The document discusses random access protocols for wireless networks including pure ALOHA and slotted ALOHA, describing how frames are transmitted and the vulnerable time periods that can cause collisions. It also covers carrier sense multiple access (CSMA)

1. 12.1
Multiple Access
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2. 12.2
Data link layer divided into two functionality-oriented sublayers

3. 12.3
Taxonomy of multiple-access protocols discussed in this chapter

4. 12.4
RANDOM ACCESSRANDOM ACCESS
InIn random accessrandom access oror contentioncontention methods, no station ismethods, no station is
superior to another station and none is assigned thesuperior to another station and none is assigned the
control over another. No station permits, or does notcontrol over another. No station permits, or does not
permit, another station to send. At each instance, apermit, another station to send. At each instance, a
station that has data to send uses a procedure definedstation that has data to send uses a procedure defined
by the protocol to make a decision on whether or notby the protocol to make a decision on whether or not
to send.to send.
ALOHA
Carrier Sense Multiple Access
Carrier Sense Multiple Access with Collision Detection
Carrier Sense Multiple Access with Collision Avoidance
Topics discussed in this section:Topics discussed in this section:

5. 12.5
To avoid access conflict or to resolve it when itTo avoid access conflict or to resolve it when it
happens, each station follows a procedure that answerhappens, each station follows a procedure that answer
the following questions:the following questions:
 When can the station access the medium.When can the station access the medium.
What can the station do if the medium is busy.What can the station do if the medium is busy.
How can the station determine the success or failureHow can the station determine the success or failure
of the transmission.of the transmission.
What can the station do if there is an access conflict.What can the station do if there is an access conflict.

6. 12.6
ALOHA
ALOHA, the earliest random access method, was developed at the University of Hawaii
in early 1970. It was designed for a radio (wireless) LAN, but it can be used on any
shared medium.
Pure ALOHA
Slotted ALOHA

7. 12.7
Frames in a pure ALOHA network

8. 12.8
Procedure for pure ALOHA protocol

9. 12.9
Vulnerable time for pure ALOHA protocol
The Vulnerable time, in which there is possibility of collision

10. 12.10
Slotted ALOHA

11. 12.11
Frames in a slotted ALOHA network

12. 12.12
Figure 12.7 Vulnerable time for slotted ALOHA protocol

13. 12.13
CSMA
Carrier Sense Multiple Access

14. 12.14
Space/time model of the collision in CSMA

15. 12.15
Figure 12.9 Vulnerable time in CSMA

16. 12.16
Persistence Method
What Should a station do if the channel is busy.
What should a station do if the channel is idle.
1-Persistence Method
Non Persistence Method
p-Persistence Method

17. 12.17
Behavior of three persistence methods

18. 12.18
Flow diagram for three persistence methods

19. 12.19
Collision of the first bit in CSMA/CD

20. 12.20
Collision and abortion in CSMA/CD

21. 12.21
Flow diagram for the CSMA/CD

22. 12.22
Figure 12.15 Energy level during transmission, idleness, or collision

23. 12.23
Figure 12.16 Timing in CSMA/CA
Interframe Space (IFS)

24. 12.24
In CSMA/CA, the IFS can also be used to
define the priority of a station or a
frame.
Note

25. 12.25
In CSMA/CA, if the station finds the
channel busy, it does not restart the
timer of the contention window;
it stops the timer and restarts it when
the channel becomes idle.
Note

26. 12.26
Figure 12.17 Flow diagram for CSMA/CA

27. 12.27
CONTROLLED ACCESSCONTROLLED ACCESS
InIn controlled accesscontrolled access, the stations consult one another, the stations consult one another
to find which station has the right to send. A stationto find which station has the right to send. A station
cannot send unless it has been authorized by othercannot send unless it has been authorized by other
stations. We discuss three popular controlled-accessstations. We discuss three popular controlled-access
methods.methods.
Reservation
Polling
Token Passing
Topics discussed in this section:Topics discussed in this section:

28. 12.28
Logical ring and physical topology in token-passing access method

29. Computer Science & Engineering
Assignment
Semester- VI (I & II) Branch: CSE
Subject with Code: Computer Networks (IT-305-F)
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Q:1 Explain Lacal Area Network.
Q:2 Explain IEEE 802 standards in Detail.