1. 01204325: Data Communication and Computer Networks
Multiple Access
: Data Communication and Computer Networks
Asst. Prof. Chaiporn Jaikaeo, Ph.D.
Computer Engineering Department
Kasetsart University, Bangkok, Thailand
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

3. Sublayers of Data Link Layer

4. Multiple Access Mechanisms

5. Random Access

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

7. ALOHA Network

8. Frames in Pure ALOHA

9. ALOHA Protocol

10. Example
Calculate possible values of TB when stations on an ALOHA network are a maximum of 600 km apart
Tp = (600 × 103) / (3 × 108) = 2 ms
When K=1, TB  {0ms,2ms}
When K=2, TB  {0ms,2ms,4ms,6ms} :

11. ALOHA: Vulnerable Time

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
Smax = when G= 1/2

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

14. Slotted ALOHA

15. Slotted ALOHA: Vulnerable Time

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
Smax = when G= 1

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

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

19. Collision in CSMA B C

20. CSMA: Vulnerable Time

21. Persistence Methods
What a station does when channel is idle or busy

22. Persistence Methods

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

24. Energy Levels

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 A B C D
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?

27. CSMA/CD: Flow Diagram

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

29. contention window size is 2K-1
CSMA/CA: Flow Diagram
contention window size is 2K-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

32. Reservation Method

33. Polling Method

34. Token Passing

35. Channelization

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

37. FDMA

38. TDMA

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

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

41. CDMA: Bit Representation

42. Transmission in CDMA

43. CDMA Encoding

44. Signal Created by CDMA

45. CDMA Decoding

46. Sequence Generation Common method: Walsh Table
Number of sequences is always a power of two

47. Example: Walsh Table
Find chip sequences for eight stations

48. Example: Walsh Table
There are 80 stations in a CDMA network. What is the length of the sequences generated by Walsh Table?