Multiple Access Protocols allow multiple devices to share a common communication channel efficiently. They prevent collisions and manage access. Types include Random Access (e.g., ALOHA, CSMA), Controlled Access (Polling, Token Passing), and Channelization (FDMA, TDMA, CDMA). Each method ensures fair, organized, and reliable data transmission in networks.

1. 12.1
Chapter 12
Multiple Access
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2. 12.2

3. Multiple Access
Broadcast link used in LAN consists of multiple sending and receiving nodes
connected to or use a single shared link
Broadcast links Examples

4. Figure 12.1 Data link layer divided into two functionality-oriented sublayers
Link Layer Control (LLC)
MAC
Responsible for error and flow control
Control
Responsible framing and MAC address
and Multiple Access Control

5.  We can consider the data link layer as two sublayers.
 The upper sub layer is responsible for data link control, and the
lower sublayer is responsible for resolving access to the
shared media.
 The upper sublayer that is responsible for flow and error control
is called the logical link control (LLC) layer;
 the lower sublayer that is mostly responsible for multiple access
resolution is called the media access control (MAC) layer.
 When nodes or stations are connected and use a common link,
called a multipoint or broadcast link, we need a multiple-access
protocol to coordinate access to the link.
12.5

6. Multiple Access
 Problem: When two or more nodes transmit at the same time, their
frames will collide and the link bandwidth is wasted during collision
 How to coordinate the access of multiple sending/receiving nodes
to the shared link???
 Solution: We need a protocol to coordinate the transmission of the
active nodes
 These protocols are called Medium or Multiple Access Control
(MAC) Protocols belong to a sublayer of the data link layer called
MAC (Medium Access Control)
 What is expected from Multiple Access Protocols:
 Main task is to minimize collisions in order to utilize the bandwidth by:

Determining when a station can use the link (medium)

what a station should do when the link is busy

what the station should do when it is involved in collision

7. 12.7
Figure 12.1 Data link layer divided into two functionality-oriented sublayers

8. 12.8
Figure 12.2 Taxonomy of multiple-access protocols discussed in this chapter

9. 12.9
12-1 RANDOM ACCESS
12-1 RANDOM ACCESS
In
In random access
random access or
or contention
contention methods, no station is
methods, no station is
superior to another station and none is assigned the
superior to another station and none is assigned the
control over another. No station permits, or does not
control over another. No station permits, or does not
permit, another station to send. At each instance, a
permit, another station to send. At each instance, a
station that has data to send uses a procedure defined
station that has data to send uses a procedure defined
by the protocol to make a decision on whether or not to
by the protocol to make a decision on whether or not to
send.
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:

10. RANDOM ACCESS
RANDOM ACCESS
 Two features give this method its name. First, there is no
scheduled time for a station to transmit. Transmission is
random among the stations. That is why these methods are
called random access.
 Second, no rules specify which station should send next.
Stations compete with one another to access the medium. That
is why these methods are also called contention methods.
 In a random access method, each station has the right to access
the medium without being controlled by any other station.
However, if more than one station tries to send, there is an
access conflict-collision-and the frames will be either
destroyed or modified.
 So we need Multiple ACCESS protocols.
 A B C D
12.10

11.  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.
12.12

12. PURE ALOHA
12.13

13. 12.14

14. 12.15
Figure 12.3 Frames in a pure ALOHA network

15. 12.16
Figure 12.4 Procedure for pure ALOHA protocol

16.  A collision involves two or more stations. If all these
stations try to resend their frames after the time-out, the
frames will collide again.
 Pure ALOHA dictates that when the time-out period
passes, each station waits a random amount of time
before resending its frame. The randomness will help
avoid more collisions. We call this time the back-off time TB
 The back-off time TB is a random value that normally
depends on K (the number of attempted
unsuccessful transmissions).
 A B C D  TB
12.17

17.  In this binary exponential back-off method, for each
retransmission, a multiplier in the range 0 to 2
K
- 1 is
randomly chosen and multiplied by Tp (maximum
propagation time) or Tfr (the average time required to
send out a frame) to find TB
 Note that in this procedure, the range of the random
numbers
increases after each collision. The value of Kmax is usually
chosen as 15.
12.18

18. Vulnerable time for pure ALOHA protocol
12.19

19. PURE ALOHA
12.20
The throughput ( S) for pure ALOHA is
S = G × e
−2G
.
The maximum throughput
Smax = 0.184 when G= (1/2).
G = Average number of frames generated by the system (all stations) during one
frame transmission time

20.  if one-half a frame is generated during one frame
transmission time (in other words, one frame during two
frame transmission times), then 18.4 percent of these
frames reach their destination successfully.
 This is an expected result because the vulnerable time is 2
times the frame transmission time.
 Therefore, if a station generates only one frame in this
vulnerable time (and no other stations generate a frame
during this time), the frame will reach its destination
successfully
12.21

21. Slotted ALOHA
 Time is divided into slots equal to a frame transmission time (Tfr)
 A station can transmit at the beginning of a slot only
 If a station misses the beginning of a slot, it has to wait until the
beginning of the next time slot.
 A central clock or station informs all stations about the start of a
each slot
12.22

22. SLOTTED ALOHA
12.23

23. 12.24

24. 12.25
Figure 12.6 Frames in a slotted ALOHA network

25. 12.26
Figure 12.7 Vulnerable time for slotted ALOHA protocol

26. 12.27
 The throughput for slotted ALOHA is
S = G × e−G
.
 The maximum throughput
Smax = 0.368 when G = 1.

27.  In other words, if a frame is generated during one frame
transmission time, then 36.8 percent of these frames
reach their destination successfully.
 This result can be expected because the vulnerable time is
equal to the frame transmission time.
 Therefore, if a station generates only one frame in this
vulnerable time (and no other station generates a frame
during this time), the frame will reach its destination
successfully.
12.28

28. PURE ALOHA vs SLOTTED ALOHA
12.29

29. 12.30
Figure 12.9 Vulnerable time in CSMA

30. 12.31
Figure 12.10 Behavior of three persistence methods

31. 12.32
Figure 12.11 Flow diagram for three persistence methods

32. 12.33
Figure 12.12 Collision of the first bit in CSMA/CD

33. 12.34
Figure 12.13 Collision and abortion in CSMA/CD

34. 12.35
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 12.5
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.

35. 12.36
Figure 12.14 Flow diagram for the CSMA/CD

36. 12.37
Figure 12.15 Energy level during transmission, idleness, or collision
On a wired network, energy level is almost double during a collision.
This is how a receiver tells if there is a collision.
But on a wireless network, energy level is not that high (barely 5-10%
higher). So with wireless, we need to avoid collisions.

37. 12.38
Figure 12.17 Flow diagram for CSMA/CA
Channel idle? Don’t transmit yet!
Wait IFS time.
Still idle after IFS? Don’t transmit yet!
Now in Contention Window.
Choose random number and wait that
many slots.
Did you wait R slots and all slots were
available? Go ahead, transmit.
Now, wait time-out for a response.

38. 12.39
Figure 12.16 Timing in CSMA/CA

39. 12.40
In CSMA/CA, the IFS can also be used to
define the priority of a station or a
frame.
Note

40. 12.41
12-2 CONTROLLED ACCESS
12-2 CONTROLLED ACCESS
In
In controlled access
controlled access, the stations consult one another
, the stations consult one another
to find which station has the right to send. A station
to find which station has the right to send. A station
cannot send unless it has been authorized by other
cannot send unless it has been authorized by other
stations. We discuss three popular controlled-access
stations. We discuss three popular controlled-access
methods.
methods.
Reservation
Polling
Token Passing
Topics discussed in this section:
Topics discussed in this section:

41. 12.42
Figure 12.18 Reservation access method

42. 12.43
Figure 12.19 Select and poll functions in polling access method

43. 12.44
Figure 12.20 Logical ring and physical topology in token-passing access method

44. 12.45
12-3 CHANNELIZATION
12-3 CHANNELIZATION
Channelization
Channelization is a multiple-access method in which
is a multiple-access method in which
the available bandwidth of a link is shared in time,
the available bandwidth of a link is shared in time,
frequency, or through code, between different stations.
frequency, or through code, between different stations.
In this section, we discuss three channelization
In this section, we discuss three channelization
protocols.
protocols.
Frequency-Division Multiple Access (FDMA)
Time-Division Multiple Access (TDMA)
Code-Division Multiple Access (CDMA)
Topics discussed in this section:
Topics discussed in this section:

45. 12.46
We see the application of all these
methods in Chapter 16 when
we discuss cellular phone systems.
Note

46. 12.47
Figure 12.21 Frequency-division multiple access (FDMA)

47. 12.48
In FDMA, the available bandwidth
of the common channel is divided into
bands that are separated by guard
bands.
Note

48. 12.49
Figure 12.22 Time-division multiple access (TDMA)

49. 12.50
In TDMA, the bandwidth is just one
channel that is timeshared between
different stations.
Note

50. 12.51
In CDMA, one channel carries all
transmissions simultaneously.
Note

51. 12.52
Figure 12.23 Simple idea of communication with code

52. 12.53
Figure 12.24 Chip sequences

53. 12.54
Figure 12.25 Data representation in CDMA

54. 12.55
Figure 12.26 Sharing channel in CDMA

55. 12.56
Figure 12.27 Digital signal created by four stations in CDMA

56. 12.57
Figure 12.28 Decoding of the composite signal for one in CDMA

57. 12.58
Figure 12.29 General rule and examples of creating Walsh tables

58. 12.59
The number of sequences in a Walsh
table needs to be N = 2m
.
Note

59. 12.60
Find the chips for a network with
a. Two stations b. Four stations
Example 12.6
Solution
We can use the rows of W2 and W4 in Figure 12.29:
a. For a two-station network, we have
[+1 +1] and [+1 −1].
b. For a four-station network we have
[+1 +1 +1 +1], [+1 −1 +1 −1],
[+1 +1 −1 −1], and [+1 −1 −1 +1].

60. 12.61
What is the number of sequences if we have 90 stations in
our network?
Example 12.7
Solution
The number of sequences needs to be 2m
. We need to
choose m = 7 and N = 27
or 128. We can then use 90
of the sequences as the chips.

61. 12.62
Prove that a receiving station can get the data sent by a
specific sender if it multiplies the entire data on the
channel by the sender’s chip code and then divides it by
the number of stations.
Example 12.8
Solution
Let us prove this for the first station, using our previous
four-station example. We can say that the data on the
channel
D = (d1 c
⋅ 1 + d2 c
⋅ 2 + d3 c
⋅ 3 + d4 c
⋅ 4).
The receiver which wants to get the data sent by station 1
multiplies these data by c1.

62. 12.63
Example 12.8 (continued)
When we divide the result by N, we get d1 .