OSI LAYER MODEL

 OSI LAYER MODEL

The Open Systems Interconnection (OSI) model defines seven levels through which computer systems communicate across a network. It was the first standard model for network communications, and it was adopted in the early 1980s by all major computer and telecommunications corporations.

The present Internet is built on the simpler TCP/IP paradigm rather of the more complex OSI model. The OSI 7-layer model, on the other hand, is still extensively used because it helps visualise and understand how networks work, as well as isolate and diagnose networking problems.

OSI was introduced in 1983 by representatives from major computer and telecommunications corporations, and it was recognised as a worldwide standard by ISO in 1984.

PHYSICAL LAYER

The physical layer contributes to the definition of the data connection's electrical and physical requirements. This layer is in charge of preparing physical network devices for data acceptance. It sends single bits from one node to the next. This layer also receives raw data, which is then sent to higher levels.

The link between a device and a physical transmission media is established at this level. Protocols and other higher-layer concerns are not addressed by the physical layer.

The examples are : network adapters, ethernet, repeaters, networking hubs.

DATA LINK LAYER

The data connection layer is in charge of repairing any errors that may arise at the physical layer. The layer allows you to choose the protocol for connecting and disconnecting two network devices.

It is a layer that assists in the definition of logical addressing so that any endpoint may and should be addressed.

The layer also contributes to the implementation of packet routing across a network. It supports you in choosing the best choice path for transferring data from one area to another.

The data link layer has two types of sublayers:

• Media Access Control Layer : Connection between the Logical Link Control layer and the network's physical layer. It is used to send packets over the network.
• Logical Link Control Layer: This layer is in charge of transferring packets to the Network layer of the receiver. It extracts the network layer protocol address from the header. It also controls the flow.

NETWORK LAYER

The Data link layer is in charge of routing and forwarding packets.

Routers are layer 3 devices, defined in this layer and used to offer routing services within an internetwork.

Network layer protocols are the protocols that are used to route network traffic. IP and IPv6 are examples of protocols.

Network Layer Functions: 

• Internetworking : The network layer's primary responsibility is internetworking. It establishes a logical link between many devices.
• Addressing : The source and destination addresses are added to the frame header by the network layer. Addressing is a method of identifying a device on the internet.
• Routing : Routing is a major component of the network layer that calculates the best optimal path from the source to the destination.
• Packetization : When a Network Layer gets packets from an upper layer and turns them to packets. Packetizing is the name given to this technique. The internet protocol is responsible for this (IP).

TRANSPORT LAYER

The Transport layer, which is a Layer 4, ensures that messages are sent in the correct order and that no data is duplicated.

The transport layer's primary responsibility is to finish the data transfer.

It receives data from the upper layer and divides it into smaller units called segments.

This layer is known as an end-to-end layer because it provides a point-to-point connection between the source and destination in order to deliver data reliably.

This layer employs two protocols:

• Transmission Control Protocol
• User Datagram Protocol

Functions of Transport Layer:

• Service-point addressing
• Segmentation and reassembly
• Connection control
• Flow control
• Error control

SESSION LAYER

The Session layer is responsible for establishing, maintaining, and synchronising communication between devices.

Session layer functions include:

• Dialog control: The session layer works as a dialogue controller, creating a dialogue between two processes or allowing communication between two processes that can be half-duplex or full-duplex.
• Synchronization: When transmitting data in a sequence, the session layer inserts certain checkpoints. If a mistake occurs in the middle of data transmission, the transmission will restart from the checkpoint. This is referred to as synchronisation and recovery.

PRESENTATION LAYER

A Presentation layer is primarily concerned with the syntax and semantics of the information transferred between the two systems. It serves as a data translator for a network. This layer is an operating system component that translates data from one presentation format to another. The syntax layer is another name for the Presentation layer.

Presentation layer functions include:

• Translation : the interchange of information between two systems in the form of character strings, integers, and so on. The presentation layer handles the interoperability between the various encoding schemes used by different computers. At the receiving end, it translates the data from a sender-dependent format to a common format and then from the common format to a receiver-dependent format.
• Encryption : To guarantee privacy, encryption is required. Encryption is the process of transforming the information sent by the sender into another form and sending the resulting message through the network.
• Data compression : The process of compressing data, which minimises the number of bits that must be communicated. Data compression is critical in multimedia applications such as text, audio, and video.

APPLICATION LAYER

An application layer acts as a portal via which users and application processes can access network services.

It deals with concerns like network transparency, resource allocation, and so on.

Although an application layer is not an application, it does fulfil application layer functions.

This layer delivers network services to end users.

Application layer functions :

• File transfer, access, and management (FTAM) : which allows a user to access files on a remote computer, retrieve files from a computer, and manage files on a remote computer.
• Mail services: An application layer handles email forwarding and storage.
• Directory services: A directory service is an application that provides distributed database sources and is used to deliver global information about various things.