LRG Networks.com
LWC Training Corp.

Network Online Course

Lesson 2 - Communication Standards

B. OSI Reference Model

OSI Reference Model

OSI Reference Model

For a greater understanding of data communications and networking, the Open Systems Interconnect (OSI) seven layer reference model of networking is usually used. The model was designed by the International Standards Organization (ISO) in order to clarify the relationship of various network components to each other.

The purpose of the model

The OSI model is a model of how communications might work. It was designed by the International Standards Organization with input from many other organizations including standards committees, manufacturers and governments. The model includes all of the functions necessary for communications and divides them by functionality into seven divisions or layers. The purpose of this model include the following OSI seven layer model
Figure 14: The OSI Reference model of communications

Some basic rules

The OSI model has some basic rules. They include

What the layers do

The following is a discussion of each layer. Figure 14 illustrates the seven layers in their correct order.

Application layer

The application layer is responsible for providing network services to users and their applications. It is not, however, the user’s application or the operating system, such as Windows.

Presentation layer

The presentation layer deals with syntax and grammatical rules for presenting data to the application layer (not the user). This includes encoding, decoding and otherwise converting data. It is responsible for the following.

Session layer

A session is an agreement to communicate between two entities and the session layer controls the setup, termination and other mechanics of this conversation.

Transport layer

The transport layer provides extra connection services including error correction.

Network layer

The network layer is primarily responsible for getting information to the correct computer on the correct network.

Data Link layer

The data link layer creates the entity (the packet or frame) which is put onto the transmission media.

Physical layer

The physical layer is not the transmission media (cable) itself, but is responsible for the specifications for the media and the electrical signal that goes on it.
Data flow through the OSI model
Figure 15: Data flow through the OSI model

Making a request

The process of moving data through the OSI model is illustrated above. It shows a request coming from a user at a workstation to a file server. The data, for example, the request to open a Word file, starts from the user. In fact, it starts with the Word program itself. It is passed to each layer where it is acted upon until it finally emerges as a packet on the network cable. It is received by the network interface card of the destination machine and then is passed through successive layers until it emerges at the top and is passed to the program running there.

Notice two things about this process. There may not be a distinct piece of software for each of these layers. The functions of several layers may be combined into a single program. Secondly, layers may be missing entirely if their functions are not needed. OSI is just a model which is useful for learning about data communications and one which developers should try to design for. It does not necessarily represent the real world.


Encapsulation
Figure 16: Encapsulation

Encapsulation

The process of moving data through successive layers is called encapsulation. Each layer takes the frame from the layer above. This becomes the new data or “payload”. It then adds information fields to the front of the data and possibly, an error correcting field to the back and then passes it on to the layer below where the complete frame becomes the new data. Because beginning and ending fields frame the data, this process is often called “framing”. Because the data is cradled within or enclosed inside, the process is also called “encapsulation”.

At the receiving end, the process is reversed. At each layer, the framing fields for that layer are stripped off. Error correcting information, if any, is calculated. Then the data is passed up to the next layer and the process is repeated.

Although the process is cumbersome with a great deal of housekeeping involved, performance can still be excellent because it operates at electronic speeds. Improving the speed of communications is a priority for all those involved in this process, including the developers, vendors and administrators of the systems. An important element of fine tuning, however, is understanding the process as outlined above.

Exercise 2-1: OSI Model Quiz

Exercise a: Label the layers of the OSI model

Exercise b: Match the OSI layer with the function it provides

_________ Mediate access to communications channel; Organize data into frames

_________ Data representation (ASCII); Data compression and encryption

_________ Establish and maintain connections; Deal with upper layer errors

_________ Ensure reliability of delivered packets; Data flow and transmission errors

_________ Specifications for physical channel; Carry signals for all higher layers

_________ User interface; User applications

_________ Network addressing and routing; Packet assembly and disassembly

[Top of page][On to next section]






Menu

Home Network Home Preface Lesson 2 Intro

This page

Exercise 2-1

Lesson Index

A. Why standards? C. Net devices Review/Exercise

rames.
  • It identifies specific computers on the network.
  • It detects errors.

    • Physical layer

    The physical layer is not the transmission media (cable) itself, but is responsible for the specifications for the media and the electrical signal that goes on it.
    Data flow through the OSI model
    Figure 15: Data flow through the OSI model

    Making a request

    The process of moving data through the OSI model is illustrated above. It shows a request coming from a user at a workstation to a file server. The data, for example, the request to open a Word file, starts from the user. In fact, it starts with the Word program itself. It is passed to each layer where it is acted upon until it finally emerges as a packet on the network cable. It is received by the network interface card of the destination machine and then is passed through successive layers until it emerges at the top and is passed to the program running there.

    Notice two things about this process. There may not be a distinct piece of software for each of these layers. The functions of several layers may be combined into a single program. Secondly, layers may be missing entirely if their functions are not needed. OSI is just a model which is useful for learning about data communications and one which developers should try to design for. It does not necessarily represent the real world.


    Encapsulation
    Figure 16: Encapsulation

    Encapsulation

    The process of moving data through successive layers is called encapsulation. Each layer takes the frame from the layer above. This becomes the new data or “payload”. It then adds information fields to the front of the data and possibly, an error correcting field to the back and then passes it on to the layer below where the complete frame becomes the new data. Because beginning and ending fields frame the data, this process is often called “framing”. Because the data is cradled within or enclosed inside, the process is also called “encapsulation”.

    At the receiving end, the process is reversed. At each layer, the framing fields for that layer are stripped off. Error correcting information, if any, is calculated. Then the data is passed up to the next layer and the process is repeated.

    Although the process is cumbersome with a great deal of housekeeping involved, performance can still be excellent because it operates at electronic speeds. Improving the speed of communications is a priority for all those involved in this process, including the developers, vendors and administrators of the systems. An important element of fine tuning, however, is understanding the process as outlined above.

    Exercise 2-1: OSI Model Quiz

    Exercise a: Label the layers of the OSI model

    Exercise b: Match the OSI layer with the function it provides

    _________ Mediate access to communications channel; Organize data into frames

    _________ Data representation (ASCII); Data compression and encryption

    _________ Establish and maintain connections; Deal with upper layer errors

    _________ Ensure reliability of delivered packets; Data flow and transmission errors

    _________ Specifications for physical channel; Carry signals for all higher layers

    _________ User interface; User applications

    _________ Network addressing and routing; Packet assembly and disassembly

    [Top of page][On to next section]






    Menu

    Home Network Home Preface Lesson 2 Intro

    This page

    Exercise 2-1

    Lesson Index

    A. Why standards? C. Net devices Review/Exercise