1. UNIT 3 MULTIPLE ACCESS Adapted from lecture slides by Behrouz A. Forouzan © The McGraw-Hill Companies, Inc. All rights reserved

2. OUTLINE Multiple access mechanisms Random access Controlled access Channelization 2

3. SUBLAYERS OF DATA LINK LAYER 3

4. MULTIPLE ACCESS MECHANISMS 4

5. RANDOM ACCESS

6. Also called contention-based access No station is assigned to control another 6

7. ALOHA NETWORK 7

8. FRAMES IN PURE ALOHA 8

9. ALOHA PROTOCOL 9

10. EXAMPLE Calculate possible values of T B when stations on an ALOHA network are a maximum of 600 km apart T p = (600 × 10 3 ) / (3 × 10 8 ) = 2 ms When K=1, T B  {0ms,2ms} When K=2, T B  {0ms,2ms,4ms,6ms} : 10

11. ALOHA: VULNERABLE TIME 11

12. ALOHA: THROUGHPUT Assume number of stations trying to transmit follow Poisson Distribution The throughput for pure ALOHA is S = G × e −2G where G is the average number of frames requested per frame-time The maximum throughput  S max = 0.184 when G= 1/2 12

13. EXAMPLE A pure ALOHA network transmits 200-bit frames on a shared channel of 200 kbps. What is the throughput if the system (all stations together) produces  1000 frames per second  500 frames per second  250 frames per second 13

14. SLOTTED ALOHA 14

15. SLOTTED ALOHA: VULNERABLE TIME 15

16. SLOTTED ALOHA: THROUGHPUT The throughput for Slotted ALOHA is S = G × e −G where G is the average number of frames requested per frame-time The maximum throughput  S max = 0.368 when G= 1 16

17. EXAMPLE A Slotted ALOHA network transmits 200-bit frames on a shared channel of 200 kbps. What is the throughput if the system (all stations together) produces  1000 frames per second  500 frames per second  250 frames per second 17

18. CSMA Carrier Sense Multiple Access  "Listen before talk" Reduce the possibility of collision  But cannot completely eliminate it 18

19. COLLISION IN CSMA 19 B C

20. CSMA: VULNERABLE TIME 20

21. PERSISTENCE METHODS What a station does when channel is idle or busy 21

22. PERSISTENCE METHODS 22

23. CSMA/CD Carrier Sense Multiple Access with Collision Detection Station monitors channel while sending a frame 23

24. ENERGY LEVELS 24

25. CSMA/CD: MINIMUM FRAME SIZE Each frame must be large enough for a sender to detect a collision Worst case scenario:  "A" is transmitting  "D" starts transmitting just before A's signal arrives 25 ABCD Long enough to hear colliding signal from D

26. EXAMPLE A CSMA/CD network has a bandwidth of 10 Mbps. If the maximum propagation time is 25.6 μs, what is the minimum size of the frame? 26

27. CSMA/CD: FLOW DIAGRAM 27

28. CSMA/CA Carrier Sense Multiple Access with Collision Avoidance Used in a network where collision cannot be detected  E.g., wireless LAN 28 IFS – Interframe Space

29. CSMA/CA: FLOW DIAGRAM 29 contention window size is 2 K -1 After each slot: - If idle, continue counting - If busy, stop counting

30. CONTROLLED ACCESS

31. CONTROL ACCESS A station must be authorized by someone (e.g., other stations) before transmitting Three common methods:  Reservation  Polling  Token passing 31

32. RESERVATION METHOD 32

33. POLLING METHOD 33

34. TOKEN PASSING 34

35. CHANNELIZATION

36. Similar to multiplexing Three schemes  Frequency-Division Multiple Access (FDMA)  Time-Division Multiple Access (TDMA)  Code-Division Multiple Access (CDMA) 36

37. FDMA 37

38. TDMA 38

39. CDMA One channel carries all transmissions at the same time Each channel is separated by code 39

40. CDMA: CHIP SEQUENCES Each station is assigned a unique chip sequence Chip sequences are orthogonal vectors  Inner product of any pair must be zero With N stations, sequences must have the following properties:  They are of length N  Their self inner product is always N 40

41. CDMA: BIT REPRESENTATION 41

42. TRANSMISSION IN CDMA 42

43. CDMA ENCODING 43

44. SIGNAL CREATED BY CDMA 44

45. CDMA DECODING 45

46. SEQUENCE GENERATION Common method: Walsh Table  Number of sequences is always a power of two 46