Written by Tharun B

OSI Model

We speak. Birds chirp. But how do applications communicate with each other over a network? To understand, OSI model is used by IT professionals. OSI stands for Open Systems Interconnection, that is used to observe how networks send and receive data on a network. The main concept of OSI is that the process of communication between two endpoints in a network can be divided into seven distinct groups of related functions, or layers. Here we discuss about the seven layers of OSI Model, coming from the top.

Layer 7: Application Layer

This is the top layer in the model in which the users directly interact. The applications like Chrome, Skype, Firefox, etc. are included in this layer. The applications offer access to different sources like mail, remote logins and many more. Ex: HTTP, HTTPS, DNS, FTP (File Transfer Protocol), TELNET, SMTP etc.

Layer 6: Presentation Layer

This layer makes the data presentable for the application layer or the network i.e. it prepares the data that can be used by the applications. It is also used to define the form in which data is exchanged between the two communicating entities. Two devices may communicate using different encryption methods. So as to make the transmitted data understandable by both the devices, presentation layer transforms the received data in to the form in which the application can understand. To be specific, presentation layer performs the duty of encrypting, decrypting and transformation of data. Ex: SSL, SSH, JPEG, etc.

Layer 5: Session Layer

When two devices like servers or computers wish to speak, there should be a slot created for them. The time between the start and end of the communication is a slot. More technically speaking, the slot is known as a session. So, the time between the opening of a connection and closing the connection is referred to as a session. This layer determines how long a device should be actively listening for a response or send a response, simply helps in synchronization. Session creation includes validating credentials or login passwords, etc.

It also creates checkpoints to synchronize the transfer of data. For example, suppose we are transferring 100 bits of data. The session layer puts checkpoints at every, say 5 bits, point so that if there is any disconnection or crash of data transfer can resume from the last checkpoint. In other terms, known as Interruption control. This would save time and efficiency instead of starting the transfer from scratch. Ex: APIs, Sockets, etc.

Layer 4: Transport Layer

This layer is responsible for end-to-end communication between the devices. The data from the session layer is broken down into chunks called segments of data for easy transmission. The receiving application’s transport layer is responsible for assembling the chunks to complete data. This layer also maintains flow control and error control. Flow control ensures that a faster connection does not overwhelm the one with a slower connection. Error control makes the transmission of data to be complete without any loss of chunks. In case of any loss, it requests for re-transmission. It also controls the congestion. Ex: TCP, UDP.

Layer 3: Network Layer

This layer is responsible for transfer of data into and other networks.  The network layer breaks up segments from the transport layer into smaller units, called packets, known as packetization on the sender’s device, and reassembling these packets on the receiving device known as depacketization. If we are supposed to connect with the server in some other country, there are millions of paths that can be taken. But by using routers, the efficient path can be easily found. This process of finding the best path for the data to reach its destination is known as routing. Ex: IP.

Layer 2: Data Link Layer

The functionality is the same as the network layer with a small difference. The network layer controls over different networks whereas the data link layer has control on the same network. It also helps in correcting the mistakes at the physical layer. Ex: Ethernet. The data link layer is subdivided into two types of sub layers:

  1. Media Access Control (MAC) layer– It is responsible for controlling how device in a network gain access to medium and permits to transmit data.
  2. Logical link control layer– This layer is responsible for identity and encapsulating network-layer protocols and allows you to find the error.

Layer 1: Physical Layer

This layer involves only physical equipment required in communication. It represents the electrical and physical specifications of the system. It converts the data to the binary digits, 0s and 1s, that can be understandable by the devices. Ex: Hubs.

There are other models as well like TCP/IP. But OSI model is used as standard model for networking. The main difference between both is that OSI model has seven layers but TCP/IP has five layers in which both session, presentation and application layers duty is performed by a single layer: Application layer.

Neologism and Related terms:

  1. Switching: It is the process of forwarding the network packets coming from one port to the other port it is intended to. It is performed by a switch. More about switches in the references.
  2. Routing: It is the process of selecting a path (route in networking) for the network traffic across the networks.
  3. Congestion: It is the state in which the traffic becomes so high(than its capability) resulting in the increase of response time from the network.
  4. Handshake: It is the identification of the devices for each other and allowing further communication.
  5. Framing: It is a function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.

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