DATA COMMUNICATIONS
© Copyright Brian Brown, 1995-2000. All rights reserved.

Part 18: Open Systems Interconnect [OSI] Model

OSI Model | Sending Data | Summary

Introduction
This section briefly discusses OSI, which is a 7 layer model for the exchange of data between computers.

Objectives
At the end of this section you will be able to

• state the organization responsible for the ISO model
• list each of the layers of the ISO model and describe their function
• describe how data is transferred between two hosts using the OSI model

OSI Model
In 1983, the International Standards Organization (ISO) developed a model that would allow the sending and receiving of data between two computers. It works on a layer approach, where each layer is responsible for performing certain functions.

When we think of how to send data from one computer to another, there are many different things involved. There are network adapters, voltages and signals on the cable, how the data is packaged, error control in case something goes wrong, and many other concerns. By dividing these into separate layers, it makes the task of writing software to perform this much easier.

In the Open Systems Interconnect model, which allows dissimilar computers to transfer data between themselves, there are SEVEN distinct layers.

7. Application Layer
   Provides Applications with access to network services.
   
   
6. Presentation Layer
   Determines the format used to exchange data among networked computers.
   
   
5. Session Layer
   Allows two applications to establish, use and disconnect a connection between them called a session. Provides for name recognition and additional functions like security which are needed to allow applications to communicate over the network.
   
   
4. Transport Layer
   Ensures that data is delivered error free, in sequence and with no loss, duplications or corruption. This layer also repackages data by assembling long messages into lots of smaller messages for sending, and repackaging the smaller messages into the original larger message at the receiving end.
   
   
3. Network Layer
   This is responsible for addressing messages and data so they are sent to the correct destination, and for translating logical addresses and names (like a machine name FLAME) into physical addresses. This layer is also responsible for finding a path through the network to the destination computer.
   
   
2. Data-Link Layer
   This layer takes the data frames or messages from the Network Layer and provides for their actual transmission. At the receiving computer, this layer receives the incoming data and sends it to the network layer for handling.

   The Data-Link Layer also provides error-free delivery of data between the two computers by using the physical layer. It does this by packaging the data from the Network Layer into a frame that includes error detection information. At the receiving computer, the Data-Link Layer reads the incoming frame, and generates its own error detection information based on the received frame data. After receiving all of the frame, it then compares its error detection value with that of the incoming frames, and if they match, the frame has been received correctly.

   A frame looks like,

   

   The Data-Link Layer actually consists of two separate parts, the Medium Access Control (MAC) and Logical Link Control Layer (LLC). Example MAC layers are Ethernet 802.3 and Token Ring 802.5

   Bridges are an example of devices which works at the MAC layer.

1. Physical Layer
   Controls the transmission of the actual data onto the network cable. It defines the electrical signals, line states and encoding of the data and the connector types used. An example is 10BaseT. Repeaters are an example of devices that work at the Physical Layer.

   For Ethernet 802.3, the Physical Layer can be represented as

   • 10Base5
   • 10Base2
   • 10BaseT
   • 10BaseF

Sending Data Via the OSI Model
Each layer acts as though it is communicating with its corresponding layer on the other end.

In reality, data is passed from one layer down to the next lower layer at the sending computer, till its finally transmitted onto the network cable by the Physical Layer. As the data it passed down to a lower layer, it is encapsulated into a larger unit (in effect, each layer adds its own layer information to that which it receives from a higher layer). At the receiving end, the message is passed upwards to the desired layer, and as it passes upwards through each layer, the encapsulation information is stripped off .

Summary
The OSI model defines a model consisting of 7 layers. This allows the exchange of data between different computers using the OSI model.

© Copyright Brian Brown, 1995-2000. All rights reserved.