The OSI Model

The OSI (Open System Interconnection) Reference Model is a series of protocols, which attempt to define and regulate the data communication processes. The OSI model describes a networking framework of different protocols through a total of 7 layers.

The OSI model describes how data communication actually takes place. It divides the whole process into seven layers. These layers are fitted with the protocol standards that are developed by the International Organization for Standardization (ISO) and other bodies, including the ANSI (American National Standards Institute), ITU (International Telecommunications Union), and the IEEE (Institute of Electrical and Electronic Engineers).  The OSI reference model is modular. It has seven layers and control is transferred from the upper layers to the lower layers.

Seven Layers Of The OSI Reference Model

The seven layers of the OSI reference model are shown below:

Functions of Each Layer:

Application Layer (Layer 7)

The application layer supports end-user and application processes. In this layer, communication partners are identified, user authentication, quality of service, privacy and any other data syntax constraints are considered. This layer offers application services for e-mail, file transfers, and other type of network software services. Everything is application specific at this layer. FTP and Telnet for example are applications, which exist completely in the application level.

Presentation Layer (Layer 6)

Presentation layer converts data into the form, which is accepted by the application layer. This layer offers independence from differences in the data representation by converting from network to application format and vice versa. Formatting and encryption of data is done at this level. It also provides liberty from compatibility problems. It is also known as the syntax layer.

Session Layer (Layer 5)

The main function of the session layer is for establishing, managing and terminating connections between various applications. This layer is responsible in setting up, coordinating, and terminating conversations, exchanging the dialogues between the several applications at each end. The session layer also deals with connection coordination.

Transport Layer (Layer 4)

Transport layer offers transparent data transfer between hosts or end systems. This layer is responsible for flow control and end-to-end error recovery. It also ensures that data must be transferred completely.

Network Layer (Layer 3)

Network layer is responsible for the routing and switching technologies. It creates logical paths, also known as virtual circuits, which are used for transmitting data from one node to other. Various functions of this layer are: routing and forwarding, addressing, error handling, internetworking, packet sequencing, and congestion control.

Data Link Layer (Layer 2)

At the data link layer, data is encoded and decoded in the form of bits. It provides transmission protocol knowledge and management. This layer also handles flow control, frame synchronization and errors that occur in the physical layer. This layer is divided into 2 sub layers: LLC (Logical Link Control) and MAC (The Media Access Control). The LLC sub layer manages flow control, frame synchronization and error checking. The MAC sub layer checks how a system on the network can access data and how it gets permission for transferring that data.

Physical Layer (Layer 1)

The physical layer transmits the bit stream in the form of electrical impulses, and light or radio signals. It offers the hardware for transmitting and receiving data on a channel, including defining cards, cables and physical aspects. ATM, RS232 and Fast Ethernet are the protocols having physical layer components.

Many protocols can exist at each layer.  For more information on this and other CCNA related materials, I encourage you to take a look at our study guides and resources.

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A simple way to remember the 7 layers is to use the following sentence: All People Seem To Need Data Processing where the A is for Application, P for Presentation, S for Session, and so on.  It was a lifesaver for me come exam time.