This document discusses multiple access mechanisms for shared communication channels, including random access methods like ALOHA and slotted ALOHA, and controlled access methods like polling, token passing, and channelization techniques like FDMA, TDMA, and CDMA. It provides examples and explanations of how each technique works, key parameters that impact performance, and calculations related to throughput and other metrics.

1. Multiple Access
01204325: Data Communication
and Computer Networks
Asst. Prof. Chaiporn Jaikaeo, Ph.D.
chaiporn.j@ku.ac.th
http://www.cpe.ku.ac.th/~cpj
Computer Engineering Department
Kasetsart University, Bangkok, Thailand
Adapted from lecture slides by Behrouz A. Forouzan
© The McGraw-Hill Companies, Inc. All rights reserved

2. 2
Outline
 Multiple access mechanisms
 Random access
 Controlled access
 Channelization

3. 3
Sublayers of Data Link Layer

4. 4
Multiple Access Mechanisms

5. Random Access

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

7. 7
ALOHA Network

8. 8
Frames in Pure ALOHA

9. 9
ALOHA Protocol

10. 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. 11
ALOHA: Vulnerable Time

12. 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 = 0.184 when G= 1/2

13. 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. 14
Slotted ALOHA

15. 15
Slotted ALOHA: Vulnerable Time

16. 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 = 0.368 when G= 1

17. 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. 18
CSMA
 Carrier Sense Multiple Access
 "Listen before talk"
 Reduce the possibility of collision
 But cannot completely eliminate it

19. 19
Collision in CSMA
B
C

20. 20
CSMA: Vulnerable Time

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

22. 22
Persistence Methods

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

24. 24
Energy Levels

25. 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. 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. 27
CSMA/CD: Flow Diagram

28. 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. 29
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. 31
Control Access
 A station must be authorized by someone
(e.g., other stations) before transmitting
 Three common methods:
 Reservation
 Polling
 Token passing

32. 32
Reservation Method

33. 33
Polling Method

34. 34
Token Passing

35. Channelization

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

37. 37
FDMA

38. 38
TDMA

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

40. 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. 41
CDMA: Bit Representation

42. 42
Transmission in CDMA

43. 43
CDMA Encoding

44. 44
Signal Created by CDMA

45. 45
CDMA Decoding

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

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

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