OSI Model OSI Model

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Module 5 OSI Model
OSI Model
Understanding Network Basics
Understanding Network Basics
OSI Model
Module 5 OSI Model
Module 5 OSI Model
♦Overview
►It is not possible to view how two computers communicate on a network, but
one can see the model which depicts how communication takes place. The
most common model that depicts communication over a network is the OSI
(Open Systems Communication) Model.
►The OSI model divides network communications into seven layers: Physical,
Data Link, Network, Transport, Session, Presentation, and Application.
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Data Link, Network, Transport, Session, Presentation, and Application.
►The OSI model only represents what happens theoretically between
communicating computers on a network. It does not specify what hardware or
software should be supported at each layer.
►This module deals with the OSI model, its layers, and how data is packaged
into frames and packets while being transmitted in between layers and finally
sent on to the network. Understanding the OSI model is crucial for a
proficient network design and troubleshooting.
♦Lessons covered in this module
►OSI Model and associated Functions.
Module 5 OSI Model
Lesson 5.1 OSI model and their
Functions
♦Introduction
►In the early days, when computers were first connected using a network,
sharing of resources was the primary objective. To perform this, International
Standards Organization (ISO) wanted to design a standard model. This model
can then help the vendors to create the other interpretable devices. As a result
of this, OSI Reference model was released in 1984.The OSI Reference Model
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of this, OSI Reference model was released in 1984.The OSI Reference Model
describes how the data is transmitted from the application layer of the
sender’s computer to the application layer of the receiver’s computer. Each
layer of the OSI Model is described in detail in this lesson.
Module 5 OSI Model
Lesson 5.1 OSI model and their
Functions
♦Topics covered in this lesson are ►Open System Interconnection Reference Model
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Module 5 OSI Model
Topic 1 OSI Reference Model
♦Open System Interconnection Reference (OSI) Model
►The Open System Interconnection (OSI) model defines a networking
framework for implementing protocols in seven layers developed by the
International Standards Organization.
►The model is called ISO OSI (Open Systems Interconnection) Reference
Model because it deals with connecting open systems for communication
with other systems.
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with other systems.
►This model is the best known and the most commonly used guide to describe
networking environments.
►The OSI model is an architecture which divides network communication into
seven layers.
►Each layer covers different network activities, equipments or protocols and
has its own functions. A new layer is created when different functions are
introduced.
►The protocols help the other systems to communicate with each other so as to
utilize the functions provided by those layers.
Module 5 OSI Model
Topic 1 OSI Reference Model
►There must be a minimum number of layers, so that the complexity of the
model can be reduced. The data can then be transmitted between interfaces
easily.
►Two or more functions should not be integrated to a single layer. Hence, the
OSI model is designed to have seven layers.
The Purpose of the OSI Model
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The Purpose of the OSI Model
►It breaks the network communication into simpler parts that are easier to
develop.
►It allows different types of network hardware and software to communicate
with each other.
►It prevents the changes in one layer from affecting the other layers, so that
they can develop more quickly.
Module 5 OSI Model
Topic 1 OSI Reference Model
♦A brief review of the OSI Model
The Open Systems Interconnect (OSI) model is a layered approach
to networking. It has seven layers which has the following order:
►Application layer
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►Presentation layer
►Session layer
►Transport layer
►Network layer
►Data link layer
►Physical layer
Module 5 OSI Model
Topic 1 OSI Reference Model
♦Layer 7 -Application layer
►The application layer is the entrance point that programs use to access the
OSI model and utilize network resources. It is responsible for interacting with
the actual user application and is responsible for providing application
services such as file transfer, e-mail and network services. It includes
network software which directly serves the user with the user interface and
application features. It is used when one machine starts communicating with
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application features. It is used when one machine starts communicating with
another. Application layer interacts with the application software and
represents the service that supports user application and establishes user
interface to the network services. This layer is usually made available by
using an Application Programmer Interface (API). Some of the common
application layer protocols are HTTP, FTP, SMTP and Telnet.
Module 5 OSI Model
Topic 1 OSI Reference Model
►The application layer performs a variety of common functions like
•It provides network services directly to applications.
•Determining the resource availability of intended communication.
•Synchronizing communication.
•Authentication of intended communication partners.
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•Data transmission
Module 5 OSI Model
Topic 1 OSI Reference Model
♦Layer 6 -Presentation layer ►The presentation layer is one of the simplest layers which modifies and
translates the data to ensure that it is presented appropriately for the end
user.
►The presentation layer has major functions. They are:
•Data Conversion: The sending device can convert text or data into general
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format American Standard Code for Information Interchange (ASCII) for
transmission over a network. The receiving computer converts this format
(ASCII) to a format required by the destination application. It translates the data
from a format from the application layer into an intermediate format.
•Data Compression: reduces the number of bits that need to be transmitted on the
network.
•Data Encryption: Data is converted into encoded form, which cannot be read by
unauthorized persons. It encrypts the data for security purposes.
Module 5 OSI Model
Topic 1 OSI Reference Model
♦Layer 5 -Session layer
►The session layer primarily starts at the beginning and end of a transmission.
At the beginning of the transmission, it makes known its intent to transmit. At
the end of the transmission, the Session layer determines the successful
transmission. This layer allows session establishment, maintenance and
termination between processes running on different stations.
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►The session layer has some of the protocols which are implemented. They
are:
•Network File Transfer (NFS)
•Structured Query Language (SQL)
•Remote Procedure Call (RPC)
•AppleTalk Session Protocol (ASP)
Module 5 OSI Model
Topic 1 OSI Reference Model
♦Layer 4 -Transport layer
►The transport layer provides the upper layers with a communication channel
to the network. This layer collects and reassembles any packets, organizing
the segments for delivery and ensuring the consistency of data delivery by
detecting and attempting to rectify the problems that are created.
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detecting and attempting to rectify the problems that are created.
►The transport layer ensures that the messages are delivered error-free, in
sequence, and with no losses or duplications.
►The transport layer decides the type of service to be provided to the session
layer and establishes connection across the network.
►Transport layer protocol is generally categorized into
•Connection-oriented
•Connectionless protocols
Module 5 OSI Model
Topic 1 OSI Reference Model
♦Connection-oriented
The connection-oriented communication is a process which involves
establishing a connection between the two hosts. It transmits the data through
the devices at the end points which uses protocols to establish connection before
sending the data. Transmission control protocol is a connection-oriented
protocol.
♦The four steps in connection-oriented communication are as follows

It establishes connection between the sending and receiving station.
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It establishes connection between the sending and receiving station.
►Synchronization.
►When connection between the two devices is complete, transfer of information
begins.
►It sends the data packets to the destination station and lets the other end of the station
to know the data is receiving.
►The destination station then sends the data packet back to the source to let the source
know that it received the notification message. The sender will always be notified of
the data packets failed delivery.
►Two devices communicate with each other and ensure that packets are delivered
error free, in sequence with no losses or duplications.
Module 5 OSI Model
Topic 1 OSI Reference Model
♦Connectionless protocols ►In the connectionless communication, no initial connection is established
between the two end points. The device transmits the data from one end to
the other end without ensuring that the recipient is prepared to receive the
data. Internet Protocol and User datagram protocol are connectionless
protocol.
Functions of Transport layer
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Functions of Transport layer
►Flow control
►Flow control is a method of controlling the rate at which the device sends
data. The flow control has three standard flavors. They are:
•Buffering
•Congestion Notification
•Windowing
Module 5 OSI Model
Topic 1 OSI Reference Model
♦Buffering ►Buffering is one of the simplest methods used in conjunction with the other
methods of flow control. The input buffer usually can hold enough data
packets, but sometimes the data flows at faster rate than the receiving device
can process it.

Congestion Notification
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Congestion Notification
►This is a signaling technique used by data transmission systems in order to
indicate the status of network congestion. Devices that are communicating
data across a network rely on congestion notification to decide when to send
or delay the transmission of data packets.
♦Windowing ►Windowing is a technique used to deliver the data packets to the recipient in
the same order in which they were transmitted to have a consistent,
connection-oriented data transfer.
Module 5 OSI Model
Topic 1 OSI Reference Model
♦Layer 3 -Network layer
►Network layer is one of the layers in OSI model that defines how
internetworks or interconnected network function. Network layer added
headers containing logical address of the device. Network layer is concerned
with getting data from one computer to another computer even if it is on a
remote network.
The main purpose of network layer is to decide which physical path the
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The main purpose of network layer is to decide which physical path the
information should follow from its source to its destination.
►The network layer provides functional and procedural means of transferring
data from source to destination over one or more networks.
►Two important aspects required to send the data from one device to another
are
•Logical address related source and destination.
•Path through which the data has to be sent to reach the destination.
Module 5 OSI Model
Topic 1 OSI Reference Model
►The important functions of network layer are:
•Routing and forwarding data.
•It is concerned with physical and logical addressing of device, error detection, flow
control, segmentation and sequencing of data.
•Internet Protocol (IP) handles physical and logical addressing of workstations.
♦Functions of network layer

Logical Addressing: Network layer is also responsible for logical addressing. It is
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Logical Addressing: Network layer is also responsible for logical addressing. It is
also known as network addressing or layer 3 addressing.
►Routing: The main function of the network layer is to move the data packets through
a series of device in a network.
►Fragmentation: The network layer has to send message to the data link layer for
transmission. But the data link layer has limits on the length that can be sent from the
network layer.
►Error Handling:The network layer also includes error detection and recovery.
Special protocols are used at the network layer to allow devices that are logically
connected and which also allow to exchange information about the status of hosts on
the network or the devices.
Module 5 OSI Model
Topic 1 OSI Reference Model
♦Layer 2 -Data Link layer
►Data Link Layer is concerned with unique identification of each device and
provides physical means of data transfer.
►It data link layer provides a system through which network devices can share
the communication channel. This function is called media
-
access control
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the communication channel. This function is called media
-
access control
(MAC). It also provides error-free transfer of data frames from one node to
another over the physical layer, allowing the layers above it to assume almost
error-free transmission over the link.
►The data link layer is mainly divided into two sub layers:
•Logical link layer
•Media access control
Module 5 OSI Model
Topic 1 OSI Reference Model
►Logical link layer (LLC)
•The logical link layer manages the communication between devices over a
single link of a network and enables multiple higher-layer protocols to share a
single physical data link.
►Media access layer (MAC)

Media access layer manages protocol access to the physical network medium
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Media access layer manages protocol access to the physical network medium
and determines the hardware addresses.
►Data link layer mainly deals with the following aspects:
•Arbitration
•Physical addressing
•Error detection
•Data framing
Module 5 OSI Model
Topic 1 OSI Reference Model
►Arbitration: Arbitration is the method to access a single data channel when
multiple hosts are using simultaneously.
►Physical addressing: The physical address is designed to uniquely identify
the devices. It is the required component to transfer the data packets from
one end to the other end of the device, specified with the appropriate
physical addresses.

Error detection: Data link layer determines the error problems which occur
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Error detection: Data link layer determines the error problems which occur
during the transmission of data. Introducing a trailer called FCS, which
detects the error before it sends the data packets to the remote machine, does
it. FCS (Frame Check Sequence) uses a Cyclic Redundancy Check (CRC) is
often used to allow the station receiving data to detect if it was received
correctly.
►Data framing: The data link layers of both the machines must be using the
same frame types, so that the data packets will recognize the actual data
contents.
Module 5 OSI Model
Topic 1 OSI Reference Model
♦Layer 1 -Physical layer
►The physical layer is the lowest layer of the OSI model and is concerned with
the transmission and reception of the unstructured raw bit stream over a
physical medium.
►It describes the electrical/optical, mechanical, and functional interfaces to the
physical medium, and carries the signals for all of the higher layers.
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►The physical layer is the most basic network layer, providing only the means
of transmitting raw bits.
►Some of the functions of physical layer involve
•Modification of the data into simple digital signal pattern as ones and zeros.
•Transmission of raw bit stream over physical cable.
•Establishment and termination of connection to communication medium such as
cables.
•Communication resources are effectively shared among multiple users.
•Conversion of digital data and signals to transmit over network.
Module 5 OSI Model
Topic 1 OSI Reference Model
♦The following table describes the different layers and its functions operating
in the corresponding layers.
LayerFunctions of the corresponding Layers
Application Layer
(layer 7)
￿
Interface between the user and the Application
￿
Provides Services that support User application like Email, File
transfer etc
￿
Provides Application Programming Interface
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￿
Provides Application Programming Interface
Presentation Layer
(Layer 6)
￿
The presentation layer is concerned with the syntax and semantics of
the information transmitted.
￿
Encoding data in a standard, agreed upon way
￿
Data compression and Encryption works in this Layer
￿
Responsible for Protocol Conversion
Session Layer
(Layer 5)
￿
Handles Remote Procedure Calls
￿
Establishes and Maintains Connection
￿
Provides Synchronization between Communicating Computers
￿
Provides Token Management and Dialogue Control
Module 5 OSI Model
Topic 1 OSI Reference Model
Transport Layer
(Layer 4)
￿
Ensures Error Free Delivery of Packets
￿
Prevents Duplication and Omission of packets, Flow Control
￿
TCP/SPX provides connection oriented service and safe delivery of
packets
￿
UDP provides connection less service and safe delivery of packets is not
ensured
Network Layer
(Layer 3)
￿
Conversion of Logical address into physical address (ARP/RARP)
￿
Routing of Packets is carried out by this layer
￿
Error reporting regarding packet delivery (ICMP), It determines the best
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￿
Error reporting regarding packet delivery (ICMP), It determines the best
route
Datalink Layer
(Layer 2)
￿
Controls flow of dataPerforms error checking
￿
Organizes Data in to Logical Frames
￿
The Hardware Address (MAC Address) is resolved here
￿
Communicates through switches, bridges and Intelligent hubs
Physical Layer
(Layer 1)
￿
signals are transmitted and bits are represented
￿
Determines the Cards, cables used for transmission
￿
Defines the electrical and the Mechanical Interfaces available for data
transfer
￿
Communicates through hubs, switches, cables, connectors
Module 5 OSI Model
Topic 1 OSI Reference Model
♦Communication in an OSI Reference Model
►The OSI reference model shows the communication between the layers and
the data transmission between the two systems. It is divided into the upper
three layers and lower four layers.
►The upper three layers of the OSI model deal with the application issues
which require access to resource on the network and usually are implemented
only in software. The lower four layers provide the basic communication
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only in software. The lower four layers provide the basic communication
service using the various protocols. Protocols are set of rules that are used for
communication between any two systems in a network.
Module 5 OSI Model
Topic 1 OSI Reference Model
♦Data Encapsulation
►The interaction between the protocols operating at the different layers of the
OSI model takes the form of each protocol adding headers to the information
it receives from the above layer.
►The information from the receiving computers flows upwards discarding the
corresponding header at each layer.
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Module 5 OSI Model
Topic 1 OSI Reference Model
♦The following table shows how the data is converted in each layer
between the systems in the OSI Model.
LayersData Conversion
Application–UserInteractionData
Presentation–DataRepresentationandEncryptionData
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Session—InterHostCommunicationData
Transport–EndtoEndConnectionsandReliabilitySegments Network–PathDeterminationandIPLogicalAddressing
Packets
Datalink–MACandLLC(Physical,Addressing)Frames
Physical–MediaSignalandBinarytransmissionBits
Module 5 OSI Model
Topic 1 OSI Reference Model
♦The data conversion in each layer takes place as follows:
►The data is created in the application, presentation and session layers from
the user’s input.
►The transport layer converts the data into segments.
►The network layer converts the segments into packets or datagrams.

The data
-
link layer converts the packets into frames.
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The data
-
link layer converts the packets into frames.
►The physical layer converts the frames into bits.
Module 5 OSI Model
♦Summary
♦The Open System Interconnection (OSI) model defines a networking framework
for implementing protocols in seven layers which is developed by the
International Standards Organization (ISO).
♦The OSI model has a layered architecture which divides network communication
into seven layers.
♦The OSI reference model has seven layers which has the following order:
►Application

Presentation
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Presentation
►Session
►Transport
►Network
►Data link
►Physical
♦The major functions of network layer are logical addressing, routing,
fragmentation and error handling.
♦Data link layer is mainly divided into two sub layers –Logical link layer and
Media access control.