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An overview of the OSI model and its layers. It explains the functions of each layer and their responsibilities. The document also covers topics such as layered architecture, peer-to-peer processes, and encapsulation. The learning outcome of this topic is to provide students with the basics of computer networks.
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Contents of Topic: Behrouz A. Forouzan
Learning Outcome: Students get to know about basics of
Computer Network
UNIT No. 1
2-1 LAYERED TASKS
We use the concept of layers in our daily life. As an
example, let us consider two friends who communicate
through postal mail. The process of sending a letter to a
friend would be complex if there were no services
available from the post office.
Topics discussed in this section:
Sender, Receiver, and Carrier
Hierarchy
2. 4
Figure 2.1 Tasks involved in sending a letter
2. 5
ISO is the organization.
OSI is the model.
Note
2. 7
Figure 2.2 Seven layers of the OSI
model
2. 8
Figure 2.4 An exchange using the OSI
model
2.
2-3 LAYERS IN THE OSI MODEL
In this section we briefly describe the functions of each
layer in the OSI model.
Topics discussed in this section:
Physical Layer
Data Link Layer
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application Layer
2.
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
Note
2. 13
physical layer defines the characteristics of the interface
between the devices and the transmission medium. It also
defines the type of transmission medium.
stream of bits (sequence of Os or 1s) with no interpretation.
To be transmitted, bits must be encoded into signals--
electrical or optical. The physical layer defines the type of
encoding (how Os and I s are changed to signals).
Other responsibilities of the Physical layer
Physical topology. The physical topology defines how devices
are connected to make a network. Devices can be connected by
using a mesh topology (every device is connected to every other
device), a star topology (devices are connected through a central
device), a ring topology (each device is connected to the next,
forming a ring), a bus topology (every device is on a common
link), or a hybrid topology (this is a combination of two or more
topologies).
Transmission mode. The physical layer also defines the
direction of transmission between two devices: simplex, half-
duplex, or full-duplex. In simplex mode, only one device can
send; the other can only receive. The simplex mode is a one-way
communication. In the half-duplex mode, two devices can send
and receive, but not at the same time. In a full-duplex (or simply
duplex) mode, two devices can send and receive at the same time.
Other responsibilities of the Physical layer
Figure 2.6 Data link
layer
2. 17
Figure 2.7 Hop-to-hop
delivery
2. 19
Other responsibilities of the data link layer