OSI Review

OSI Model

The OSI isn’t a physical model, rather, it is a set of guidelines that will allow many different operating system platforms to transfer data over a network. The OSI has seven different layers and is divided into two groups. The top three layers (5-7) are used to communicate with the application and the end user. The bottom four layers (1-4) define how the data is transmitted from node to node. The OSI model is a hierarchical structure that consists of seven layers all of which define the requirements to communicate over a network. Because the OSI model is layered, each layer operates independent of the other, this is ideal for implementing new technology without interruption. For example, a packet filter.

OSI Layers

(Layer 7) Application Layer

This is the layer where the users actually communicate with the computer. A great example of someone using the application layer is when you use a browser to access the internet. Every time you click a link or enter a search in a search engine, you are giving a set of parameters to be used to access your network.

(Layer 6) Presentation Layer

The Presentation Layer modifies the data from the Application Layer to be used effectively by all of the other layers. It also translates your incoming network requested data into a useable form for your application.

(Layer 5) Session Layer

The session layer controls sessions between nodes (computers) across a network. It establishes, manages and terminates connections between applications.

(Layer 4) Transport Layer

The Transport Layer allows the transfer of data between end users. It is responisble for end-to-end error recovery and flow control.

(Layer 3) Network Layer

The Network Layer allows the data to be routable through many different physical networks and provides a means for transferrring variable length data sequences.

The Network Layer is responsible for:

addressing

determining routes for sending

managing network problems such as packet switching, data congestion and routing

(Layer 2) Data Link Layer

The Data Link Layer provides the physical transmission of the data and handles errors in the physical layer. The Logical Link Layer (LLC) is responisble for frame synchronization, flow control, and error checking. Most importantly this layer encodes and decodes data packets into bits.

(Layer 1) Physical Layer

The Physical Layer defines the physical medium and electrical specifications of a network. The Physical Layer is responsible for the transmission of raw bit streams and defines the techniques to transfer the bit stream through cable.