Open System Interconnection Model

The OSI model depicts how data communications should take place. It splits the functions or processes into seven groups that are described as layers.

For clear understanding, The OSI model defines standards for:

• The way in which devices communicate between each other.
• The means used to inform devices when to send data and when not to transmit data.
• The methods which ensure that devices have a correct data flow rate
• The means used to ensure that data is passed to, and received by the intended recipient.
• The manner in which physical transmission media is arranged and connected.

The International Standards Organization (ISO) developed a seven-layer Open Systems (OSI) model to serve as a standard model for network architectures. Dividing data communications functions into seven distinct layers promotes the development of modular network architectures, which assists the development, operation, and maintenance of complex telecommunications networks.

Let's discuss each of these layers in detail.

Layer 7 or Application Layer

The application layer of OSI layer architecture is closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications and provides user services by file transfer, file sharing, etc. Database concurrency and deadlock situation controls are undertaken at this layer level. This is the layer at which communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified.

Layer 6 or Presentation Layer

This layer at times referred as Syntax Layer also, is usually a part of an operating system, that converts incoming and outgoing data from one presentation format to another (for example, from a text stream into a popup window with the newly arrived text). The presentation service data units are then encapsulated into Session Protocol Data Units, and moved down the stack. It further controls on screen display of data, transforms data to a standard application interface. Encryption, data compression can also be undertaken at this layer level.

Layer 5 or Session Layer

This layer sets up, coordinates, and terminates conversations, exchanges, and dialogs between the applications at each end. It deals with session and connection coordination. It provides for full-duplex, half-duplex, or simplex operation, and establishes check pointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for "graceful close" of sessions also.

Layer 4 or Transport Layer

This layer ensures reliable and transparent transfer of data between user processes, assembles and disassembles message packets, and provides error recovery and flow control. Multiplexing and encryption are undertaken at this layer level. This means that the Transport Layer can keep track of the segments and retransmit those that fail.

Layer 3 or Network Layer: 

The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks, while maintaining the quality of service requested by the Transport Layer. The Network Layer makes a choice of the physical route of transmission, creates a virtual circuit for upper layers to make them independent of data transmission and switching, establishes, maintains, terminates connections between the nodes and ensure proper routing of data.

Layer 2 or Data Link Layer

The Data Link Layer responds to service requests from the Network Layer and issues service requests to the Physical Layer. The Data Link Layer is the protocol layer which transfers data between adjacent network nodes in a wide area network or between nodes on the same local area network segment. This layer is also a hardware layer which specifies channel access control method and ensures reliable transfer of data through the transmission medium. It provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer.

Layer 1 or Physical Layer

The Physical Layer is a hardware layer which specifies mechanical features as well as electromagnetic features of the connection between the devices and the transmission. In particular, it defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, cable specifications, Hubs, repeaters, network adapters, Host Bus Adapters (HBAs used in Storage Area Networks) and more. 

Let us now conclude this topic by understanding some functions of this model. These are listed in the points below:

1. Establishment and termination of a connection to a communications medium.
2. Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.
3. Modulation or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and optical fiber) or over a radio link.

Well, that’s all on the different layers of the Open System Interconnection model. We shall discuss Transmission Control Protocol/ Internet Protocol in the next article. Stay tuned for that.

And like always, if you have any queries, doubts or suggestions, let me know. Just click on ask a question on the left panel of your screen.

Thank You.