Download OSI Model

Semester 1
Chapter 2
The O S I Model
(Open System Interconnection)
The OSI Model
• The OSI model describes
how information or data
moves from one
computer or end system
through a network to
another computer or end
system.
• The different layers refer
to information flow.
Why a Layered Network Model ?
Application
Reduces Complexity
Presentation
Standardizes Interfaces
Session
Facilitates Modular Engineering
Transport
Ensures Interoperable Technology
Network
Data Link
Physical
Accelerates Evolution
Simplifies Teaching & Learning
The purpose of the OSI model is to:
• provide a standard way to describe how data
travels on a network.
• replace a number of competing standards
with a single standard.
• divide networks into seven layers and thus
simplify the process.
The OSI Model
7
Application
Network processes to applications
6
Presentation
Data Representation
5
Session
4
Transport
End-to-end connections
3
Network
Addresses and best path
2
Data Link
Access to media
1
Physical
Binary Transmission
Interhost Communication
The OSI Model
7
Application
6
Presentation
5
Session
4
Transport
3
Network
2
Data Link
1
Physical
The application layer
provides network services to
user applications. For
example, a word processing
application is serviced by
file transfer services at this
layer.
The OSI Model
7
Application
6
Presentation
5
Session
4
Transport
3
Network
2
Data Link
1
Physical
This layer provides data
representation and code
formatting. It ensures that
the data that arrives from
the network can be used by
the application, and it
ensures that information
sent by the application can
be transmitted on the
network.
The OSI Model
7
Application
6
Presentation
5
Session
4
Transport
3
Network
2
Data Link
1
Physical
The Presentation layer is
also said to be concerned
with data structures and
negotiation of data transfer
syntax.
The OSI Model
7
Application
6
Presentation
5
Session
4
Transport
3
Network
2
Data Link
1
Physical
This layer establishes,
maintains, and manages
sessions between
applications.
It manages data exchange
between presentation layer
entities.
The OSI Model
7
Application
6
Presentation
5
Session
4
Transport
3
Network
2
Data Link
1
Physical
This layer segments and
reassembles data into a
data stream. This layer
uses the TCP protocol.
The OSI Model
7
Application
6
Presentation
5
Session
4
Transport
3
Network
2
Data Link
1
Physical
This layer determines the
best way to move data from
one place to another. The
router operates at this layer.
This layer uses the Internet
Protocol addressing scheme.
The OSI Model
7
Application
6
Presentation
5
Session
4
Transport
3
Network
2
Data Link
1
Physical
This layer provides
physical transmission
across the medium. It
handles error notification,
physical addressing,
network topology, and
flow control. This layer
uses the Media Access
Control (MAC) address.
The OSI Model
7
Application
6
Presentation
5
Session
4
Transport
3
Network
2
Data Link
1
Physical
This layer provides the
electrical and functional
specifications for activating
and maintaining the physical
link between systems. This
layer uses the physical media
like twisted pair, coaxial, and
fiber-optic cable.
The OSI Model
7
Application
6
Presentation
5
Session
4
Transport
3
Network
2
Data Link
1
Physical
This layer is also said to
provide Data transmission
across the network media.
This is accomplished
through the use of wires,
connectors, and voltages.
The OSI Model
7
Application
6
Presentation
5
Session
4
Transport
3
Network
2
Data Link
1
Physical
Host Layers
Media Layers
The OSI Model - Remembering
order of the Layers:
Application
7 • All
Presentation
6 • People
Session
5 • Seem
Transport
4 • To
Network
3 • Need
Data Link
2 • Dr.
Physical
1 • Pepper
OSI model - Remembering order
of the Layers (Bottom to top):
•
•
•
•
•
•
•
Please
Do
Not
Tell
Sales
People
Anything
•
•
•
•
•
•
•
1 - Physical
2 - Data Link
3 - Network
4 - Transport
5 - Session
6 - Presentation
7 - Application
Comparing the OSI & TCP/IP models
• Similarities:
• Both have layers
• both have application layers,
though they include very
different services
• both have comparable
transport and network layers
• packet-switched (not circuitswitched) technology is
assumed
Comparing the OSI & TCP/IP models
• Differences - TCP/IP:
• Combines the presentation
and session layer issues into
its application layer
• Combines the OSI data link
and physical layers into one
layer (a.k.a. Network Interface)
• Appears simpler because it
has fewer layers
• In the TCP/IP model, the Transport Layer is responsible
for reliability, flow control, and error correction.
Encapsulation
• Encapsulation is
the wrapping of
data in a
particular
protocol header.