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If you are a beginner learner of cybersecurity or networking, you have probably heard the term OSI model in computer networks used in the context of network communication. Although the typical internet user will probably never need to know how things are running behind the scenes of the world wide web, it is still an interesting concept and could be useful for anyone who wishes to dabble in the world of networking. Let us understand what is OSI model and OSI model layers are.
What is OSI Model in Computer Network? [Definition]
The OSI model in computer network or OSI stands for Open System Interconnection model is a conceptual model that describes the functionality of the networking system with no regard to the underlying technology infrastructure. The OSI model was introduced in 1984. It is divided into seven abstract layers of data communication and standardized protocols into appropriate groups of networking functionality.
The purpose of the OSI reference model is to guide vendors and developers so the digital communication products and software programs they create will interoperate, and to facilitate clear comparisons among communications tools. It is designed to be an abstract model and teaching tool, the OSI model remains a useful tool for learning about today's network technologies such as Ethernet and protocols like the Internet Protocol.
OSI Model Layers
As we mentioned previously, the OSI model is divided into 7 different layers and here we are explaining these 7 layers of OSI model.
Layer 1: Physical Layer
At Layer 1, the Physical layer in the OSI model is responsible for the ultimate transmission of digital data bits from the Physical layer of the sending (source) device over network communications media to the Physical layer of the receiving (destination) device. Data are transmitted through the type of signaling supported by the physical medium: electric voltages, radio frequencies, pulses of infrared or ordinary light. Some technologies include Ethernet cables and Token Ring networks. Additionally, hubs and other repeaters are standard network devices that function at the Physical layer, as are the cable connectors.
Layer 2: Data Link layer
The Data Link layer in OSI model manages the data transmission between physically connected devices like switches. The raw data from the physical layer after being checked for physical transmission errors are packed into data frames that contain the protocols to route information between the appropriate nodes. The Data Link Layer is divided into two sublayers Media Access Control (MAC) and Logical Link Control (LLC). The Data Link layer manages physical addressing schemes such as MAC addresses for Ethernet networks, controlling access of any various network devices to the physical medium. While LLC is responsible for the flow controls and error controls that ensure error-free and accurate data transmission between the network nodes.
Layer 3: Network Layer
The Network layer adds the concept of routing above the Data Link layer. When data arrives at the Network layers, the source and destination addresses contained inside each frame are examined to determine if the data has reached its final destination. If the data has reached the final destination, this Layer formats the data into packets ready to be delivered up to the Transport layer. Otherwise, the Network layer updates the destination address and pushes the frame back down to the lower layers. To support routing, the Network layer maintains logical addresses such as IP addresses for devices on the network. The Network layer also manages the mapping between these logical addresses and physical addresses. In IP networking, this mapping is accomplished through the Address Resolution Protocol (ARP).
Layer 4: Transport Layer
The transport layer in the OSI model ensures reliable and complete delivery of data packets. This layer provides flow control, error control, and congestion control which keeps the packets error-free and sends the information to the other layers. Layer 4 ensures that a packet is sent in response to a specific process by using the service-point addressing function. The most common protocols include the Transmission Control Protocol (TCP) for connection-oriented data transmission and User Datagram Protocol (UDP) for connectionless data transmission.
Layer 5: Session Layer
The Session Layer in OSI model manages the sequence and flow of events that initiate and tear down network connections. A common example includes HTTPS sessions which allow users to visit and browse websites for a specific time period. Layer 5 is responsible for a wide array of functions including setting up, destroying, and re-establishing session activities, authorization, and authentication of communication between specific apps and servers. The most commonly used protocols include Point-to-point Tunneling Protocol (PPTP), Session Control Protocol (SCP), Remote Procedure Call protocol (RPC), and Session Description Protocol (SDP).
Layer 6: Presentation Layer
The presentation layer in OSI model is responsible for data compression, data translation, character code translation, encryption, and decryption of data. Layer 6 is responsible for translating incoming data into a syntax that the application layer of the receiving device can understand. If the devices are communicating over an encrypted connection, layer 6 is responsible for adding the encryption on the sender’s end as well as decoding the encryption on the receiver's end so that it can present the application layer with unencrypted, readable data. This layer is primarily responsible for preparing data so that it can be used by the application layer; in other words, layer 6 makes the data presentable for applications to consume.
Layer 7: Application Layer
The application layer in the OSI model directly interacts with the user data. Software applications rely on the application layer to initiate communications. This does not mean that the client software is part of the application layer but is responsible for the protocols and data manipulation that the software relies on to present meaningful data to the user. The most common protocols used in this layer are HTTP (HyperText Transfer Protocol) and SMTP (Simple Mail Transfer Protocol is an email communication protocol).
We hope that this blog helped you understand more about the OSI model and its 7 layers of OSI reference model. We went through all the layers of the OSI model, however, where can this information be used? From a practical standpoint, the OSI model is most useful when designing non-TCP networks. You can comment with your suggestions and feedback in our comment box.
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