The OSI Reference Model

The OSI Model, was formed by the International Organization for Standardization (ISO) in 1982 with the goal of producing a standard reference model for hardware and software connection of digital equipment.

OSI is the acronym for Open Systems Interconnection and defines how the vast majority of the digital networks on earth function.

The concept of how a modern day network operates can be understood by dissecting it into seven layers – this seven layer model is the OSI Model.

One thing you need to be aware of, the OSI Reference Model provides guidelines for creation of network standards and does not define a network standard.

The seven layers of the OSI Model are:

1. The Physical Layer

The physical layer is the layer of low-level networking equipment, such as some hubs, cabling, and repeaters. The physical layer is never concerned with protocols or other such higher-layer items.

It defines the relationship between a device and a physical transmission medium (e.g., a copper or fiber optical cable, radio frequency). It is responsible for transmission and reception of unstructured raw data in a physical medium.

2. The Data Link Layer

The data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link

3. The Network Layer

The network layer provides the functional and procedural means of transferring variable length data sequences (called datagrams) from one node to another connected to the same network.

Its deciding which physical path the data should take based on network conditions, priority of service, and other factors.

4. The Transport Layer

The transport layer provides the functional and procedural means of transferring variable-length data sequences from a source to a destination host via one or more networks, while maintaining the quality of service functions.

It ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.

5. The Session Layer

The session layer controls the dialogues (connections) between computers. It allows session establishment between processes running on different stations. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures.

6. The Presentation Layer

The presentation layer formats the data to be presented to the application layer.

It establishes context between application-layer entities, in which the application-layer entities may use different syntax and semantics if the presentation service provides a mapping between them. If a mapping is available, presentation service data units are encapsulated into session protocol data units, and passed down the protocol stack.

7. The Application Layer

The application layer is the OSI layer closest to the end user and serves as the window for users and application processes to access network services.