Understanding OSI Model Basics

The International Standards Organization (ISO) began developing the OSI model in 1974 and finally adopted the model in 1977. The OSI model is not something you can buy, yet it is built into nearly all networking devices. It is a theoretical model that defines how networks are built and function from the ground up. By understanding the OSI model, you can develop a better understanding of networking protocols and standards. Even though networks are implemented differently, they are all based on the same or similar reference model. The OSI model defines seven layers and the function of the data flow within each layer.

International Standards Organization (ISO)

An organization dedicated to defining global communication and informational exchange standards. The American National Standards Institute (ANSI) is the American representative to the ISO.

network

Two or more computers connected for the purpose of sharing resources such as files or printers.

protocol

A set of rules for communication between two devices.

standard

An agreed-upon set of rules, procedures, and functions that are widely accepted.

The smallest unit of data begins at the Application layer. As data moves down the OSI model, more information is added at each layer to enable network communication to take place. At the receiving computer, the layers are stripped off at corresponding layers, called peer layers. The process is then reversed for the reply.

Using this type of model has three main advantages:

• Breaking down a large concept such as a network into smaller parts (in this case, layers) makes it easier to understand.

• Compartmentalizing network functions enables you to easily replace specific technologies without having to replace the entire network.

• Devices that are built around a common model should work together, or interoperate, regardless of the manufacturer that built the device.

encryption

The process of encoding data to prevent unauthorized access.

error control

A mechanism for assuring that data received is in the same condition and format that it was sent.

flow control

A feature of the Transport layer that manages the amount of data being transmitted between sender and receiver. If the receiving computer is unable to accept more data, it notifies the sending computer to pause transmission.

packet

Data that has been encapsulated with information from the Transport and Network layers of the OSI model.

internetwork

Two or more networks that are connected by using a router.

bridge

A Layer 2 device that enables networks using different Layer 2 protocols to communicate with one another. A bridge can also minimize traffic between two networks by passing through only those packets that are addressed to the other network.

The functions of the seven layers of the OSI model are:

OSI Layer	Function	Example of Layer Function
Application	Supports the communication of applications with the network.	Examples of Application layer services include sending and receiving messages and transferring files.
Presentation	Formats and translates data.	An example of a Presentation layer service is the compression or encryption of data.
Session	Manages communication sessions between service requesters and service providers. This layer manages communications by establishing, synchronizing, and maintaining the connection between sender and receiver.	At the Session layer, a web server may close the session with a user after a specified period of time has elapsed without activity.
Transport	Is usually associated with reliable end-to-end communication. It is the layer responsible for managing connections, error control, and flow control between sender and receiver.	When part of a web page does not download due to heavy traffic, the Transport layer will resend the missing data.
Network	Is responsible for moving packets over an internetwork so that they can be routed to the correct network.	The IP address of a device is used on the Internet to determine the route that information travels to get to the correct sender and receiver.
Data-Link	Includes information about the source and destination hardware addresses of the communicating devices. It also specifies lower-layer flow control and error control.	Devices that use Data-Link layer protocols are network cards and bridges.
Physical	Is responsible for the transmission of data over a physical medium.	Cables and physical connection specifications are defined at the Physical layer.