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1. © 2003, Cisco Systems, Inc. All rights reserved.

2. © 2003, Cisco Systems, Inc. All rights reserved. 2
OSI Reference Model

3. © 2003, Cisco Systems, Inc. All rights reserved. INTRO v1.0a—1-3
ISO / OSI / IOS
ISO : International Organization
for Standardization
OSI : Open System Interconnect
（Reference Model）
IOS : Internetwork Operating System

4. © 2003, Cisco Systems, Inc. All rights reserved. INTRO v1.0a—1-4
The OSI Reference Model
• The ISO created and released the OSI reference
model in 1984 to provide vendors with a set of
standards to ensure greater compatibility and
interoperability between various types of network
technologies.
• The OSI reference model reduces complexity,
standardizes interfaces, facilitates modular
engineering, ensures interoperable technology,
accelerates evolution, and simplifies teaching and
learning.

5. © 2003, Cisco Systems, Inc. All rights reserved. INTRO v1.0a—1-5
The OSI Reference Model
Application
Presentation
Session
Transport
Network
Data Link
Physical
7
6
5
4
3
2
1
• Reduces complexity
• Standardizes interfaces
• Facilitates modular engineering
• Ensures interoperable technology
• Accelerates evolution
• Simplifies teaching and learning

6. © 2003, Cisco Systems, Inc. All rights reserved. INTRO v1.0a—1-6
The Seven Layers of the OSI Model
Network Processes to Applications
• Provides network services to
application processes (such as
electronic mail, file transfer,
and terminal emulation)
Application
Presentation
Session
Transport
Network
Data Link
Physical
7
6
5
4
3
2
1

7. © 2003, Cisco Systems, Inc. All rights reserved. INTRO v1.0a—1-7
The Seven Layers of the OSI Model
Data Representation
• Ensures data is readable by
receiving system
• Formats data
• Structures data
• Negotiates data transfer syntax
for application layer
Network Process to Applications
Application
Presentation
Session
Transport
Network
Data Link
Physical
7
6
5
4
3
2
1

8. © 2003, Cisco Systems, Inc. All rights reserved. INTRO v1.0a—1-8
The Seven Layers of the OSI Model
Application
Presentation
Session
Transport
Network
Data Link
Physical
7
6
5
4
3
2
1
Interhost Communication
• Establishes, manages, and
terminates sessions between
applications
Network Process to Applications
Data Representation

9. © 2003, Cisco Systems, Inc. All rights reserved. INTRO v1.0a—1-9
The Seven Layers of the OSI Model
Application
Presentation
Session
Transport
Network
Data Link
Physical
7
6
5
4
3
2
1
End-to-End Connections
• Handles transportation issues
between hosts
• Ensures data transport reliability
• Establishes, maintains and
terminates virtual circuits
• Provides reliability through fault
detection and recovery
information flow control
Interhost Communication
Network Process to Applications
Data Representation

10. © 2003, Cisco Systems, Inc. All rights reserved. INTRO v1.0a—1-10
The Seven Layers of the OSI Model
Application
Presentation
Session
Transport
Network
Data Link
Physical
7
6
5
4
3
2
1
Data Delivery
• Provides connectivity and path
selection between two host systems
• Routes data packets
• Selects best path to deliver data
Network Process to Applications
Data Representation
End-to-End Connections
Interhost Communication

11. © 2003, Cisco Systems, Inc. All rights reserved. INTRO v1.0a—1-11
The Seven Layers of the OSI Model
Application
Presentation
Session
Transport
Network
Data Link
Physical
7
6
5
4
3
2
1
Access to Media
• Defines how data is formatted for
transmission and how access to the
network is controlled
Network Process to Applications
Data Representation
End-to-End Connections
Data Delivery
Interhost Communication

12. © 2003, Cisco Systems, Inc. All rights reserved. INTRO v1.0a—1-12
The Seven Layers of the OSI Model
Application
Presentation
Session
Transport
Network
Data Link
Physical
7
6
5
4
3
2
1
Binary Transmission
• Defines the electrical, mechanical,
procedural, and functional
specifications for activating,
maintaining, and deactivating the
physical link.
Network Process to Applications
Data Representation
End-to-End Connections
Data Delivery
Access to Media
Interhost Communication

13. © 2003, Cisco Systems, Inc. All rights reserved. INTRO v1.0a—1-13
Data Encapsulation
HDR = header
Sender
Bits
User Data
User Data
User Data
User Data
User Data
User Data
User Data
L2
HDR
L3
HDR
L4
HDR
L5
HDR
L6
HDR
L7
HDR
FCS
L7
HDR
L7
HDR
L7
HDR
L7
HDR
L7
HDR
L6
HDR
L6
HDR
L6
HDR
L6
HDR
L5
HDR
L5
HDR
L5
HDR
L4
HDR
L4
HDR
L3
HDR
Application
Presentation
Session
Transport
Network
Data Link
Physical
7
6
5
4
3
2
1

14. © 2003, Cisco Systems, Inc. All rights reserved. INTRO v1.0a—1-14
Data De-Encapsulation
HDR = header
Receiver
Bits
User Data
User Data
User Data
User Data
User Data
User Data
User Data
L2
HDR
L3
HDR
L4
HDR
L5
HDR
L6
HDR
L7
HDR
FCS
L7
HDR
L7
HDR
L7
HDR
L7
HDR
L7
HDR
L6
HDR
L6
HDR
L6
HDR
L6
HDR
L5
HDR
L5
HDR
L5
HDR
L4
HDR
L4
HDR
L3
HDR
Application
Presentation
Session
Transport
Network
Data Link
Physical
7
6
5
4
3
2
1

15. © 2003, Cisco Systems, Inc. All rights reserved. INTRO v1.0a—1-15
Peer-to-Peer Communication
Receiver
Sender
Network
Bits
Frames
Packets
Segments
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Presentation
Session
Transport
Network
Data Link
Physical

16. © 2003, Cisco Systems, Inc. All rights reserved. INTRO v1.0a—1-16
Application
Transport
Network
Data Link
TCP/IP Model
Physical

17. © 2003, Cisco Systems, Inc. All rights reserved. INTRO v1.0a—1-17
TCP/IP Protocol Stack vs. OSI Model
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Transport
Internet
Network
Access
TCP/IP Protocol Stack OSI Model
Networks
Protocols
Data Flow
Layers
Application
Layers

18. © 2003, Cisco Systems, Inc. All rights reserved. INTRO v1.0a—1-18
Summary
• The OSI reference model reduces complexity,
standardizes interfaces, facilitates modular
engineering, ensures interoperable technology,
accelerates evolution, and simplifies teaching and
learning.
• Each layer of the OSI model has a set of unique
functions. The seven layers of the OSI model are
the application, presentation, session, transport,
network, data-link, and physical layers.
• Encapsulation is the process in which data is
wrapped in a particular protocol header before
network transit.