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Medium Access Control

Medium Access Control

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Medium Access Control

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  1. Medium Access Control

  2. Channel Allocation • Static channel allocation in LANs and MANs • FDMA, TDMA, CDMA • Dynamic channel allocation in LANs and MANs • MAC protocols: with collisions, polling, token

  3. Static Channel Allocation • Delay for one fast channel: • Delay for multiple FDM slower channels:

  4. Poisson Process • Probability of k arrivals in time t: • Probability that interarrival time exceeds t:

  5. M/M/1 Queue • Queue equations • Delay for multiple FDM slower channels:

  6. Delay • Little’s formula • Delay is:

  7. Static Channel Allocation • Delay for one fast channel: • Delay for multiple FDM slower channels:

  8. Dynamic Channel Allocation • (a) Single channel (b) Multiple channel • (a) Collision (b) Collision-free 3. (a) Continuous Time.(b) Slotted Time. 4. (a) Carrier Sense.(b) No Carrier Sense.

  9. Multiple Access Protocols • ALOHA • Carrier Sense Multiple Access (CSMA) protocols • CSMA/CD • CSMA/CA • Collision-Free protocols • Reservation based • Token based

  10. Pure ALOHA In pure ALOHA, frames are transmitted at completely arbitrary times.

  11. Pure ALOHA Vulnerable period for the shaded frame.

  12. ALOHA Throughput • Throughput is S=GP0, where P0 is the probability of successful transmission. • The k frames per f frame slots is P0=e-fG • For pure ALOHA f=2, for slotted ALOHA f=1, so:

  13. Pure and Slotted ALOHA Throughput versus offered traffic for ALOHA systems.

  14. Carrier Sense Multiple Access • 1-Persistant CSMA • Nonpersistant CSMA • P-Persistant CSMA

  15. Persistent and Nonpersistent CSMA Comparison of the channel utilization versus load for various random access protocols.

  16. CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, or idle.

  17. Wireless LAN: CSMA with Collision Avoidance A wireless LAN. (a) A transmitting. (b) B transmitting.

  18. Wireless LAN: CSMA-CA The MACA protocol. (a) A sending an RTS to B. (b) B responding with a CTS to A.

  19. DOCSIS (Data Over Cable Service Interface Specification

  20. Collision-Free Protocols:Reservations The basic bit-map protocol.

  21. Collision-Free Protocols: Bidding The binary countdown protocol. A dash indicates silence.

  22. Collision Free Protocols: Fiber Distributed Data Interface (FDDI) • Station transmits only when it has a token • Timers count the time while the token is away • Two timers determine how much data a station may transmit, so that the token delay is limited

  23. Limited-Contention Protocols Acquisition probability for a symmetric contention channel.

  24. Adaptive Tree Walk Protocol The tree for eight stations.

  25. WDMA Networks • Stations may be connected to the optical star coupler or onto the optical ring and utilize multiple wavelengths. • Stations are equipped with the tunable/fixed transmitters and tunable/fixed receivers. • Stations are tuned to the common control channel where they compete for the medium.

  26. Ethernet • Ethernet, IEEE 802.3 • 10Base (10Mbps) • Fast Ethernet (100Mbps) • Gigabit Ethernet

  27. Ethernet MAC Sublayer Protocol Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.

  28. CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, or idle.

  29. Performance of Non-Persistant CSMA/CD Rom and Sidi, Multiple Access Protocols, Springer Verlag, 1990

  30. Rom and Sidi, Multiple Access Protocols, Springer Verlag, 1990 Performance of 1-Persistant CSMA/CD

  31. Comparison of CSMA and CSMA/CD Rom and Sidi, Multiple Access Protocols, Springer Verlag, 1990

  32. Ethernet Performance

  33. Ethernet Performance Efficiency of Ethernet at 10 Mbps with 512-bit slot times.

  34. Back-Off Mechanism • After a collision, user accesses medium with probability 1/W where W is the window size. • With each collision W doubles.

  35. 10Mbps Ethernet Cabling The most common kinds of Ethernet cabling.

  36. 10Mbps Ethernet Cabling Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T.

  37. Ethernet Cabling Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

  38. 10Mb Ethernet Coding (a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.

  39. 10 Mb Ethernet Collision Detection 10Base5 cabling, Kadambi, Crayford and Kalkunte, Gigabit Ethernet, Prentice Hall, 1998

  40. 10 Mb Ethernet Collision Detection 10Base2 and 10BaseT cabling, Kadambi, Crayford and Kalkunte, Gigabit Ethernet, Prentice Hall, 1998

  41. Fast Ethernet The original fast Ethernet cabling.

  42. Fast Ethernet • Auto negotiation enables communication with 10Mb Ethernet • Manchester code → 4B/5B code • Full duplex mode is optional with using PAUSE command

  43. Switched Ethernet A simple example of switched Ethernet.

  44. Gigabit Ethernet (a) A two-station Ethernet. (b) A multistation Ethernet.

  45. Gigabit Ethernet Gigabit Ethernet cabling.

  46. Gigabit Ethernet • Prioritization of fiber over copper • 4B/5B coding → 8B/10B coding • Full duplex mode is preferred with PAUSE message • Carrier extension, and frame bursting introduced in half-duplex mode

  47. IEEE 802.2: Logical Link Control (a) Position of LLC. (b) Protocol formats.

  48. IEEE 802.2: Logical Link Control (a) Position of LLC. (b) Protocol formats.

  49. A Sample HFC System narrowcast Downstream: 500 MHz shared by ~50,000 (broadcast) 200 MHz by 1200 (narrowcast) Upstream: ~37 MHz shared by 300 HOME o o o o o o o o lup lb ln(4ln/fiber) Secondary Hub Fiber Node o o o o o o o o o o RF Spectrum on coax: return 80 broadcast channels30 QAM channels (~150 video channels) broadcast 5-42 MHz 550 MHz 750 MHz Sheryl Woodward, AT&T Labs-Research

  50. Justification for Using Shared Medium • Equivalent circuit rate (ECR) on a cable with many users is the rate of a dedicated link that would provide the same e.g. average delay (similar results is obtained for 90th percentile page delay). By Shankar, Jiang and Mishra: where tON is the transmission tim, and tOFF is the think time, r is the channel rate, tON/(tON+tOFF)<<1, on periods have an exponential distribution.