1. Chapter 13
Multiple Access

2. Figure 13.1 Multiple-access protocols

3. 13.1 Random Access/ Contention
Multiple Access
CSMA
Carrier Sense Multiple Access
CSMA/CD
Carrier Sense Multiple Access with Collision Detection
CSMA/CA
Carrier Sense Multiple Access with Collision Avoidance

4. CSMA/ Collision Detection CSMA/ Collision Avoidance
Evolution of random-access methods
or Contention method
CSMA/ Collision Detection
Media Access
(ALOHA)
Carrier Sense
Multiple Access
CSMA/ Collision Avoidance
No station is superior to another station
None is assigned the control over another
No station permits, or does not permit, another station to send
station that has data to send uses a procedure defined by protocol to make a decision on whether or not to send
The decision depends on the state of medium (idle or busy)

5. MA (Media Access) ALOHA network
When can the station access the medium ?
What can the station do if the medium is busy?
How can the station determine the success or failure of the transmission?
What can the station do if there is an access conflict?

6. Media Access/ ALOHA protocol
Station sends frame whenever it has a frame to send.
there is only one channel to share, there is possibility of collision between frames from different stations
Pure Aloha

7. Procedure for ALOHA protocol
Kmax : Maximum No. of retransmission attemps
Tb : Back-off time
K: backoff = # attempt
K=0
Tb = Random x Tp_max
Random = [0, 1]
Twaiting = 2 x Tp_max
K>Kmax
Tp_max = max.
propagation
time
K=K++

8. Carrier Sense Multiple Access
CSMA
The chance of collision can be reduced
if station sense medium before trying to use it.
CSMA requires that each station first listen to the medium before sending
( “sense before transmit” or “listen before talk” )

9. Persistence Methods What should station do if channel if busy?
What should station do if channel if idle?
Three methods have been devised to answer these questions
Nonpersistent method
1-Persistent method
p-persistent method

10. Persistence strategies

11. Flow diagram for three persistence methods
Highest chance of collision
Reduce efficiency of N/W
Channel has time slot : >= Max. propagation time
Combine advantages of two methods
Reduce chance of collision
improve efficiency
Behavior of three persistence methods

12. Collision
Delay
Delay

13. Collision in CSMA Possibility of collision still exists
because of Propagation delay
Collision in CSMA
CSMA can reduce the possibility of collision, but it cannot eliminate

14. CSMA/CD procedure Collision Detection K=0 Nonpersistence p-persistence
CSMA does not specify procedure following collision
CSMA/CD augments algorithm to handle collision
station monitors medium after sending frame
transmission was successful, station is finished
there is collision, frame is send again
K=0
Tb = Random x Tp_max
Random = [0, 2K - 1]
Nonpersistence
p-persistence
K>Kmax
K=K++
Kmax is normally 15

15. Timing in CSMA/CA
In wireless network, much of sent energy is lost in transmission : not useful for effective collision detection
We need to avoid collisions on wireless network because they cannot be detected
CDMA/CA has three strategies
Interframe space (IFS)
Contention window
Acknowledgement

16. CSMA/CA procedure

17. Control Access
Reservation
Polling
Token Passing

18. Reservation access method

19. Controlled Access Protocol
Polling

20. Polling

21. Selecting

22. Controlled Access Protocol
TOKEN Passing

23. Token-passing network
predecessor
successor
Token

24. Token-passing procedure

25. 13.3 Channelization FDMA: Frequency Division Multiple Access
TDMA: Time Division Multiple Access
CDMA: Code Division Multiple Access

26. Channelization Use Multiplexing Techniques
FDMA: Frequency Division Multiple Access
the bandwidth is divided into channels.
TDMA: Time Division Multiple Access
the bandwidth is just one channel that is timeshared.
CDMA
one channel carries all transmissions simultaneously

27. Frequency-division multiple access (FDMA)

28. Time-division multiple access (TDMA)

29. Channelization
CDMA

30. Simple idea of communication with code

31. Chip sequences

32. Encoding rules

33. CDMA multiplexer

34. Digital signal created by four stations in CDMA

35. CDMA demultiplexer

36. Decoding of the composite signal for one in CDMA

37. PN Sequence Generation (W1 and W2N)
Walsh Table
Walsh Code or Hadamard Matrix

38. Sequence generation
Element W2 W2 W2 W2
N=1
N=2

39. Example 1
Check to see if the second property about orthogonal codes holds for our CDMA example.
Solution
The inner product of each code by itself is N. This is shown for code C; you can prove for yourself that it holds true for the other codes.
C . C = [+1, +1, -1, -1] . [+1, +1, -1, -1] = = 4
If two sequences are different, the inner product is 0.
B . C = [+1, -1, +1, -1] . [+1, +1, -1, -1] = = 0

40. Example 2
Check to see if the third property about orthogonal codes holds for our CDMA example.
Solution
The inner product of each code by its complement is -N. This is shown for code C; you can prove for yourself that it holds true for the other codes.
C . (-C ) = [+1, +1, -1, -1] . [-1, -1, +1, +1] = = -4
The inner product of a code with the complement of another code is 0.
B . (-C ) = [+1, -1, +1, -1] . [-1, -1, +1, +1] = = 0