The OSI model is a layered framework for the design of network that allows communication between different systems without changing the underlying hardware and software. It was introduced in 1970 by International Standards Organization (ISO) as a standard covering all aspects of network communication. The OSI model is composed of seven ordered layers as follows:

1)    Layer 1 – physical
2)    Layer 2 – data link
3)    Layer 3 – network
4)    Layer 4 – transport
5)    Layer 5 – session
6)    Layer 6 – presentation
7)    Layer 7 – application

There is a simple method to remember all these layers by using a sentence “Please Do Not Take Sales Persons Advice”. First letter of each word corresponds to a layer name sequentially. The sending and receiving device uses all those seven layers for message communication.

Each layer defines a family of functions distinct from those of the other layers. Within a machine, each layer calls the services of the layer just below it. Between two machines, Layer x on one machine communicates with layer x on another machine. Each layer in the sending device adds its own information to the message it receives from upper layer and passes the whole package to its lower layer. Let’s describe the functions of each layer.

1) Physical Layer: It is responsible for movement of a bit stream over a physical medium. It defines the functions that physical devices and interfaces have to perform for transmission. The physical layer data consists of a stream of 0s and 1s.

Physical layer also defines the type of encoding for data transmission, the transmission rate and the direction of transmission between two devices.

2) Data Link Layer: Its function is to move data frames from one station to other. Data link layer divides the stream of bits into number of frames. Also the physical address of the receiver device is added to the data frame. It provides an error control mechanism to detect and retransmit corrupt or lost frames.

3) Network Layer: The network layer is responsible for the delivery of individual data packets from the source-to-destination across multiple networks. It adds the logical address of the sender and receiver to the packet. It also provides routing facility.

4) Transport Layer: It is responsible for process-to-process delivery of the entire message. Since a computer may run several programs at same time. So the transport layer includes a port address to deliver the message to the correct process of the receiving computer.

It also reassembles the segmented parts of the message at the destination node. And provides flow control and error control mechanisms.

5) Session Layer: It establishes, maintains and synchronizes the interaction between communicating devices. It is also called the dialog controller.

6) Presentation Layer: It is responsible for translation, compression and encryption of information.

7) Application Layer: It is responsible for providing services to the user. It Provides user interfaces and support for services like email, remote file access and shared database management services.

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