OSI Model

The OSI model was developed by the ISO in an attempt to standardize the way in which systems communicate across networks. The hope was to create a standard protocol set which would allow the product of any vendor to be able to network with other vendor's products. While the protocol set did not catch on, the model did. At the time of the OSI model's creation, the TCP/IP protocol suite was already in place as a widely used networking protocol. While TCP/IP has it's own model which is still commonly used today when examining and understanding networking issues, the protocol suite has become an integral part of the OSI model.

The OSI model is made up of seven layers. Each layer represents a different step in communicating over a network. At each layer, there are a set of protocols that operate to achieve that layer's responsibility. The objective of the OSI model is to outline how the protocols at each layer need to function so that they can work with systems that may be developed by other vendors. This concept is known as an open network. An open network architecture is one that no vendor owns, and if implemented, it provides a standard way of operating. It is this open network design that lets a computer that uses an Intel processor communicate with another that uses AMD.

The seven layers of the OSI model, moving from the top to the bottom are: Application, Presentation, Session, Transport, Network, Data link, and Physical. Each layer has a different function when creating a

message to be sent over a network. When a system needs to create a message to send, it begins at the top of the model and works down. Each layer adds information to the message as it travels down the model. At each stop the message grows in size. When the receiving system gets the message, the message moves in reverse up the model. As it moves up the model, each layer removes the information that was added by it's counterpart layer on the other system. Once each layer removes the data that pertains to it, it passes the message on to the next layer. This process where layers and protocols communicate with their counterparts across systems is known as encapsulation. Encapsulation is based on the idea that a layer only needs to know how to do it's job and how to pass the message on to the next layer. The session layer is not concerned with how the physical layer is going to put the electrical signals onto the wire, and vice versa.

Each layer has different responsibilities and functions it performs, as well as a format it expects the message to be in. Each layer also has a connection point, or interface, that allows it to communicate with three other layers: 1) the layer above it, 2) the layer below it, and 3) the same layer on the target machine. A layer provides control functions by adding information to the message in the form of headers and footers on the data packet. This tells the corresponding layer on the target machine how the message is to be handled.

The benefit of encapsulating the responsibilities of these layers is it allows products from different vendors to work together within the single model in a predictable manner. If a vendor designs a protocol for the session layer that is based on the OSI model, other vendors, and consumers, can be certain it will function properly with other open system protocols.