Do you know what is the OSI model in computer networks?
Introduction
The Open Systems Interconnections (OSI) model is a conceptual model which consists seven layers of steps in communicating over a computer network. This OSI model is actually a set of standards and protocols that are imposed in communicating over a computer network. Because of these set of standards, different computer networks can communicate with each other as they follow the same standards. OSI model is the first set of standards that is used in computer network. It was in the early 1980s when this OSI model was used in effect in the existing computer networks.
Why we need OSI model?
This OSI model is very helpful when it comes to managing or troubleshooting a computer network. That is because the network manager or the person in responsible to the computer network can directly identify the particular problem that is encountered in the network as this model works layered architecture. Therefore, in the process of debugging the network manager does not need to surf the whole network to find the problem, in fact he only needs to do adjustments in a particular layer. As these layers follow abstraction from each other layers, changes made in a particular layer does not affect the other layers. This property is very helpful for a network administrator in managing a network.
Layers in the OSI model
We will go through these layers in the top to bottom order from application layer to the physical layer.
Application Layer (Layer 7)
Application layer is the closest layer to the end user. It is mostly used by end user software such as file transfer applications, email clients and web browsers. This layer is the main interface between the computer network and the user. The user sends and receives data through this layer. The communication is initiated via this layer. Actually, the client application software does not belong to the application layer, it is only responsible for handling data which are pulled into the software and pushed out of the software. Some of the protocols that are used in here are Simple Mail Transfer Protocol (SMTP), File Transfer Protocol (FTP) and Hyper Text Transfer Protocol (HTTP).
Presentation Layer (Layer 6)
In the presentation layer, it starts the process of transferring the raw data that is being sent by the application layer to a format which the network can transmit. In the other way around, if we consider when the data is sent from the presentation layer to the application layer in the receiver’s end, the presentation layer is responsible of transferring data to a format the application layer can process. This layer is responsible of compressing, translating, and encrypting data. Through encryption and compression done in this layer the security of the data and the efficiency of transferring the data over a network is increased.
Session Layer (Layer 5)
The session layer is responsible in establishing concluding communication channels between the devices inside a network. The time between establishment of a channel and the concluding of a channel is known as a session. The duty of the session layer is to open a session when it is needed and keeping it open while it is needed and closing the session when the work is done. The session layer is also capable of synchronizing the data transfers through having checkpoints in the data that is being transmitted. Because of this, even if there was a disconnection or crash in the network happens while transferring a set of data, it does not need to send the set of data from the beginning, it only needs to send data starting from the last checkpoint. This is a huge plus point in network communication as it saves resources and time.
Transport Layer (Layer 4)
The responsibility of the transport layer is to perform the error control and the flow control of data that is being transferred inside a network. Through error controlling the transport layer checks whether the receiving end has received the data correctly. Flow controlling is done in order to maintain the data flow rate according to the speed of the network and to match the receiving device’s speed. In the transport layer, the data that is sent from the session layer is divided into blocks called segments. And also, in the receiving end, the transport layer is responsible of reassembling the segments when the data is received from the network layer.
Network Layer (Layer 3)
The network layer breakdowns the data segments into smaller chunks of units called data packets which are received from the transport layer. In the receiving end it is responsible of reassembling the packets. The service of the network layer is more important when there is a connection between two different networks. When the data transmission is happening in the same network it is not that important as this layer is responsible of routing the data. The main functionality of the network layer is routing the data through routers of the network. This layer is responsible of choosing the most efficient route in sending a packet of data through a network. In order to do this task, it uses Internet Protocol (IP) addresses to find the destination.
Data Link Layer (Layer 2)
The data link layer is responsible of node-to-node data transmission through creating and terminating a connection specially inside the network or between physically connected devices. This layer breakdowns the data packets into frames which are received from the network layer. This layer has two main parts as Logical Link Control (LLC) and Media Access Control (MAC). LLC is responsible of synchronizing frames, error checking and identifying network protocols. MAC is responsible of defining permissions in transmitting and receiving data, and using MAC addresses in connecting devices. MAC address is an unique identifier designated to a network interface controller which helps in identifying a hardware component inside a network.
Physical Layer (Layer 1)
The physical layer is the last layer of this OSI model. There it is consist of physical equipment which are required to transmit the data from one network to another. In this layer the data frames which are received from the data link layer are transferred into a stream of bits which contains 1s and 0s in order to be transmitted to the destination node. This layer defines the hardware device standards or the wireless communication frequencies of the communication link takes place between the sender and the receiver.
Here is a bonus point for you. Although we talk about the OSI model, the modern internet is based on a different model called TCP/IP model which also very similar to OSI model but much simpler in comparison. Although OSI model is established by International Standard Organization (ISO), the TCP/IP model is created by Advanced Research Project Agency Network (ARPANET).
