Chapter 12

Multiple Access

12.1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
 Figure 12.1 Data link layer divided into two functionality-oriented sublayers

Types of Links:

Dedicated, Point to Point

Common, Multipoint

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

12.3
 12-1 RANDOM ACCESS

In random access or contention methods, no station is

superior to another station and none is assigned the
control over another. No station permits, or does not
permit, another station to send. At each instance, a
station that has data to send uses a procedure defined
by the protocol to make a decision on whether or not to
send.
Topics discussed in this section:
ALOHA
Carrier Sense Multiple Access
Carrier Sense Multiple Access with Collision Detection
Carrier Sense Multiple Access with Collision Avoidance
12.4
 Collisions

To avoid a collision or to resolve it

when it happens, the used protocol
should answer the following:

 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 a collision?

12.5
 ALOHA
 Multiple Access (MA) procedure
 Designed for a radio (wireless) LAN, but it
can be used on any shared medium.
 When a station sends data, another
station may attempt to do so at the same
time. The data from the two stations
collide and become garbled.

12.6
 Pure ALOHA
 The original ALOHA protocol.
 Each station sends a frame whenever it has a
frame to send.
 The pure ALOHA protocol relies on ACKs from the
receiver to know if the transmission was
successful.
 Resends the frame after a time-out period.
 Each station waits a random amount of time
before resending its frame.
 Back-off time;
Binary exponential back-off
 Maximum no. of transmission attempts
 The randomness will help avoid more collisions.

12.7
 Figure 12.3 Frames in a pure ALOHA network

12.8
 Figure 12.4 Procedure for pure ALOHA protocol

12.9
 Example 12.1

The stations on a wireless ALOHA network are a

maximum of 600 km apart. If we assume that signals
propagate at 3 × 108 m/s, we find
Tp = (600 × 103 ) / (3 × 108 ) = 0.002 s = 2 ms.
Now we can find the value of TB for different values of
K.

a. For K = 1, the range is {0, 1}. The station needs to|

generate a random number with a value of 0 or 1. This
means that TB is either 0 ms (0 × 2) or 2 ms (1 × 2),
based on the outcome of the random variable.

12.10
 Example 12.1 (continued)

b. For K = 2, the range is {0, 1, 2, 3}. This means that TB

can be 0, 2, 4, or 6 ms, based on the outcome of the
random variable.

c. For K = 3, the range is {0, 1, 2, 3, 4, 5, 6, 7}. This

means that TB can be 0, 2, 4, . . . , 14 ms, based on the
outcome of the random variable.

d. We need to mention that if K > 10, it is normally set to

10.

12.11
 Figure 12.5 Vulnerable time for pure ALOHA protocol

12.12
 Example 12.2

A pure ALOHA network transmits 200-bit frames on a

shared channel of 200 kbps. What is the requirement to
make this frame collision-free?

Solution
Average frame transmission time Tfr is 200 bits/200 kbps or
1 ms. The vulnerable time is 2 × 1 ms = 2 ms. This means
no station should send later than 1 ms before this station
starts transmission and no station should start sending
during the one 1-ms period that this station is sending.

12.13
 Note

The throughput for pure ALOHA is

S = G × e −2G .
G: Average no. of frames generated during one frame
transmission time
S: Average no. of successful transmissions

The maximum throughput

Smax = 0.184 when G= (1/2).

12.14
 Example 12.3
A pure ALOHA network transmits 200-bit frames on a
shared channel of 200 kbps. What is the throughput if the
system (all stations together) produces
a. 1000 frames per second b. 500 frames per second
c. 250 frames per second.
Solution
The frame transmission time is 200/200 kbps or 1 ms.
a. If the system creates 1000 frames per second, this is 1
frame per millisecond. The load is 1. In this case
S = G× e−2 G or S = 0.135 (13.5 percent). This means
that the throughput is 1000 × 0.135 = 135 frames. Only
135 frames out of 1000 will probably survive.
12.15
 Example 12.3 (continued)
b. If the system creates 500 frames per second, this is
(1/2) frame per millisecond. The load is (1/2). In this
case S = G × e −2G or S = 0.184 (18.4 percent). This
means that the throughput is 500 × 0.184 = 92 and
that only 92 frames out of 500 will probably survive.
Note that this is the maximum throughput case,
percentagewise.

c. If the system creates 250 frames per second, this is (1/4)

frame per millisecond. The load is (1/4). In this case S =
G × e −2G or S = 0.152 (15.2 percent). This means that the
throughput is 250 × 0.152 = 38. Only 38 frames out of
250 will probably survive.
12.16
 Slotted ALOHA
 Was invented to improve the efficiency of
pure ALOHA.
 The time is divided into slots of Tfr and
the stations are forced to send only at the
beginning of the time slot.
 Collision might occur at the beginning of
the time slot.
 Vulnerable time = Tfr

12.17
 Figure 12.6 Frames in a slotted ALOHA network

12.18
 Note

The throughput for slotted ALOHA is

S = G × e−G .
G: The average no. of frames generated during one
frame transmission time
S: Average no. of successful transmissions

The maximum throughput

Smax = 0.368 when G = 1.

12.19
 Figure 12.7 Vulnerable time for slotted ALOHA protocol

12.20
 Example 12.4
A slotted ALOHA network transmits 200-bit frames on a
shared channel of 200 kbps. What is the throughput if the
system (all stations together) produces
a. 1000 frames per second b. 500 frames per second
c. 250 frames per second.
Solution
The frame transmission time is 200/200 kbps or 1 ms.
a. If the system creates 1000 frames per second, this is 1
frame per millisecond. The load is 1. In this case
S = G× e−G or S = 0.368 (36.8 percent). This means
that the throughput is 1000 × 0.368 = 368 frames.
Only 386 frames out of 1000 will probably survive.
12.21
 Example 12.4 (continued)
b. If the system creates 500 frames per second, this is
(1/2) frame per millisecond. The load is (1/2). In this
case S = G × e−G or S = 0.303 (30.3 percent). This
means that the throughput is 500 × 0.303 = 151.
Only 151 frames out of 500 will probably survive.

c. If the system creates 250 frames per second, this is (1/4)

frame per millisecond. The load is (1/4). In this case
S = G × e −G or S = 0.195 (19.5 percent). This means
that the throughput is 250 × 0.195 = 49. Only 49
frames out of 250 will probably survive.

12.22
 Carrier Sense Multiple Access (CSMA)

 Sense before Transmit

 The possibility of collision still exists
because propagation delay.

12.23
 Figure 12.8 Space/time model of the collision in CSMA

12.24
 Figure 12.9 Vulnerable time in CSMA

12.25
 Persistence Methods
 Persistence Methods
 What should a station do if the channel is
busy?
 What should a station do if the channel is
idle?

12.26
 Figure 12.10 Behavior of three persistence methods

12.27
 Figure 12.11 Flow diagram for three persistence methods

12.28
 Carrier Sense Multiple Access with
Collision Detection (CSMA/CD)
 CSMA/CD modifies the CSMA algorithm to handle
the collision
 In this method, a station monitors the medium after
it sends a frame to see if the transmission was
successful. If so, the transmission is finished. If,
however, there is a collision, the frame is sent
again.
 Minimum Frame Size
 There is a restriction on the frame size.
 Before sending the last bit of the frame, the sending
station must detect a collision, if any, and abort the
transmission.
 This is because the station, once the entire frame is sent,
does not keep a copy of the frame and does not monitor
the line for collision detection.

Tfr ≥ 2Tp

12.29
  CSMA/CD Flow Diagram
 Energy Level
 The level of energy in a channel can have
three values.
 Zero level -> the channel is idle.
 Normal level -> a station has successfully
captured the channel and is sending its frame.
 Abnormal level -> there is a collision and the
level of energy is twice the normal level.
 A station that has a frame to send or is
sending a frame needs to monitor the energy
level to determine if the channel is idle, busy,
or in collision mode.

12.30
 Throughput
 The throughput of CSMA/CD is greater
than that of pure or slotted ALOHA.
 The maximum throughput occurs at a
different value of G and is based on the
persistence method and the value of p in
the p-persistent approach.

12.31
 CSMA/CD
Operation

12.32
 Figure 12.12 Collision of the first bit in CSMA/CD

12.33
 Figure 12.13 Collision and abortion in CSMA/CD

12.34
 Example 12.5

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?
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.
12.35
 Figure 12.14 Flow diagram for the CSMA/CD

12.36
 Figure 12.15 Energy level during transmission, idleness, or collision

12.37
 Carrier Sense Multiple Access with
Collision Avoidance (CSMA/CA)
 In wireless networks, Collisions can not be
detected effectively. Why?
 Collision is avoided through the use of
three strategies:
 The Interframe Space (IFS)
 When an idle channel is found, the station waits for a period
of time called IFS.
 The Contention Window
 Amount of time divided into slots.
 A station that is ready to send chooses a random number of
slots as its wait time.
 Number of slots -> binary exponential back-off strategy.

12.38
 Acknowledgments

 There still may be a collision resulting in

destroyed data.
 In addition, the data may be corrupted
during the transmission.
 To guarantee that the receiver has
received the frame.

12.39
 Figure 12.16 Timing in CSMA/CA

12.40
 Note

In CSMA/CA, the IFS can also be used to

define the priority of a station or a
frame.

12.41
 Note

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.

12.42
 Figure 12.17 Flow diagram for CSMA/CA

12.43
 12-2 CONTROLLED ACCESS

In 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. We discuss three popular controlled-access
methods.

Topics discussed in this section:

Reservation
Polling
Token Passing

12.44
 Reservation
 A station needs to make a reservation
before sending data

 Time is divided into intervals

 In each interval, reservation frame

precedes the data frames

12.45
 Figure 12.18 Reservation access method

12.46
 Polling
 Topologies with one primary station and
many secondary stations
 The primary device is always the initiator
of a session
 Select and Poll functions
 If the primary wants to receive data, it asks
the secondaries if they have data to send
(Poll).
 If the primary want to send data, it tells the
secondary to get ready to receive (Select).
12.47
 Figure 12.19 Select and poll functions in polling access method

12.48
 Token Passing
 Logical Ring topology; predecessor vs.
successor
 Token control packet
 Token management:
 Stations must be limited in the time they can have
possession of the token.
 The token must be monitored to ensure it has not
been lost or destroyed.
 Assigning priorities to the stations and to types of
data being transmitted.

12.49
Figure 12.20 Logical ring and physical topology in token-passing access method

12.50
 12-3 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.
In this section, we discuss three channelization
protocols.

Topics discussed in this section:

Frequency-Division Multiple Access (FDMA)
Time-Division Multiple Access (TDMA)
Code-Division Multiple Access (CDMA)

12.51
 Figure 12.21 Frequency-division multiple access (FDMA)

12.52
 Note

In FDMA, the available bandwidth

of the common channel is divided into
bands that are separated by guard
bands.

12.53
 Figure 12.22 Time-division multiple access (TDMA)

12.54
 Note

In TDMA, the bandwidth is just one

channel that is timeshared between
different stations.

12.55
 Note

In CDMA, one channel carries all

transmissions simultaneously.

12.56
 Figure 12.23 Simple idea of communication with code

12.57