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1. THE OSI MODEL
(Open Systems Interconnection)
GORVAM SADDAR

2.  The Open Systems InterconnectionThe Open Systems Interconnection ((OSIOSI))
is the layered model for the networkis the layered model for the network
system, Designed bysystem, Designed by ISOISO (( InternationalInternational
Organization StandardizationOrganization Standardization ). OSI). OSI
enables devices in the network toenables devices in the network to
communicate with each other.communicate with each other.

3. The OSI 7-layer Model
All
People
Seem
To
Need
Data
Processing
Away
Pizza
Sausage
Throw
Not
Do
Please
• Defined in 1984 and become an international standard

4. Relationship of OSI layers
Physical
Communication

5. 1. Physical Layer
• Responsible for transmitting
bits from one computer to
another
Example protocols: Ethernet, Token Ring (Physical part)
Appl.
Pres.
Sess.
Tran.
Netw.
Data.
Phys.
Appl.
Pres.
Sess.
Tran.
Netw.
Data.
Phys.
Converts bits into electronic
signals for outgoing messages.
Defines the way in which two or
more devices can be connected
physically.

6. 2. Data Link Layer
• Provides for flow of data over a
single link from one device to
another.
• Packets placed into frames at
this layer
• Provide error-free frame
transfer by acknowledgment
and retransmission
Example protocols: Ethernet, Token Ring
Appl.
Pres.
Sess.
Tran.
Netw.
Data.
Phys.
Appl.
Pres.
Sess.
Tran.
Netw.
Data.
Phys.

7. 3. Network Layer
• Logical address to physical
address translation
For TCP/IP running on Ethernet
Logical address: IP address
158.132.148.99
Physical address: Ethernet address
00 00 E2 15 1A CA
• Determine the route from
source to destination computer
Example protocols: IP, IPX
Appl.
Pres.
Sess.
Tran.
Netw.
Data.
Phys.
Appl.
Pres.
Sess.
Tran.
Netw.
Data.
Phys.

8. 4. Transport Layer
• Ensure packets are delivered
error free, in sequence
• Translate between packets and
message
• Control flow of data and
acknowledges successful receipt
of data.
1 2 3
Sending
Example protocols: TCP, SPX, NWLink
Appl.
Pres.
Sess.
Tran.
Netw.
Data.
Phys.
Appl.
Pres.
Sess.
Tran.
Netw.
Data.
Phys.

9. 5. Session Layer
Example application program: Winsock, UNIX Socket
Appl.
Pres.
Sess.
Tran.
Netw.
Data.
Phys.
Appl.
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Data.
Phys.
• Establishment, maintaining and
release of session
• Provide dialog management
• Regulate which side transmit,
when, for how long (Sync.)
• Provide synchronization
between user tasks

10. 6. Presentation Layer
Example application program: redirector (NT), SSL
Appl.
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Sess.
Tran.
Netw.
Data.
Phys.
Appl.
Pres.
Sess.
Tran.
Netw.
Data.
Phys.
• Translate data from
Application Layer to the
format suitable for session
layer (the network)
• Provide data encryption,
compression
• Changing or converting
character set, graphic, and file
format

11. 7. Application Layer
Example protocols: FTP, Telnet, HTTP
• Entry point for application to
access network
• Directly support user
applications
• E.g. File transfer, email
• General capabilities:
Network access, flow control,
Error recovery
Appl.
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Sess.
Tran.
Netw.
Data.
Phys.
Appl.
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Tran.
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Data.
Phys.

12. Example: File Transfer
PictureTel
PictureTel
FTP ServerFTP Client
Network
• The most traditional network task
• Implemented by a simple Application Layer
protocol called FTP