Www.bpsharma.in eLearning Networking OSI Reference Model

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Introduction to OSI Reference Model

eLearning >> Networking

Introduction

This module introduces you to the 7 layers of OSI model and explains how data is encapsulated for data transmission.

After reading this module, you will be able to:

Identify reasons why the networking industry uses a layered network model.Name the layers of the OSI model and give their relative position in the stack.Identify the functions of each layer of the OSI reference model.Characterize the differences between data communicated by the lower layers and by the uppe r layers.Define and explain the steps of data encapsulation and decapsulation.

Network Model Overview

Network Model Overview

In order for a computer to send information to another computer, and for that computer to receive and understand the information, there has to exist aset of rules or standards for this communication process. These standards ensure that varying de vices a nd products can communicate with eachother over any network. This set of standards is called a model.

The International Standards O rganization (ISO) has been instrumental in c reating an industry wide model, or framework, for defining the rulesnetworks should emp loy to ensure reliable communications. This network mode l is broken into layers, with each laye r having a distinctive job in thecommunication process.

Consider the following analogy to help put the idea of layered models and their standards in perspective:

Network Model Analogy

In the network mode l analogy be low, a courier service is used to represent the network model. Each step in the p rocess o f sending a package fromAtlanta to Toronto requires a specific job to be done at a specific level.

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Illustration with Images TextRajeev in Atlanta tells Nidhi in Toronto that he is g oing to se nd her a packa ge throughthe O SI Courier Service. Nidhi in Toronto resp onds, "Great - I'll be looking for it."

Bill prepares the weigh bill for the courier service and ca lls for pickup.

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The O SI Courier Service picks up the package from Bill's Atlanta office and takes o ffin a truck.

The package g ets transferred to an airplane for shipment to Toronto.

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The airplane arrives in Toronto where it is offloaded, sorted and placed on a truckbound for Nidhi's o ffice.

The package is delivered to Nidhi via the truck.

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Nidhi acknowledges to Bill that she received the package.

Why the Network Model is Layered

In the courier example above, each step in the process of sending the package from Atlanta to Toronto required a specific job to be done at aspecific level. Just as the package had to go though each step o f the process , transferring and sharing data from one machine on a network toanother requires stepping through a process. To standardize the process, a model for how data is to be sent, sorted, transported and delivered mustbe used.

This process is called a network model. Network models exist to provide a framework, or blueprint, for implementing standards and protocolsthat allow machines and devices to communicate. Processes and tasks are separated into logical groups called layers.

Network Model Advantages

This sec tion provides advantages for the network des ign, architecture and implementation. These include:

Reduces complexity - by dividing the processes into groups, or layers, implementation of network architecture is less complex

Provides co mpatibility - standardized interfaces allow for "plug-and-play" compatibility and multi-vendor integration

Facilitat es modularizat ion - deve lopers "swap" out new technologies at each layer keeping the integrity of the network architecture

Accelerates evolution of technology - deve lopers focus on technology at one layer while preventing the changes from affecting anotherlayer

Simplifies learning - processes broken up into groups divides the complexities into smaller, manageable chunks

Available Network Models

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There are a va riety of networked mode ls currently be ing implemented. However, in this section, the focus will be on the OSI model, while alsoreferencing the TCP/IP model pe riodically.

What are some advantages in using a network model? (check all that apply)

Increases complexity

Facilitates modularizationProvides compatibility

Complicates learning

Correct Answer(s): 2, 3

OSI Network Model

Open Systems Interconnection(OSI) Model Overview

The Open Systems Interconnection (OSI) model began as a reference model, but has since been implemented. It was created by the InternationalOrganiza tion for Standardiz ation (ISO) to provide a logical framework for how data communication processe s should interact across networks.Standards were created for the computer industry allowing different networks to work together efficiently.

OSI Model Layers

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Modularity refers to the exchange of protocols a t a pa rticular level without interfering with or damaging the relationship or tasks of another level.

Within a layer, protocols can be exchanged, allowing communications to still take place seamlessly. These exchanges may need to occur based onhardware from different vendors or for a variety of different reasons.

Illustration with Images TextConsider the courier examp le. Modularity at the transportation level implies that it really doesn't matter howthe package gets to the airplane.

It could travel from the sender by truck to the plane or by boat. Either way, the sender still sends thepackage and expects it to get to Toronto. The plane still flies the package to Toronto regardless of how itwas delivered to the pane.

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The term "open" in OSI was used because the intent of the model was to providea way for networks to be interconnected without regard to the underlying hardware,as long as the communications software adhered to the standards.

True

False

Correct Answer: 1

The OSI model organizes communication protocols into how many layers?

4

16

7

12

Correct Answer: 3

The 7 OSI Layers

The 7 OSI Layers

The OSI Model consists of the following seven layers:

1. Application2. Presentation3. Session4. Transport5. Network6. Data Link7. Physical

What do the 7 layers really do?

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When data is being transferred a nd shared over a network, it must pass through each of the 7 layers in one machine, from the app lication layerdown, before z ipping across a network and working its way up the layers in the receiving machine.

The OSI Model

The main use of the model is to help network designers understand the functionality involved with the development and flow of data communications.Included within this mode l are the protocols for network properties a nd transmission methods.

The model is divided into 7 layers, with individual characteristics and tasks within each layer. Each layer must communicate with the layer directlyabove and below through a series of standards and protocols.

Illustration with Images Text

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Application Layer: Provides network se rvices to user app lications. It is responsible for exchanging informationbetween programs running on the machine, such as an e-mail program, and other services running on a network,such as a print server or another computers' applica tion.

Presentation Layer: Concerned with how data is converted and formatted for data transfer. Examples of formatconversions include ASCII text for documents and .gif and JPG for images. This layer performs code conversion,data translation, compression and encryption.

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Session Layer: Determines how two devices establish, maintain and manage a connection - how they talk to eachother. These connections are called sessions.

Transport Layer: Responsible for breaking the data into segments, establishing an end-to-end logical connectionbetween machines, and p roviding for error handling.

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Network Layer: Responsible for determining addressing on the network, determining the routes that information willtake on its journey, and managing network traffic congestion. Data at this level is package d into packe ts.

Data Link Layer: Provides the link for how data, packaged into frames is communicated through hardware to betransported ac ross a medium. It communicates with network cards, manages physical layer communications betweenconnecting systems and handles e rror notification.

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Physical Layer: Specifies how data is processed into bits and physically transferred over medium, such as cables.It's resp onsible for ac tivating and maintaining the physical link between systems.

Which of the OSI layers provides segmentation and re-assembly

Applications

Presenation

Session

Transport

Network

Data Link

Physical

Correct Answer: 4

What is the information structure p roduced at OSI level 3 ca lled?

Segment

Data

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Packet

Frame

Correct Answer: 3

Which of layers p rovides code formatting?

Applications

Presentation

Session

Transport

Network

Data Link

Physical

Correct Answer: 2

Which of layers p rovides the electrical and mechanical connections betweensystems?

Applications

Presenation

Session

Transport

Network

Data Link

Physical

Correct Answer: 7

Data EncapsulationPDFmyURL.com


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Data Encapsulation Overview

This topic defines and explains the steps of data encapsulation and decapsulation in the data communication process.

Data Encapsulation

If we refer back to the example of the courier service, each layer required a separate process to ensure reliable delivery and transport of thepacka ge from Atlanta to Toronto. In order for this to happe n, the packag e need ed to:

1. Have a de stination address and tracking number attached2. Be sorted with the other package p ickups for Toronto3. Placed on a truck bound for a specific plane carrying the packages to Toronto

4. Picked up by a truck and taken to be soted by Toronto des tination, and5. Delivered to the Toronto office.

Along the way, the o riginal destination address served as a reference, however, other information was attached as well, such as theidentification numbers for which truck or which plane. These add itions did not change the package contents or the de stination addressinformation, they merely provided additional routing information needed at the time.

Therefore, this analogy shows how the OSI model is modular, pe rmiting modifications or replacement of each individual layer without affectingthe integrity of the data or mod ifications that may be made to other layers in the OSI layer stack.

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Data Encapsulation Example

There are 5 basic steps of encapsulation that need to occur:

Illustration with Images TextStep 1. When you send an e-mail, the information in the message is convertedto usable data in the top three layers (Application, Presentation, Session) andpasse d to the Transport Layer.

Step 2. At the Transport Layer, protocol information is attached as a header tothe data, ensuring that the computer at the other end will be able tocommunicate. The data and header is then packaged into a segment.

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Step 3. The segment is transferred to the Network Layer where an additionalheader containing source a nd destination logical address is attached. Thesegment and head er is package d into a pa cket and transferred to the DataLink Layer.

Step 4. At the Data Link Layer, both a header and trailer with additionalinformation is attached and packaged as a frame. The frame providesinformation that allows connection to the next network device on the link.

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Step 5. The frame is converted into a series of 0's and 1's (bits) fortransmission ac ross the network medium at the Physica l Layer.

As information structures descend the OSI model layers, they become nested inthe information structures below. What is this called?

Segmentation

Fragmentation

Encapsulation

Multiplexing

Correct Answer: 3

Summary

In Summary...

After reading through the OSI Reference ModelModule, you should have a better understanding of the background of layered models, why theyexist, and what purpose they serve.

In review, see if you are able to answer the following:

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Identify reasons why the networking industry uses a layered network model.

Name the layers of the OSI model and give their relative position in the stack.

Identify the functions of each layer of the OSI reference model.

Characterize the differences between data communicated by the lower layers and by the uppe r layers.

Define and explain the steps of data encapsulation and decapsulation.

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Www.bpsharma.in eLearning Networking OSI Reference Model