The OSI Model
A Framework for Communications
David A. Abarca
July 19, 2005
Background
Background
Top to
Bottom
The Layers
• Open System Interconnection (OSI) Model:
– Was developed in 1984 by the International
Application
Presentation
Session
Organization for Standardization (ISO), a group that
provides a worldwide common language for many
Transport
Network
Data Link
technology industries,
– Is only a framework for communications between
Physical
computers, not a method of communications.
Summary
Background
Background
Top to
Bottom
The Layers
Application
Presentation
Session
•
Open System Interconnection (OSI) Model
– The Model defines the separate functions necessary to process
data communications between devices.
– The Model uses 7 separate layers, one layer to represent each
function.
Transport
– The layers are stacked on top of each other and pass data from
Network
Data Link
Physical
Summary
the top of the “stack” to the bottom.
– When the data reaches the bottom of the “stack”, the data is
transferred across a cable, or other media, and then back up the
“stack” to the other device.
Communications from Top to Bottom
Background
Top to
Bottom
• Each layer performs a specific network communication
function which allows devices, such as computers, to
The Layers
communicate with other devices on a network.
Application
Presentation
Session
Transport
Network
Data Link
• Application Layer
All
• Presentation Layer
People
• Session Layer
Seem
• Transport Layer
To
• Network Layer
Need
• Data Link Layer
Data
• Physical Layer
Processing
Physical
Summary
Communications from Top to Bottom
Background
Top to
Bottom
The Layers
Application
• The 7 layers are divided into two groups:
– The Upper layers
• Are implemented by software.
Presentation
Session
Transport
Network
• Address the user’s ability to interact with other
applications.
– The Lower layers
Data Link
• Are implemented by both software and hardware.
Physical
Summary
• Handle the transport of data over the network including
the physical medium, or cable (or wireless medium).
Communications from Top to Bottom
Background
Top to
Bottom
The Layers
Application
Presentation
Session
Transport
Network
Data Link
Physical
Summary
Information (data) is packaged by adding a header and, in some cases a
trailer, then the package is handed off to the next layer where the
process is repeated.
User A
User B
H Data T
H Data T
H A Layer Data T
H
Data
T
H
P Layer Data
H
Data
H
S Layer Data
Data
H
H
T Layer Data
H
Data
H
N Layer Data
H
Data
DL Layer Data
T
T
T
T
T
T
T
T
After the data is transmitted and received, the process is reversed and the
headers and trailers are stripped off the lower layer data package and
handed up to the next layer where the process is repeated.
Application Layer
Background
Top to
Bottom
The role of the Application Layer is to provide services that allow software
applications (Word, Excel, Outlook, etc.) to transfer data (files), email, web
pages, and other network software on the network.
The Layers
Application
Interface with the user’s applications
Uses protocols including TELNET, HTTP, FTP
Presentation
Session
Transport
Network
Data Link
Physical
Summary
One Data File
Presentation Layer
Background
Top to
Bottom
The role of the Presentation Layer is to transform data into the form that
the application layer can accept. This layer converts, compresses, formats
and encrypts data to be sent across a network
The Layers
Application
Presentation
Provides Translation and Encryption
Specs. defined include: GIF, TIFF, JPEG, and ASCII.
Session
Transport
Network
Data Link
Physical
Summary
One Data File
Session Layer
Background
Top to
Bottom
The role of the Session Layer is to establish, maintain and terminate
connections, or “sessions”, between application. The Session Layer also
reports error in data transmission to the other upper layers.
The Layers
Application
Presentation
Session
Provides logon and password services
and provides error checking
Transport
Network
Data Link
Physical
Summary
One Data File
Transport Layer
Background
Top to
Bottom
The Layers
The role of the Transport Layer is to convert the data from the upper layers
into segments. The Transport Layer is responsible for delivering ALL the
segments to ensure the complete message is sent. Error checking is a
major responsibility of this layer and helps to ensure complete data transfer.
Application
Presentation
Session
Transport
Reliability and Error Checking
Protocols: TCP, UDP, NETBEUI, SPX.
Network
Data Link
Physical
Summary
One
Segments
Data File
Network Layer
Background
Top to
Bottom
The Layers
The role of the Network Layer is to convert the segments from the
Transport Layer into datagrams without regard to their contents.
Creation of logical paths, routing and delivery across a network using
Layer 3 Switches and Routers are also functions of this layer.
Application
Presentation
Session
Transport
Network
Software Addressing,
IP, IPX, AppleTalk, ICMP, X.25
Data Link
Physical
Summary
Datagrams
Segments
Data Link Layer
Background
Top to
Bottom
The Layers
The role of the Data Link Layer is to encode and decode the data packets
from the Network Layer into bits, and is responsible for reassembling bits
taken of the wire. This layer makes sure they are in the correct order
and requests retransmission of the data in case an error occurs.
Application
Presentation
Session
Transport
Network
Data Link
Hardware Addressing
NICs, Bridges, Hubs
Physical
Summary
Data
Datagrams
Packets
Physical Layer
Background
Top to
Bottom
The role of the Physical Layer is to transport the bits of data (1s and 0s)
across the media used for a particular network as electrical or optical
impulses, or as radio signals.
The Layers
Application
Presentation
Session
Transport
Network
Data Link
Physical
Summary
Wire, Wireless
Fiber-Optics
1s and 0s
Data Packets
Summary
Background
Top to
Bottom
The Layers
The OSI Model provides the framework for communications by using the
User A concept of 7 stacked layers to handle the necessary
Application
network communication functions
Presentation
Session
Transport
to accomplish the task
of transferring data
Network
across a
Data Link
Physical
network.
Summary
Questions ?
User B