OSI LAYERS

June 23, 2019

The OSI layer is an acronym for Open Systems Interconnection Model. The OSI model is a conceptual model which characterizes and standardizes the communication functions of a telecommunication or computing system without regard to its underlying internal structure and technology.

There are seven major layers in the OSI layers. These are:

  • Layer 7: The application layer
  • Layer 6: The presentation layer
  • Layer 5: The session layer
  • Layer 4: The transport layer
  • Layer 3: The network layer
  • Layer 2: The data-link layer
  • Layer 1: The physical layer

The OSI Model was developed by the International Standards Organization (ISO). It predominantly divides network communication into seven layers. Layers 1-4 are referred to as the lower layers which are mostly concern with the transportation of data. Layers 5-7, referred to as the upper layers, deal with application-level data. In the OSI model, control is passed from one layer to the next, starting at the application layer (Layer 7), and proceeding to the bottom layer, in order of the hierarchy. The OSI model takes the task of inter-networking and divides that up into what is referred to as a vertical stack that consists of 7 layers as mentioned above.

7 Layers of the OSI Diagram

The above diagram gives us a comprehensive idea about what the OSI layer consists of, and how the data flows within it. Now that we have a basic understanding of what the OSI layer is, and what it is made up of, we will look at the functionality of each of the layers in details.

THE APPLICATION LAYER

The Application Layer, a.k.a Layer 7, is responsible for application and end-user processes. Communication methods are defined, quality of service is maintained, privacy and validity of the user is taken care of, and any constraints on data syntax are identified. Everything at this layer is in respect to the application. This layer provides application services for file transfers, e-mail, and other network software services. Both Telnet as well as FTP exist only in the transport layer.

THE PRESENTATION LAYER

Th Presentation Layer takes care of the problem of different forms of data representation by translating the data from application to network format, and vice versa. The main functionality of the presentation layer is to transform data into the form that is understandable by the application layer. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. This layer is also known as the syntax layer. Some examples of the presentation layer are: ASCII, TIFF, GIF.

THE SESSION LAYER

The Session Layer establishes, manages and terminates connections between multiple applications. The session layer manages the lifespan of conversations between applications from their inception, right upto their termination. Some examples are: NFS, RPC.

TRANSPORT LAYER

Indeniably one of the most popular terms in computer networks. The Transport Layer is in charge of clear, transparent transport of data between two hosts. It is also responsible for any data loss and recovery of the same. Example: TCP/IP, UDP.

THE NETWORK LAYER

The Network Layer  enables efficient switching and routing pathways for data, creation of logical pathways, known as virtual circuits. The main functions of the network layer are Routing and forwarding,  addressing, internetworking, error handling,packet sequencing.

THE DATA LINK LAYER

The main objective of the Data Link Layer, is to encode data packets and also decode them into bits. It contains a comprehensive library of transmission protocol knowledge and also dwells in management and handling of discrepencies in the physical layer, flow control and frame synchronization. The data link layer is broken down into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control(LLC) layer. The MAC sub layer manages how a computer on the network gains access to the data and permission to transmit it. The LLC layer looks after the frame synchronization, flow control and error checking.

THE PHYSICAL LAYER

The Physical Layer, as the name suggests, conveys the bit stream through the network at the electrical and mechanical level. It deals directly with the hardware side of things at the user end.