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1. Unit 3
Network & Transport
Layers

3. Multiple Access

4. Figure Data link layer divided into two functionality-oriented sublayers

5. Figure Taxonomy of multiple-access protocols discussed in this chapter

6. RANDOM ACCESS
•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
•uses a procedure defined by the protocol to make a
decision on whether or not to send.
ALOHA
Carrier Sense Multiple Access
Carrier Sense Multiple Access with Collision Detection
Carrier Sense Multiple Access with Collision Avoidance

7. ALOHA
 Packet Radio
 When station has frame, it sends
 Station listens (for max round trip time)plus small
increment
 If ACK, fine. If not, retransmit
 If no ACK after repeated transmissions, give up
 Frame check sequence (as in HDLC)
 If frame OK and address matches receiver, send ACK
 Frame may be damaged by noise or by another
station transmitting at the same time (collision)
 Any overlap of frames causes collision
 Max utilization 18%

8. Procedure for ALOHA protocol

9. Pure ALOHA
In pure ALOHA, frames are transmitted at
completely arbitrary times.

10. Pure ALOHA (2)
Vulnerable period for the shaded frame.

11. Slotted ALOHA
 Time in uniform slots equal to frame
transmission time
 Need central clock (or other sync
mechanism)
 Transmission begins at slot boundary
 Frames either miss or overlap totally
 Max utilization 37%

12. CSMA
 Propagation time is much less than transmission time
 All stations know that a transmission has started
almost immediately
 First listen for clear medium (carrier sense)
 If medium idle, transmit
 If two stations start at the same instant, collision
 Wait reasonable time (round trip plus ACK
contention)
 No ACK then retransmit
 Max utilization depends on propagation time (medium
length) and frame length
 Longer frame and shorter propagation gives better utilization

13. Figure Vulnerable time in CSMA

14. Figure Behavior of three persistence methods

15. Figure Flow diagram for three persistence methods

16. Figure Collision of the first bit in CSMA/CD

17. Figure Collision and abortion in CSMA/CD

18. CSMA/CD
 With CSMA, collision occupies medium for
duration of transmission
 Stations listen whilst transmitting
 If medium idle, transmit
 If busy, listen for idle, then transmit
 If collision detected, jam then cease
transmission
 After jam, wait random time then start again
 Binary exponential back off

19. CSMA/CD procedure

20. CSMA/CA
 No collisions
 After finding the line idle, the station waits
an IFG( interframe gap)
 Then waits for a random amount of time
sends the frame and sets timer
 Station waits for an ack from receiver
 If no ack, something is wrong
 Station increments the value of the backoff
parameter, waits for a backoff amount of
time and resenses the line

21. CSMA/CA procedure

22. Figure Timing in CSMA/CA

23. In CSMA/CA, the IFS can also be used to
define the priority of a station or a frame.
Note

24. CONTROLLED ACCESS
• the stations consult one another to find which station
has the right to send
• A station cannot send unless it has been authorized
by other stations.
Reservation
Polling
Token Passing

25. Figure Reservation access method

26. Figure Select and poll functions in polling access method

27. Figure Logical ring and physical topology in token-passing access method

28. CHANNELIZATION
Channelization is a multiple-access method in which
the available bandwidth of a link is shared in time,
frequency, or through code, between different stations.
Frequency-Division Multiple Access (FDMA)
Time-Division Multiple Access (TDMA)
Code-Division Multiple Access (CDMA)

29. Figure Frequency-division multiple access (FDMA)

30. In FDMA, the available bandwidth
of the common channel is divided into
bands that are separated by guard
bands.
Note

31. Figure Time-division multiple access (TDMA)

32. In TDMA, the bandwidth is just one
channel that is timeshared between
different stations.
Note

33. In CDMA, one channel carries all
transmissions simultaneously.
Note

34. Figure Simple idea of communication with code

35. Figure Chip sequences

36. Figure Digital signal created by four stations in CDMA