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 upper 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 a set of rules or standards for this communication process.  These standards ensure that varying devices and products can communicate with each other over any network. This set of standards is called a model.

The International Standards Organization (ISO) has been instrumental in creating an industry wide model, or framework, for defining the rules networks should employ to ensure reliable communications. This network model is broken into layers, with each layer having a distinctive job in the communication process.

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

Network Model Analogy
 
In the network model analogy below, a courier service is used to represent the network model. Each step in the process of sending a package from Atlanta to Toronto requires a specific job to be done at a specific level.

Illustration with Images Text
Rajeev in Atlanta tells Nidhi in Toronto that he is going to send her a package through the OSI Courier Service. Nidhi in Toronto responds, "Great - I'll be looking for it."
Bill prepares the weigh bill for the courier service and calls for pickup.
The OSI Courier Service picks up the package from Bill's Atlanta office and takes off in a truck.
The package gets transferred to an airplane for shipment to Toronto.
The airplane arrives in Toronto where it is offloaded, sorted and placed on a truck bound for Nidhi's office.
The package is delivered to Nidhi via the truck.
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 a specific level.  Just as the package had to go though each step of the process, transferring and sharing data from one machine on a network to another requires stepping through a process. To standardize the process, a model for how data is to be sent, sorted, transported and delivered must be used.

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

Network Model Advantages
 

This section provides advantages for the network design, architecture and implementation. These include:

 
  • Reduces complexity - by dividing the processes into groups, or layers, implementation of network architecture is less complex
  • Provides compatibility - standardized interfaces allow for "plug-and-play" compatibility and multi-vendor integration
  • Facilitates modularization - developers "swap" out new technologies at each layer keeping the integrity of the network architecture
  • Accelerates evolution of technology - developers focus on technology at one layer while preventing the changes from affecting another layer
  • Simplifies learning - processes broken up into groups divides the complexities into smaller, manageable chunks
Available Network Models
 
There are a variety of networked models currently being implemented. However, in this section, the focus will be on the OSI model, while also referencing the TCP/IP model periodically.

What are some advantages in using a network model? (check all that apply)
 
 
Increases complexity
 
Facilitates modularization
 
Provides 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 International Organization for Standardization (ISO) to provide a logical framework for how data communication processes should interact across networks. Standards were created for the computer industry allowing different networks to work together efficiently.

OSI Model Layers
 

There are 7 layers in the OSI model. Each layer is responsible for a particular aspect of data communication. For example, one layer may be responsible for establishing connections between devices, while another layer may be responsible for error checking during transfer.

The layers of the OSI model are divided into two groups: the upper layer and lower layer. The upper layers focus on user applications and how files are represented on the computers prior to transport. For the most part, network engineers are more concerned with the lower layers. It's the lower layers that concentrate on how the communication across a network actually occurs.

The Term "Open" in OSI
 

The term “open” in OSI was used because the intent of the model was to provide a way for networks to be interconnected without regard to the underlying hardware, as long as the communications software adhered to the standards. The term has since come to imply modularity as well.

Modularity
 

Modularity refers to the exchange of protocols at a particular 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 on hardware from different vendors or for a variety of different reasons.


Illustration with Images Text
Consider the courier example. Modularity at the transportation level implies that it really doesn't matter how the 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 the package and expects it to get to Toronto. The plane still flies the package to Toronto regardless of how it was delivered to the pane.

The term "open" in OSI was used because the intent of the model was to provide a 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. Application
  2. Presentation
  3. Session
  4. Transport
  5. Network
  6. Data Link
  7. Physical
What do the 7 layers really do?
 

When data is being transferred and shared over a network, it must pass through each of the 7 layers in one machine, from the application layer down, before zipping 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 model are the protocols for network properties and transmission methods.

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


Illustration with Images Text

Application Layer: Provides network services to user applications. It is responsible for exchanging information between 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' application.

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

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

Transport Layer: Responsible for breaking the data into segments, establishing an end-to-end logical connection between machines, and providing for error handling.

Network Layer:  Responsible for determining addressing on the network, determining the routes that information will take on its journey, and managing network traffic congestion. Data at this level is packaged into packets.

Data Link Layer: Provides the link for how data, packaged into frames is communicated through hardware to be transported across a medium. It communicates with network cards, manages physical layer communications between connecting systems and handles error notification.

Physical Layer: Specifies how data is processed into bits and physically transferred over medium, such as cables. It's responsible for activating 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 produced at OSI level 3 called?
 
 
Segment
 
 
Data
 
Packet
 
 
Frame
  Correct Answer: 3


Which of layers provides code formatting?
 
 
Applications
 
Presentation
 
 
Session
 
 
Transport
 
 
Network
 
 
Data Link
 
 
Physical
  Correct Answer: 2


Which of layers provides the electrical and mechanical connections between systems?
 
 
Applications
 
 
Presenation
 
 
Session
 
 
Transport
 
 
Network
 
 
Data Link
 
Physical
  Correct Answer: 7

 

Data Encapsulation

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 the package from Atlanta to Toronto. In order for this to happen, the package needed to:

  1. Have a destination address and tracking number attached
  2. Be sorted with the other package pickups for Toronto
  3. 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 destination, and
  5. Delivered to the Toronto office.

Along the way, the original destination address served as a reference, however, other information was attached as well, such as the identification numbers for which truck or which plane. These additions did not change the package contents or the destination address information, they merely provided additional routing information needed at the time.

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

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

Illustration with Images Text

Step 1. When you send an e-mail, the information in the message is converted to usable data in the top three layers (Application, Presentation, Session) and passed to the Transport Layer.

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

Step 3. The segment is transferred to the Network Layer where an additional header containing source and destination logical address is attached. The segment and header is packaged into a packet and transferred to the Data Link Layer.

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

Step 5. The frame is converted into a series of 0's and 1's (bits) for transmission across the network medium at the Physical Layer.


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

 
 
Segmentation
 
 
Fragmentation
 
Encapsulation
 
 
Multiplexing
  Correct Answer: 3

 
Summary

In Summary...
 

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

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

 
  • 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 upper layers.
  • Define and explain the steps of data encapsulation and decapsulation.


Feedback

Please provide feedback ...
 
Your feedback and recommendation will greatly help us and your friends navigate the content on this site. Send your comments and suggestion on bpsharma_in@yahoo.co.in