7 OSI Model layers with explanation

What is OSI Model –

• OSI Stands for Open System Interconnection
• It was developed by ISO (International Standards Organization)
• It is a framework that describes how different networking technologies work together
• It is not a protocol that must be followed in order to communicate.
• Each layer in this model has specific functions

Physical Layer

The Physical Layer is the first layer in the OSI model, and it deals with the physical transmission of data over a communication channel. It defines the physical specifications of the hardware required to connect two devices, such as cables, connectors, and interfaces. The main functions of the Physical Layer are:

1. Bit synchronization: It provides a method for the sender and receiver to synchronize the timing of the transmitted bits so that they can be correctly interpreted.
2. Encoding: It defines the way data is represented in bits so that it can be transmitted over the channel. It may involve techniques like Manchester encoding or Differential Manchester encoding.
3. Transmission mode: It defines how bits are transmitted over the channel, such as simplex, half-duplex, or full-duplex.
4. Physical media: It defines the type of physical media used for communication, such as twisted-pair copper cables, fiber-optic cables, or wireless.
5. Line coding: It converts digital signals to electrical or optical signals that can be transmitted over the physical media.
6. Physical addressing: It defines the physical addressing scheme used to identify devices on the network, such as the MAC address.

The Physical Layer provides a means for transmitting raw data bits over a communication channel. It does not provide any error detection or correction mechanisms, and it is concerned only with the physical transmission of data. The functions of the Physical Layer are implemented in hardware, such as network interface cards (NICs), repeaters, and hubs.

Data Link

The Data Link Layer is the second layer in the OSI model, and it provides reliable communication between two devices that are directly connected. The Data Link Layer is responsible for the transmission of data frames between two nodes over the physical layer. The main functions of the Data Link Layer are:

1. Framing: The Data Link Layer divides the data into smaller units called frames. Each frame contains a header and a trailer, which provide information about the frame, such as the source and destination addresses.
2. Flow control: The Data Link Layer regulates the flow of data between the sender and the receiver. It ensures that the sender does not overwhelm the receiver with data.
3. Error control: The Data Link Layer provides error detection and correction mechanisms. It uses techniques like checksums and cyclic redundancy checks (CRC) to detect errors in the transmitted frames. It may also use retransmission of frames to correct errors.
4. Access control: The Data Link Layer provides a mechanism for controlling access to the physical media shared by multiple devices. It uses techniques like Carrier Sense Multiple Access with Collision Detection (CSMA/CD) for Ethernet networks.
5. Addressing: The Data Link Layer uses physical addressing to identify the source and destination devices on the network. In Ethernet networks, the Media Access Control (MAC) address is used for addressing.

The Data Link Layer ensures the reliable transmission of data over a physical link between two devices. The functions of the Data Link Layer are implemented in hardware, such as network interface cards (NICs), switches, and bridges. The Data Link Layer is an important layer in the OSI model because it provides a reliable communication channel between directly connected devices, which is a fundamental requirement for building any type of network.

Network Layer

The Network Layer is the third layer in the OSI model, and it provides logical addressing and routing functions for data between networks. The Network Layer is responsible for transmitting data between two nodes that are not directly connected. The main functions of the Network Layer are:

1. Logical addressing: The Network Layer provides logical addressing to identify the source and destination devices on the network. In IP networks, the Internet Protocol (IP) address is used for addressing.
2. Routing: The Network Layer determines the best path for data transmission from the source device to the destination device. It uses routing protocols to exchange routing information and build routing tables. The routing tables contain information about the available paths to reach different networks.
3. Fragmentation and reassembly: The Network Layer is responsible for breaking down large packets into smaller packets for transmission across the network. It also reassembles the packets at the destination.
4. Quality of Service (QoS): The Network Layer provides QoS functions, such as prioritization and traffic shaping, to ensure that data is delivered according to specific requirements.
5. Network congestion control: The Network Layer monitors the network for congestion and provides congestion control mechanisms to prevent network congestion.

The Network Layer provides a logical connection between devices on different networks. It ensures that data is delivered to the correct destination and takes the most efficient path through the network. The functions of the Network Layer are implemented in software, such as routers and layer 3 switches. The Network Layer is an important layer in the OSI model because it enables the communication between devices on different networks, which is a fundamental requirement for building large-scale networks.

Transport Layer

The Transport Layer is the fourth layer in the OSI model, and it is responsible for providing end-to-end communication between applications on different devices. The Transport Layer provides services to the application layer above it and uses the services of the Network Layer below it. The main functions of the Transport Layer are:

1. Segmentation and reassembly: The Transport Layer breaks down the data received from the application layer into smaller segments for transmission across the network. It also reassembles the segments at the destination.
2. Connection-oriented or connectionless communication: The Transport Layer provides two modes of communication: connection-oriented and connectionless. In connection-oriented communication, a virtual circuit is established between the sender and the receiver before data transmission. This ensures that data is delivered in the correct order and with no loss. In connectionless communication, there is no virtual circuit established, and each packet is sent independently.
3. Flow control: The Transport Layer regulates the flow of data between the sender and the receiver. It ensures that the sender does not overwhelm the receiver with data.
4. Error control: The Transport Layer provides error detection and correction mechanisms. It uses techniques like checksums to detect errors in the transmitted data.
5. Multiplexing and demultiplexing: The Transport Layer can multiplex data from multiple applications into a single stream for transmission across the network. At the destination, it demultiplexes the data and delivers it to the appropriate application.

The Transport Layer provides reliable communication between applications on different devices. The functions of the Transport Layer are implemented in software, such as Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). TCP is a connection-oriented protocol that provides reliable, ordered data transmission, while UDP is a connectionless protocol that provides unreliable, unordered data transmission.

The Transport Layer is an important layer in the OSI model because it provides end-to-end communication between applications on different devices. This enables applications to communicate with each other reliably, regardless of the underlying network technology.

Session Layer

The Session Layer is the fifth layer in the OSI model, and it is responsible for establishing, maintaining, and terminating communication sessions between applications running on different devices. A session is a logical connection between two applications, and the Session Layer manages the session by coordinating the exchange of data between them. The main functions of the Session Layer are:

1. Session establishment: The Session Layer establishes a session between two applications. It manages the negotiation of session parameters such as data transfer rates, data synchronization, and data exchange formats.
2. Session management: The Session Layer is responsible for managing the session once it is established. It ensures that data is exchanged correctly between the two applications and that the session remains active until it is terminated.
3. Session synchronization: The Session Layer provides mechanisms for synchronizing the data exchange between the two applications. It ensures that data is exchanged in the correct sequence and that there are no gaps or overlaps in the data exchange.
4. Session termination: The Session Layer terminates the session between two applications when the data exchange is complete or when an error occurs. It ensures that all data is exchanged correctly before terminating the session.

The Session Layer provides a way for applications to communicate with each other using a common protocol. The functions of the Session Layer are implemented in software, such as the Remote Procedure Call (RPC) and Network File System (NFS).

The Session Layer is an important layer in the OSI model because it provides a way for applications to establish and manage sessions with each other. This enables applications to communicate with each other in a way that is independent of the underlying network technology.

Presentation Layer

The Presentation Layer is the sixth layer in the OSI model, and it is responsible for the presentation of data to the application layer. The Presentation Layer provides services to the application layer above it and uses the services of the Session Layer below it. The main functions of the Presentation Layer are:

1. Data representation: The Presentation Layer is responsible for data representation, which involves translating data from the format used by the application layer into a common format that can be understood by both the sending and receiving devices. This process includes data compression, encryption, and character encoding.
2. Data compression: The Presentation Layer compresses data to reduce the amount of data that needs to be transmitted across the network.
3. Encryption: The Presentation Layer encrypts data to ensure its confidentiality and integrity during transmission across the network.
4. Character encoding: The Presentation Layer converts data into a common character set that can be understood by both the sending and receiving devices. This process includes converting text into ASCII, Unicode, or other character sets.
5. Data formatting: The Presentation Layer formats data for presentation to the application layer. This process includes converting data into the appropriate format for display or printing.

The Presentation Layer provides a way for applications to exchange data in a format that is independent of the underlying network technology. The functions of the Presentation Layer are implemented in software, such as the Portable Network Graphics (PNG) format and the Joint Photographic Experts Group (JPEG) format.

The Presentation Layer is an important layer in the OSI model because it provides a standardized way for applications to exchange data. This enables applications to communicate with each other regardless of the underlying network technology and ensures that data is presented in a format that can be understood by both the sending and receiving devices.

Application Layer

The Application Layer is the seventh and topmost layer in the OSI model, and it is responsible for providing services to the end-user applications. The Application Layer interacts directly with the end-user and is the layer that users interact with when they use applications like web browsers, email clients, and file transfer programs. The main functions of the Application Layer are:

1. Network services: The Application Layer provides access to network services like email, file transfer, and remote login.
2. User interfaces: The Application Layer provides user interfaces that allow users to interact with network services. This includes graphical user interfaces (GUIs) and command-line interfaces (CLIs).
3. Data exchange: The Application Layer is responsible for exchanging data between applications running on different devices. This includes exchanging data in different formats such as text, images, and audio.
4. Network virtual terminal: The Application Layer provides a network virtual terminal (NVT) that allows remote users to interact with the network as if they were physically connected to it.
5. Application-level protocols: The Application Layer implements application-level protocols like HTTP, SMTP, and FTP that allow applications to communicate with each other over the network.

The Application Layer provides a way for end-users to access network services and exchange data between applications running on different devices. The functions of the Application Layer are implemented in software, such as web browsers, email clients, and file transfer programs.

The Application Layer is an important layer in the OSI model because it provides a way for end-users to access network services and exchange data with other users over the network. This enables users to communicate with each other regardless of the underlying network technology and ensures that data is exchanged in a format that is compatible with the end-user applications.

So, in this article, we learned about the different layers of the OSI Model. In conclusion, the OSI (Open Systems Interconnection) model is a conceptual framework for understanding how computer networks function.