In the earliest era of networking, computers could usually communicate only with computers, which were from the similar manufacturer. The purpose of OSI model is to show how to facilitate communication between different systems without requiring changes to its underlying Hardware and Software.

1. OSI MODEL
Dr R Jegadeesan Prof & Head
Jyothishmathi Institute of Technology and Science,
karimnagar
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2. • In the earliest era of networking, computers
could usually communicate only with
computers, which were from the similar
manufacturer.
• The purpose of OSI model is to show how to
facilitate communication between different
systems without requiring changes to its
underlying Hardware and Software.
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3. • This model was developed by International
Standard Organization (ISO) in 1970’s
• OSI had two major components
➢ the Basic Reference Model or seven-layer
model
➢ set of specific protocols
• It is called as open system interconnection
because it is a system which is open for
communication with other systems
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4. 7 Layers of OSI model
• Layer 7. Application Layer
• Layer 6. Presentation Layer
• Layer 5. Session Layer
• Layer 4. Transport Layer
• Layer 3. Network Layer
• Layer 2. Data Link Layer
• Layer 1. Physical Layer
• PDNTSPA (Please Do Not Throw Sauce
Pizza Away)
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5. 5

6. • OSI model consists of 7 layers which is corresponds
to 3 sub groups
• First 3 layers---Physical, Data, Network layers are
network support layers
• They perform
➢physical connection
➢communication
➢transport timing from one device to another device
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7. • The last three layers i.e session,presentation
application layers are user support layers
• They perform
➢Interoperability between unrelated software
systems
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8. • The fourth layer called Transport layer
ensures that the transmitted data from lower
layers is in the required form as upper layers
need.
➢The upper three layers are implemented in
software.
➢The lower three layers are a combination of
hardware and software.
➢Physical layer is almost hardware.
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9. Physical layer
• The physical layer is responsible for movements
of individual bits from one hop (node) to the
next.
• The physical layer coordinates the functions
required to carry a bit stream over a physical
medium.
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10. The physical layer is also concerned with the
following:
• Physical characteristics of interfaces and
medium
• Representation of bits
• Data rate
• Line configuration
• Physical topology
• Transmission mode.
• Synchronization of bits
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11. • Physical characteristics of interfaces and
medium:
The physical layer defines the
characteristics of the interface between the
devices and the transmission medium. It also
defines the type of transmission medium
• Representation of bits
The physical layer data
consists of a 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 1 s are changed to signals).
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12. • Data rate
➢ The transmission rate-the number of bits sent each
second-is also defined by the physical layer.
➢ In other words, the physical layer defines the duration
of a bit, which is how long it lasts.
Line configuration
➢ The physical layer is concerned with the connection of
devices to the media.
➢ In a point-to-point configuration, two devices are
connected through a dedicated link.
➢ In a multipoint configuration, a link is shared among
several devices.
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13. • 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.
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14. • Synchronization of bits
• The sender and receiver must use the same
bit rate.
• In other words, the sender and the receiver
clocks must be synchronized.
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15. Data Link Layer
• The data link layer is responsible for moving
frames from one hop (node) to the next.
• The data link layer is often conceptually
divided into two sub layers:
➢ Logical Link Control (LLC)
➢ Media Access Control (MAC)
• It makes the physical layer appear error-free to
the upper layer (network layer)
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16. • Logical link control refers to the functions
required for the establishment and control of
logical links between local devices on a
network.
• Media Access Control (MAC).
This refers to the procedures used
by devices to control access to the network
medium. Since many networks use a shared
medium it is necessary to have rules for
managing the medium to avoid conflicts
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17. • Other responsibilities of the data link layer
include the following
• Framing
• Physical addressing.
• Flow control
• Error control
• Access control
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18. Framing
The data link layer divides the stream of bits received
from the network layer into manageable data units
called frames.
Physical addressing
If frames are to be distributed to different systems on
the network, the data link layer adds a header to the
frame to define the sender and/or receiver of the frame.
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19. • Flow control.
If the rate at which the data are absorbed by the receiver
is less than the rate at which data are produced in the
sender, the data link layer imposes a flow control
mechanism (Stop-and-Wait and Sliding Window) to
avoid overwhelming the receiver.
• Error control
The data link layer adds reliability to the physical layer
by adding mechanisms to detect and retransmit damaged
or lost frames. It also uses a mechanism to recognize
duplicate frames. Error control is normally achieved
through a trailer added to the end of the frame.
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20. • Access control.
When two or more devices are connected to
the same link, data link layer protocols
determines which device has control over the
link at any given time.
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21. Network layer
• The network layer is responsible for the
delivery of individual packets from the source
host to the destination host.
• The data link layer oversees the delivery of
the frames between two systems on the same
network (links), the network layer ensures
that each packet gets from its point of origin
to its final destination.
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22. • If two systems are connected to the same link,
there is usually no need for a network layer.
However, if the two systems are attached to
different networks (links) with connecting
devices between the networks (links), there is
often a need for the network layer to
accomplish source-to-destination delivery.
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23. Other responsibilities of the network layer
include the following
• Logical addressing
• Routing
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24. Logical addressing
➢ The physical addressing implemented by the
data link layer handles the addressing problem
locally.
➢If a packet passes the network boundary, we
need another addressing system to help
distinguish the source and destination systems.
➢ The network layer adds a header to the packet
coming from the upper layer that, among other
things, includes the logical addresses of the
sender and receiver.
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25. Routing
• When independent networks or links are
connected to create internetworks (network of
networks) or a large network, the connecting
devices (called routers or switches) route or
switch the packets to their final destination.
This is called routing.
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26. Transport Layer
• The transport layer is responsible for the delivery
of a message from one process to another.
• A process is an application program running on a
host.
• Application programs include word processors;
database programs; Web browsers; drawing,
paint, and image editing programs; and social
communication application programs
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27. • The network layer supervise source-to-
destination delivery of individual packets, it does
not recognize any relationship between those
packets. It treats each one independently, as
though each piece belonged to a separate
message, whether or not it does.
• The transport layer, on the other hand, ensures
that the whole message arrives intact and in
order, overseeing both error control and flow
control at the source-to-destination level.
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28. Other responsibilities of the transport layer include the
following
• Service-point addressing
• Segmentation and reassembly
• Connection control
• Flow control
• Error control
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29. Service-point addressing
• There may be number of programs running at the
same time on a computer.
• For this purpose, source-to-destination delivery
should also include process-to-process delivery.
• Data must be transferred from a specific process
(running program) on one computer to a specific
process or another computer and to accomplish this,
port address is required.
• Thus, transport layer includes service-point address
(also referred to as port address) in the header.
• Using these port addresses transport layer delivers the
packets to the correct process.
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30. Segmentation and reassembly
• A message is divided into transmittable
segments, with each segment containing a
sequence number.
• These numbers enable the transport layer to
reassemble the message correctly upon
arriving at the destination and to identify and
replace packets that were lost in transmission.
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31. Connection control
• Transport layer provides either connection-
oriented or connectionless service.
• In connection-oriented service, transport layer
establishes a connection with the transport layer
at the destination before transmitting segments
and terminates the connection when all the data
is transferred.
• In connectionless service, transport layer treats
each segment as individual packet and transmits
it to the transport layer at destination.
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32. • Flow control.
Data link layer provides flow control of data
across a single link. Transport layer, on the
other hand is also responsible for flow control
which is performed end to end.
• Error control
Transport layer also performs error checking.
Error control takes place end to end. Transport
layer at the sending machine ensures that the
entire message is delivered at the receiving
machine without an error.
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33. Session layer
• The session layer is responsible for dialog control
and synchronization.
• The session layer is the network dialog controller. It
establishes, maintains, and synchronizes the
interaction among communicating systems.
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34. Specific responsibilities of the session layer
include the following:
• Dialog control
• Synchronization.
➢ Dialog control
The session layer allows two systems to enter
into a dialog (communication process) These
dialogs can take place either in half-duplex
or full duplex mode. It is useful in video
conferencing.
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35. • Synchronization
The session layer allows a process to add
checkpoints, or synchronization points, to a stream
of data.
• For example
If a system is sending a file of 2000 pages, it is
advisable to insert checkpoints after every 100 pages
to ensure that each 100-page unit is received and
acknowledged independently. In this case, if a crash
happens during the transmission of page 523, the
only pages that need to be resent after system
recovery are pages 501 to 523. Pages previous to 501
need not be resent
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36. Presentation Layer
• The presentation layer is responsible for
translation, compression, and encryption.
• So this presentation layer is concerned with
the syntax and semantics of the information
exchanged between two systems
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37. Specific responsibilities of the presentation
layer include the following:
➢Translation
➢Encryption
➢Compression
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38. Translation
• The processes (running programs) in two systems are
usually exchanging information in the form of
character strings, numbers, and so on.
• The information must be changed to bit streams
before being transmitted.
• Because different computers use different encoding
systems, the presentation layer is responsible for
interoperability between these different encoding
methods.
• The presentation layer at the sender changes the
information from its sender-dependent format into a
common format.
• The presentation layer at the receiving machine
changes the common format into its receiver-
dependent format. 38

39. Encryption
• To carry sensitive information, a system must
be able to ensure privacy.
• Encryption means that the sender transforms
the original information to another form and
sends the resulting message out over the
network.
• Decryption reverses the original process to
transform the message back to its original
form.
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40. Compression
• Data compression refers to a process of
encoding data using less number of bits.
• This procedure reduces the number of bits
used to symbolize the information.
• It is important while transmitting multimedia
information such as text, audio and video
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41. Application Layer
• The application layer is responsible for
providing services to the user.
• The application layer enables the user, whether
human or software, to access the network
• It provides user interfaces and support for
services such as electronic mail, remote file
access and transfer, shared database
management, and other types of distributed
information services.
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42. Specific services provided by the application
layer include the following:
• Mail services
• File Transfer, Access, and Management.
• Network Virtual Terminal.
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43. Mail services
• Using this application e-mails can be
forwarded to another device.
File transfer, access, and management
• This application allows a user
• to access files in a remote host.
• to transfer files from a remote computer for use
in the local computer,
• and to manage or control files in a remote
computer locally.
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44. Network Virtual Terminal
• It is a software version. Using this software the
user can log on to a computer that is remotely
connected on the network.
• When the user's computer wants to communicate
with the remote host, it communicates with the
virtual terminal that converts the data in the
intermediary format used by the virtual terminal
and sends it to the remote host.
• After processing the data the remote host replies
to the terminal in the reverse order.
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47. Thank you
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