This document discusses multiple access protocols at the data link layer. It covers random access protocols like ALOHA and CSMA, as well as controlled access protocols including polling, reservation, and token passing. Random access allows any station to transmit at any time by using carrier sensing, collision detection, and random backoff times to avoid collisions. Controlled access requires stations to get permission before transmitting via polling, reservations, or a circulating token.

1. Networking Fundamentals
MULTIPLE ACCESS
12.1

2. Data link layer divided into two functionality-oriented sublayers
12.2

3. Taxonomy of multiple-access protocols
12.3

4. RANDOM ACCESSRANDOM ACCESS
• InIn random accessrandom access oror contentioncontention methods, no stationmethods, no station
is superior to another station and none is assignedis superior to another station and none is assigned
the control over another.the control over another.
• No station permits, or does not permit, anotherNo station permits, or does not permit, another
station to send.station to send.
• At each instance, a station that has data to sendAt each instance, a station that has data to send
uses a procedure defined by the protocol to make auses a procedure defined by the protocol to make a
decision on whether or not to send.decision on whether or not to send.
12.4

5. 12.5

6. Frames in a pure ALOHA network
12.6
1. Each station sends a frame when it has a frame to send. Thus likelihood of collisions.
2. If Ack timed-out, each station waits a random amount of time, TB, before resending.
3. After Kmax (normally = 15) No. of resending attempts, must give up and try later.

7. 12.7

8. Space/time model of the collision in CSMA
12.8
B’s
C’s

9. Vulnerable time in CSMA
12.9

10. Behavior of three persistence methods- What to do if channel busy or idle
12.10
After finding line idle
sends immediately, ie.
with probability 1. Max
chance of collisions
If line idle sends
immediately, else waits
random amount of
time. Reduced
collisions
If line idle sends with
prob p, else waits for
next slot.

11. Flow diagram for three persistence methods
12.11

12. 12.12
Note, quickly terminating damaged frames saves time and bandwidth.

13. Collision of the first bit in CSMA/CD
12.13

14. Collision and abortion in CSMA/CD
12.14
Qn. What about the Minimum Frame Size?

15. 12.15

16. A network using CSMA/CD has a bandwidth of 10 Mbps.
If the maximum propagation time (including the delays in
the devices and ignoring the time needed to send a
jamming signal, as we see later) is 25.6 μs, what is the
minimum size of the frame?
Example
Solution
The frame transmission time is Tfr = 2 × Tp = 51.2 μs.
This means, in the worst case, a station needs to transmit
for a period of 51.2 μs to detect the collision. The
minimum size of the frame is 10 Mbps × 51.2 μs = 512
bits or 64 bytes. This is actually the minimum size of the
frame for Standard Ethernet.
12.16

17. Flow diagram for the CSMA/CD
12.17

18. Energy level during transmission, idleness, or collision
12.18

19. 12.19

20. Timing in CSMA/CA
12.20

21. In CSMA/CA, the IFS (time allowed for a
distant frame to reach another station)
can also be used to define the priority of
a station or a frame.
Note
12.21

22. Contention window is an amount of time
divided into slots; A station ready to
transmit, chooses a random No. of slots
as its wait time.
Note
12.22

23. In CSMA/CA, if the station finds the
channel busy, it does not restart the
timer of the contention;
it stops the timer and restarts it when
the channel becomes idle.
Note
12.23

24. Flow diagram for CSMA/CA
12.24

25. 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.
12.25

26. Reservation Access Method
12.26

27. Select and poll functions in Polling Access Method
12.27

28. 12.28

29. Logical ring and physical topology in token-passing access method
12.29