OSI Model

The OSI Model (Open Systems Interconnection)  is developed by the International Standard Organization. It is a layered framework for the design of network systems that allows communication between all types of computer systems. Its primary purpose is to provide a set of structural guidelines for exchanging information between computers, workstations, and networks.

OSI model describes how information from a software application in one computer moves through a physical medium to the software application in another computer. OSI consists of seven layers, and each layer performs a particular network function. The  OSI model was developed by the International Organization for Standardization (ISO) in 1984, and it is now considered an architectural model for inter-computer communications. OSI model divides the whole task into seven smaller and manageable tasks. Each layer is assigned a particular task. Each layer is self-contained so that tasks assigned to each layer can be performed independently.

The OSI model is divided into two layers: upper layers and lower layers.

1. The upper layer of the OSI model mainly deals with application-related issues, and they are implemented only in the software. The application layer is closest to the end-user. Both the end-user and the application layer interact with the software applications. An upper layer refers to the layer just above another layer.
2. The lower layer of the OSI model deals with data transport issues. The data link layer and the physical layer are implemented in hardware and software. The physical layer is the lowest layer of the OSI model and is closest to the physical medium. The physical layer is mainly responsible for placing the information on the physical medium.

Functions of the OSI Layers

There are seven OSI layers. Each layer has different functions. A list of seven layers are given below:

1. Physical Layer
2. Data-Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer

Physical Layer

The Physical Layer is the lowermost layer in the OSI model and its major responsibility includes the actual propagation of the unstructured data bits (0’s and 1’s) across the network, from the physical layer of the sending device to the physical layer of the receiving device.

The Physical layer contains information in the form of bits. It transmits individual bits from one node to the next node. The transmission media defined by the physical layer include metallic cable, optical fiber, and the wireless radio-wave.

Physical Layer Includes −

• Bit Synchronization – Physical layer provides the bit synchronization of bits by providing a clock. This clock controls the sender and receiver providing synchronization at bit level.
• Bit Rate Control – Physical layer defines the transmission rate. The number of bits sent per second.
• Physical Topologies – The physical layer specifies how the different devices are arranged in a network (bus, ring, star, and mesh topology).
• Transmission mode – The physical layer checks if the transmission is simplex, half-duplex, or full-duplex. It defines how the data flows between the two connected devices.

Data Link Layer

It is the second layer of the OSI model. The data link layer is responsible for providing error-free communication across the physical link connecting the primary and secondary nodes within a network. It provides hop-to-hop delivery. It packages the data from the physical layer into a group called blocks.

The data link layer provides the final framing of the information signal, and it provides synchronization facilities for the orderly flow of data between the nodes.

• Framing − Breaks messages into frames and reassembles frames into messages.
• Error handling − It is used to soles the damaged, lost, and duplicate frames.
• Flow Control − It keeps a fast transmitter from flooding a slow receiver.
• Access Control − In access control, if many hosts have usage of the medium, When a single communication channel is shared by multiple devices, the MAC sub-layer of data link layer helps to determine which device has control over the channel at a given time.

Network Layer

The network layer provides details that enable data to be routed between devices in an environment using multiple networks, sub-networks, or both.

The networking components that operate at the network layer include routers and their software. It determines which network configuration is most appropriate for the function provided by the network and addresses and routes data within a network by establishing, maintaining, and terminating connectors between them.

It provides the upper layers of the hierarchy with independence from the data transmission and switching technologies used to interconnect systems.

It also provides the source and destination network addresses, subnet information, and source and destination node addresses. In this, the network is subdivided into subnet-work that is separated by routers.

• Internetworking: An internetworking is the main responsibility of the network layer. It provides a logical connection between different devices.
• Addressing: A Network layer adds the source and destination address to the header of the frame. Addressing is used to identify the device on the internet.
• Routing: Routing is the major component of the network layer, and it determines the best optimal path out of the multiple paths from source to destination.
• Packetizing: A Network Layer receives the packets from the upper layer and converts them into packets. This process is known as Packetizing. It is achieved by internet protocol (IP).

Transport Layer

We can say that the transport layer controls and ensures the end-to-end integrity of the data message propagated through the network between two devices, providing the reliable, transparent transfer of data between the endpoints.

Responsibilities of Transport Layer

• Segmentation and Reassembly − In this, a message is divided into small pieces. Reassemble the message correctly upon arriving at the destination.
• Reliability − It ensures that packets arrive at their destination. Reassembles out-of-order messages.
• Service Decisions − It is used to check what types of service to provide error-free point-to-point, datagram, etc.
• Mapping − It determines which messages belong to which connections.
• Naming − It must be translated into an internal address and route, send to node XYZ.
• Flow Control − It keeps a fast transmitter from flooding a slow receiver.
• Error Control − To retransmit the damaged segments.

Session Layer

The session layer creates communication channels between devices. It is responsible for opening sessions, ensuring they remain open and functional while the data is being transferred, and closing the session when the communication ends.

The session layer can also set checkpoints during a data transfer. If a session is interrupted, then the devices can resume data transfer from the last checkpoint.

The session layer is responsible for network availability for data storage and processes capacity. It provides the logical connection entities at the application layer.

Session Layer Responsibilities −

• Network log-on and log-off procedures
• User authentication
• Determines the type dialog available − simplex, half-duplex, and full-duplex.
• Synchronization of data flow for recovery purposes.
• Creation of dialog units and activity units.

Presentation Layer

The presentation layer prepares the data for its upper layer or the application layer. It defines how two devices should encode, encrypt, and compress the data.

• The presentation layer receives any data transmitted by the application layer and prepares it for transmission over the session layer.
• It specifies how the end-user applications should format the data.
• This layer provides for the translation between the local representation of data and the representation of data that will be used for transfer between the end-users. The result of encryption, data compression, and virtual terminals are examples of translation services.

Application Layer

The application layer is the topmost layer in the OSI model and acts as the general manager of the network by proving access to the OSI environment. This layer provides distributed information services and controls the sequence of activities within an application and also the sequence of events between the computer application and the user of the application. It communicates directly with the user’s application program.

The application layer uses HTTP, FTP, POP, SMTP, and DNS protocols that allow the software to send and receive information and present meaningful data to users.

• File transfer, access, and management (FTAM): An application layer allows a user to access the files in a remote computer, retrieve the files from a computer, and manage the files in a remote computer.
• Mail services: An application layer provides the facility for email forwarding and storage.
• Directory services: An application provides the distributed database sources and is used to provide that global information about various objects.