Ch_12 of media acess protocol and other .ppt

Chapter 12
Media
Access
Control
(MAC)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapter 12: Outline
12.1
12.1 RANDOM ACCESS
RANDOM ACCESS
12.2
12.2 CONTROLLED ACCESS
CONTROLLED ACCESS
12.3
12.3 CHANNELIZATION
CHANNELIZATION

Chapter 12: Objective
 The first section discusses random-access protocols. Four
protocols, ALOHA, CSMA, CSMA/CD, and CSMA/CA, are
described in this section. These protocols are mostly used in
LANs and WANs, which we discuss in future chapters.
 The second section discusses controlled-access protocols. Three
protocols, reservation, polling, and token-passing, are described
in this section. Some of these protocol are used in LANs, but
others have some historical value.
 The third section discusses channelization protocols. Three
protocols, FDMA, TDMA, and CDMA are described in this
section. These protocols are used in cellular telephony, which we
discuss in Chapter 16.

12.4
Figure 12.1: Taxonomy of multiple-access protocols

12.5
12-1 RANDOM ACCESS
12-1 RANDOM ACCESS
In random-access or contention no
station is superior to another station
and
none is assigned control over another. 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. This decision depends on
the state of the medium (idle or busy).

12.6
12.12.1 ALOHA
12.12.1 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. It is obvious
that there are potential collisions in this
arrangement. The medium is shared between the
stations. 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.

12.7
Figure 12.2: Frames in a pure ALOHA network

12.8
Figure 12.3: Procedure for pure ALOHA protocol

12.9
Figure 12.4: Vulnerable time for pure ALOHA protocol

12.10
Figure 12.5: Frames in a slotted ALOHA network

12.11
Figure 12.6: Vulnerable time for slotted ALOHA protocol

12.12
12.12.2 CSMA
12.12.2 CSMA
To minimize the chance of collision and, therefore,
increase the performance, the CSMA method was
developed. The chance of collision can be reduced if
a station senses the medium before trying to use it.
Carrier sense multiple access (CSMA) requires that
each station first listen to the medium (or check the
state of the medium) before sending. In other words,
CSMA is based on the principle “sense before
transmit” or “listen before talk.”

12.13
Figure 12.7: Space/time model of a collision in CSMA

12.14
Figure 12.8: Vulnerable time in CSMA

12.15
Figure 12.9: Behavior of three persistence methods

12.16
Figure 12.10: Flow diagram for three persistence methods

12.17
12.12.3 CSMA/CD
12.12.3 CSMA/CD
The CSMA method does not specify the procedure
following a collision. Carrier sense multiple access
with collision detection (CSMA/CD) augments the
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 station is finished. If, however,
there is a collision, the frame is sent again.

12.18
Figure 12.11: Collision of the first bits in CSMA/CD

12.19
Figure 12.12: Collision and abortion in CSMA/CD

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 minimum frame transmission time is Tfr = 2 × Tp =
512.2 μs. This means, in the worst case, a station needs to
transmit for a period of 512.2 μs to detect the collision. The
minimum size of the frame is 10 Mbps × 512.2 μs = 512 bits
or 64 bytes. This is actually the minimum size of the frame
for Standard Ethernet, as we will see later in the chapter.
Example 12. 5
12.20

12.21
Figure 12.13: Flow diagram for the CSMA/CD

12.22
Figure 12.14: Energy level during transmission, idleness, or collision

12.23
12.12.4 CSMA/CA
12.12.4 CSMA/CA
Carrier sense multiple access with collision
avoidance (CSMA/CA) was invented for wireless
networks. Collisions are avoided through the use of
CSMA/CA’s three strategies: the interframe space,
the contention window, and acknowledgments, as
shown in Figure 12.15. We discuss RTS and CTS
frames later.

12.24
Figure 12.15: Flow diagram for CSMA/CA

12.25
Figure 12.16: Contention window

12.26
Figure 12.17: CSMA/CA and NAV

12.27
12-2 CONTROLLED ACCESS
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 controlled-access methods.

12.28
12.2.1 Reservation
12.2.1 Reservation
In the reservation method, a station needs to make a
reservation before sending data. Time is divided into
intervals. In each interval, a reservation frame
precedes the data frames sent in that interval.

12.29
Figure 12.18: Reservation access method

12.30
12.2.2 Polling
12.2.2 Polling
Polling works with topologies in which one device is
designated as a primary station and the other devices
are secondary stations. All data exchanges must be
made through the primary device even when the
ultimate destination is a secondary device. The
primary device controls the link; the secondary
devices follow its instructions. It is up to the primary
device to determine which device is allowed to use
the channel at a given time.

12.31
Figure 12.19: Select and poll functions in polling-access method

12.32
12.2.3 Token Passing
12.2.3 Token Passing
In the token-passing method, the stations in a
network are organized in a logical ring. In other
words, for each station, there is a predecessor and a
successor. The predecessor is the station which is
logically before the station in the ring; the successor
is the station which is after the station in the ring.

12.33
Figure 12.20: Logical ring and physical topology in token-passing
access method

12.34
12-3 CHANNELIZATION
12-3 CHANNELIZATION
Channelization (or channel partition, as
it is sometimes called) is a multiple-
access
method in which the available
bandwidth of a link is shared in time,
frequency, or
through code, among different stations.
In this section, we discuss three
protocols: FDMA, TDMA, and CDMA.

12.35
12.3.1 FDMA
12.3.1 FDMA
In frequency-division multiple access (FDMA), the
available bandwidth is divided into frequency bands.
Each station is allocated a band to send its data. In
other words, each band is reserved for a specific
station, and it belongs to the station all the time.
Each station also uses a bandpass filter to confine
the transmitter frequencies. To prevent

12.36
Figure 12.21: Frequency-division multiple access (FDMA)

12.37
12.3.2 TDMA
12.3.2 TDMA
In time-division multiple access (TDMA), the
stations share the bandwidth of the channel in time.
Each station is allocated a time slot during which it
can send data. Each station transmits its data in its
assigned time slot. Figure 12.22 shows the idea
behind TDMA.

12.38
Figure 12.22: Time-division multiple access (TDMA)

12.39
12.3.3 CDMA
12.3.3 CDMA
Code-division multiple access (CDMA) was
conceived several decades ago. Recent advances in
electronic technology have finally made its
implementation possible. CDMA differs from FDMA
in that only one channel occupies the entire
bandwidth of the link. It differs from TDMA in that
all stations can send data simultaneously; there is
no timesharing.

12.40
Figure 12.23: Simple idea of communication with code

12.41
Figure 12.24: Chip sequences

12.42
Figure 12.25: Data representation in CDMA

12.43
Figure 12.26: Sharing channel in CDMA

12.44
Figure 12.27: Digital signal created by four stations in CDMA

12.45
Figure 12.28: Decoding of the composite signal for one in CDMA

12.46
Figure 12.29: General rules and examples of creating Walsh tables

Find the chips for a network with
a. Two stations
b. Four stations
Example 12.6
12.47

What is the number of sequences if we have 90 stations in
our network?
Example 12. 7
12.48

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
12.49

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