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Basic Model

Basic Model. MEDIUM. Distributed Sources. Fig.13 Generic Model for Media Access Systems. Message Delay - FDMA. Example FDMA. 50 kbps line - 1000 bit frames to be transmitted from four sources. Two Approaches. MUX. 1) All traffic on 50 kps line 2) 12.5 kbps dedicated to each source. MUX.

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Basic Model

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  1. BasicModel MEDIUM Distributed Sources Fig.13 Generic Model for Media Access Systems

  2. Message Delay - FDMA

  3. Example FDMA 50 kbps line - 1000 bit frames to be transmitted from four sources Two Approaches MUX 1) All traffic on 50 kps line 2) 12.5 kbps dedicated to each source MUX MUX MUX MUX

  4. Average delay(sec) Four 12.5 kbps lines One 50 kbps lines Load

  5. Continuation FDMA Average delay(sec) L=64 L=16 L=4 L=1 Load

  6. Message Delay - TDMA

  7. L=64 L=16 L=4

  8. Cycle time Delay

  9. Assumption: Total lineflow , L, Poisson P(Collision)= Total traffic New traffic Retransmitted traffic Define R=LM, total loadand r=lM, load due to new traffic

  10. P(Collision)= Total traffic New traffic Retransmitted traffic Define R=LM, total loadand r=lM, load due to new traffic

  11. Delay Analysis

  12. Pure ALOHA Average Delay/Retransmission Interval Slotted ALOHA r

  13. Carrier Sense Multiple Access • Collision avoided by sensing line for other users • Three strategies: • 1-persistent - Transmit when line sensed idle • p-persistent - Transmit with probability p when • line sensed idle • nonpersistent - random timeout before new attempt • Problem: Two terminals transmit within tseconds of • one another - collision results

  14. Duration of Conflict Terminal A transmits Terminal B transmits 2t-e Contention Interval e Propagation delay - t seconds

  15. nonpersistent CSMA a=0 a=.001 a=.01 round trip delay a= Throughput message transmission time a=.1 a=1 Arrival Rate

  16. Ethernet Protocol CSMA/CD Carrier Sense Multiple Access with Collision Detection line length 1 km t = 5 m sec a = .01 and 10 Mbps rate B=5000 Same technique with optical fiber? R=1 Gbps a=t/(B/R)=1 Reduced efficiency

  17. Polling model appropriate

  18. Tree Search Physical Tree Broadcast Polls Central Processor Bridge 0 1 Logical Tree 0 0 01 10 11 000 001 010 011 100 101 110 111

  19. Example 0 1 0 0 01 10 11 011 Polling - eight steps Probing - four steps Terminal with message General result for light loading Polling - N steps Probing - log N steps Adaptivity

  20. Contention Resolution in Random Access 0 1 0 0 01 10 11 011 101 Adaptivity

  21. CDMA Code Division Multiple Access (pages 271-275) User data #1 User data #1 C H A N N E L Code seq #1 Code seq #1 User data #2 User data #2 Code seq #2 Code seq #2 User data #N User data #N Code seq #N Code seq #N

  22. Two kinds of code sequences Direct sequence TB TC 1/TB =Data rate 1/TC =Chip rate Processing gain= TB /TC Hard to generate “fast” sequences

  23. Frequency Hopping TB TC 1/TB =Data rate 1/TC =Chip rate Processing gain= TB /TC

  24. Orthogonality of Code Sequences Code sequence A Code sequence B

  25. Wavelength Division Multiple Access(WDMA) All optical no electronic bottleneck • Star • Coupler Tuneable Transmitters and/or Receivers

  26. IEEE 802 LANs and MANs Standards

  27. 802.3 - Derived from Ethernet

  28. Electrically-A bus

  29. 10base10 Physical Arrangements Vapire tap coax Core 10base5 Connector 10baseT Core Hub Twisted pairs

  30. Topologies Cable snaking through offices Backbone up an elevator shaft

  31. Tree Repeater Greatest length=2.5 km and four repeaters roundtrip delay=51.2 m seconds = 512 bit times

  32. 802.3 Frame Format Bytes 7 1 2 or 6 2 or 6 2 0-1500 0-46 4 Preamble Destn Addrss Source Addrss Data Pad Cksm Start of frame delimiter Length of data field • Dotting pattern • Addressing - broadcast, multicast, local, global • Minimum frame length - 64 bytes

  33. Binary Exponential Backoff Algorithm slot time=51.2 m seconds = 512 bit times Initial transmission collision retransmission collision retransmission Process continues to ten retransmissions-1023 slots

  34. Efficiency k statons with messages each transmits in a slot with prob P slot = 2t=2line length(km)/propagation speed(km/sec)=2L/c A=P(one successful)=P(only one transmits)=kp(1-p)k-1 If p=1/k A is maximum (dA/dP=0) Amax=e-1 for k>>1 P(j slot in contention interval)=A(1-A)j-1 Average number of slots in a contention interval=1/A Average duration contention interval=2t/A=< 2t e=5.4 t P=average frame transmission time = average length(bits)/line rate(bps)=F/B Efficiency=P/(P+ 2t/A)=1/(1+2BLe/cF)

  35. 802.4 Token Bus Logical ring 75 ohm broadband coax Direction of token motion Speeds - 1,5 and 10 Mbps

  36. Four priority classes - 0(lowest), 2, 4, 6 (highest) Token passing sequence Start Class 6 stations Class 4 stations Class 2 stations Class 0 stations

  37. 802.4 Frame Format Bytes 1 1 1 2 or 6 2 or 6 0-8182 4 1 Preamble Destn Addrss Source Addrss Data Cksm frame control Start of frame delimiter Length of data field end delimiter

  38. 802.5 Token Ring Station Interface

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