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Protocol Reference Model of
OSI
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Introduction to the OSI layer
How OSI was created and why
Comparison with TCP/IP
Layers :
Application layer
Presentation layer
Session layer
Transport layer
Network layer
Data link layer
Physical layer
Conclusion
Resources
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Open Systems Interconnection (OSI)
Each layer support the layers above it and
offers services to the layers below
Each layer performs unique and specific task
A layer only has knowledge of its neighbour
layers only
A layer service is independent of the
implementation
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An attempt for a framework for developing
networking technologies
OSI became a tool for explaining the
Networking in general
Before OSI was created people created their
Software/Hardware as they wanted it to be.
There was not any compatibility. Now OSI is
used as a rule set for all vendors to create
their Software/Hardware by using the
standards.
II OSI Overview
1. OSI - layered framework for
the design of network systems
that allows communication
across all types of computer
systems.
2. The OSI 7 Layers. ( Brief
functional overview. )
3. Vertical and horizontal
communication between the
layers using interfaces. (defines
what information and services
should the layer provide to the
layer above it. )
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Each layer contains a logical grouping of
functions
Each function receive an input(one or more)
and produces an output
I History
1. The need for standardization
- many vendors, no
interoperability
- no common framework
2. ISO and CCITT came up with
OSI (Open System
Intercommunication) in 1984.
3. OSI Protocol Suite –
unaccepted by vendors and
users. (TCP won)
4. OSI – a standard, which allows
communication between
different systems without
requiring changes to the logic
of the underlying hardware and
software.
Layer abstraction and the path of the
message
OSI Overview
4. Data Encapsulation
a) PDU conception – each
protocol on the diff. layer
has its own format.
b) Headers are added
while a packet is going
down the stack at each
layer.
c) Trailers are usually
added on the second
layer.
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Pretty similar to OSI
TCP/IP has less layers(four)
Main difference in layers is after layer 4
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Applications and Services run on it
Enables human network to interface the underlying data network
Applications on that layer (E-mail clients, web browsers, Chats,
etc.) – top-stack applications (As people are on the top of the
stack)
Applications provide people with a way to create message
Application layer services establish an interface to the network
Protocols provide the rules and formats that govern how data is
treated
Protocols on the destination and the host must match
III The OSI Layers
7. Application Layer
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Provides user interfaces and
support for services
Resource sharing and device
redirection
Remote file access
Remote printer access
Inter-process communication
Network management
Directory services
Electronic messaging (such as
mail)
Network virtual terminals
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Coding and conversion of Application layer data to ensure that
data from the source device can be interpreted by the
appropriate application on the destination device.
Compression of the data in a manner that can be decompressed
by the destination device.
Encryption of the data for transmission and the decryption of
data upon receipt by the destination.
This is the layer at which application programmers consider data
structure and presentation
Examples: GIF, JPEG, TIFF, etc.
Sometimes n distinction is made between the presentation and
application layers. For example http/https. HTTP is generally
regarded as an application layer protocol although it has
Presentation layer aspects such as the ability t identify
character encoding for roper conversion
The OSI Layers
6. Presentation Layer
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Translation (connects different
computer systems)
Compression (transmission
efficiency)
Encryption (SSL security)
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Functions at this layer create and maintain
dialogs between source and destination
applications
Authentication
Permissions
Session Restoration (Checkpoint or recovery)
The OSI Layers
5. Session Layer
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Session establishment,
maintenance and termination
(Deciding who sends, and
when.)
Session support (security,
name recognition, logging )
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Tracking the individual communication between applications on
the source and destination hosts
Segmenting data and managing each piece
Reassembling the segments into streams of application data
Identifying the different applications
Conversation Multiplexing
Segments
Connection-oriented conversations
Reliable delivery
Ordered data reconstruction
Flow control
TCP – Web Browser
UDP – Video Streaming Applications
The OSI Layers
4. Transport Layer
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Connectionless and connectionoriented services
Process-Level Addressing
Multiplexing and Demultiplexing
Segmentation, Packaging and
Reassembly
Connection Establishment,
Management and Termination
Acknowledgments and
Retransmissions
Flow Control
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Addressing (IPV4)
Encapsulation (Inserts a header with source
and destination IPs)
Routing (Move a packet over the Internet)
Decapsulation (Open the packet and check
the destination host)
IP is connectionless
The OSI Layers
3. Network Layer
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Logical Addressing
Routing (where the packet is
destinated to)
Datagram Encapsulation
Fragmentation and Reassembly
(handling too big packets )
Error Handling and Diagnostics (
using status messages for
example )
End to end packet delivery
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It is the role of the OSI Data Link layer to prepare Network layer
packets for transmission and to control access to the physical media.
Allows the upper layers to access the media using techniques such as
framing
Controls how data is placed onto the media and is received from the
media using techniques such as media access control and error
detection
Frame - The Data Link layer PDU
Node - The Layer 2 notation for network devices connected to a
common medium
Media/medium - The physical means for the transfer of information
between two nodes
Network - Two or more nodes connected to a common medium
The Data Link layer is responsible for the exchange of frames
between nodes over the media of a physical network.
The OSI Layers
2. Data Link Layer
2.1. Logical Link Control (LLC )
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Establishment and control of
logical links between local
devices on a network.
2.2. Media Access Control (MAC)
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The procedures used by devices
to control access to the network
medium.
Frame sequencing
Frame acknowledgment
Addressing
Frame delimiting
Frame error checking
PDU: frame
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The role of the Physical layer is to encode the
binary digits that represent Data Link layer
frames into signals and to transmit and
receive these signals across the physical
media that connect network devices.
Copper cable
Fiber
Wireless
The OSI Layers
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Physical Layer
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Definition of Hardware
Specifications (of cables,
connectors, wireless radio
transceivers, network interface
cards )
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Encoding and Signaling (bit
representation)
Data Transmission and Reception
(half duplex, full duplex )
Topology and Physical Network
Design (mesh, ring, bus)
PDU: bit
OSI Summary
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The way people learn Networking
A standard for software
A standard for hardware
Seven layers architecture
Each layer independent on the others
Similar to TCP/IP(TCP/IP explained)
OSI is used as a model for developing
network aware applications(Here I mean that
people use its structure to model software)