocol Data Unit/Frame

  16. Wireless LAN - 802.11 • Design issues • Distance • Performance • Reliability • Security • Administration • Scalable

  17. Wireless LAN - 802.11 • Technology • LAN Emulation • Frequency Hopping • Direct Sequencing • Collision • Distribution • Frame Format • Other

  18. Wireless LAN - 802.11 • Frequency Hop Spread Spectrum • 2.4Ghz, 1 and 2 Mbps transmission • Hop over 79 channels (N.America) • Pseudo-random hopping • Interference Immunity

  19. Wireless LAN - 802.11 • Direct Sequence Spread Spectrum • 2.4Ghz, 1 and 2 Mbps transmission • 11 Chip Barker Sequnce • Interference Immunity

  20. Wireless LAN - 802.11 • Baseband Diffuse Infrared • 1 and 2 Mbps transmission • Interference Immunity • Light • Reflection

  21. Wireless LAN - 802.11 • Baseband Diffuse Infrared • 1 and 2 Mbps transmission • Interference Immunity • Light • Reflection

  22. Wireless LAN - 802.11 • Direct Sequencing • Chipping Code • Spread signal across frequency • Increase in bandwidth determined by bit size of chipping code • 802.11 chipping code 11-bits

  23. Four-bit chipping sequence

  24. Wireless LAN - 802.11 • Collision Avoidance • Unlike Ethernet, not all nodes can “see” each others signal • Use of RTS/CTS/ACK handshake between sender and receiver

  25. Example of a wireless network

  26. Wireless LAN - 802.11 • Components • Access Point • Central wireless base Unit • Client • Remote Node • Distribution System • Interconnecting LAN (Like Ethernet)

  27. Access points connected to a distribution network

  28. Node Mobility

  29. IEEE 802.11 frame format

  30. Wireless LAN - 802.11 • Other related Topics • IEEE 802.11b - 11Mbps Wireless LAN • Currently only DSS support from vendors • IEEE 802.11a - Future (51Mbps?) Wireless LAN

  31. Wireless LAN - 802.11 Other References (outside of class text): http://grouper.ieee.org/groups/ 802/11/Tutorial/index.html Stallings, W “Data and Computer Communications - 5th ed.”, Prentice-Hall, 1997

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