The OSI Reference Model Explained

OSI stands for the Open Systems Interconnection

The OSI has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other and with users. The bottom four layers define how data is transmitted from one host to the other.

·          _ Application layer (layer 7)

·          _ Presentation layer (layer 6)

·          _ Session layer (layer 5)

·          _ Transport layer (layer 4)

·          _ Network layer (layer 3)

·          _ Data Link layer (layer 2)

·          _ Physical layer (layer 1)

Each layer performs a specialized function to ensure that two hosts or devices effectively communicate, below are some of the functions performed by each layer of the OSI model:

The Application Layer (layer 7)

The application layer provides the user interface. I.e. it marks the spot where users actually communicate to the computer.  Examples of protocols that operate at the application layer include; HTTP (for examples internet browsers work at the application layer), SMTP (for example email clients), FTP (for example FTP clients) and TFTP.

The Presentation Layer (layer 6)

The Presentation layer gets its name from its purpose: It presents data to the Application layer and is responsible for data translation and code formatting. This layer is essentially a translator and provides coding and conversion functions.

Tasks like data compression, decompression, encryption, and decryption are associated with this layer.

The Session Layer (layer 5)

The Session layer is responsible for setting up, managing, and then tearing down sessions between Presentation layer entities. This layer also provides dialog control between communicating hosts. It coordinates communication between systems and serves to organize their communication by offering three different modes: simplex, half duplex, and full duplex.

The Transport Layer (layer 4)

The Transport layer provides end-to-end data transport services and can establish a logical connection between the sending host and destination host on an internetwork.

Examples of protocols operating at this layer include; TCP and UDP, Therefore the Transport layer can be connectionless (UDP) or connection-oriented (TCP).

TCP is considered a reliable mechanism of transport whereas UDP is considered unreliable. The term reliable networking means that acknowledgments, sequencing, and flow control are used. The Transport layer is responsible for providing mechanisms for multiplexing upper-layer applications, establishing sessions, and tearing down virtual circuits.

The Network Layer (layer 3)

The Network layer (also called layer 3) manages IP addressing, tracks the location of hosts on the network, and determines the best way to move data, which means that the Network layer must transport traffic between hosts that aren’t locally attached. Examples of Network layer devices are; Routers and Layer 3 Switches.

The Data Link Layer (layer 2)

The Data Link layer provides the physical transmission of the data, handles error notification, network topology, and flow control. This means that the Data Link layer will ensure that messages are delivered to the proper device on a LAN using hardware or MAC addresses and will translate messages from the Network layer into bits for the Physical layer to transmit. Switches and Bridges are examples of devices that operate at the data link layer.

The Physical Layer (layer 1)

The Physical layer does two things: It sends bits and receives bits. The Physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end systems. Examples of devices that operate at the physical layer include; Hubs, NIC (Network Interface card), cables.