OSI Model & Its Functions In Networking

What is OSI Model ?

The OSI Model stand for (Open Systems Interconnection Model) is a network framework used to describe the functions of a computer networking system. OSI model was developed by the International Organization for Standardization (ISO) in 1984. In OSI model architecture has seven layers Physical, Data Link, Network, Transport, Session, Presentation, and Application. Each layer having own specific functionality to perform in communication between computer network system. OSI model provides a standard for different computer systems to able to communicate with each other. That why OSI model seen as universal language for computer networking.

OSI model

Seven Layers of OSI Model Explained

Application Layer (Layer 7)

The Application Layer is the one at the top of OSI model, this is only layer that directly interacts with data from the user. This layer is closest to the end user, it receives information directly from users and displays incoming data it to the user. Software applications like web browsers (Google Chrome, Safari, Firefox & Opera) and email clients (Gmail, Yahoo, Outlook) are examples of communications  that rely on Layer 7. Application layer protocols include HTTP, SMTP, SSH, Telnet, DNS & FTP.

Functions of Application Layer
1. Mail Services
2. Directory Services
3. Network Virtual Terminal
4. FTAM-File transfer access and management

Presentation Layer (Layer 6)

The Presentation layer is also called as the syntax layer because is mainly concerned with the syntax and semantics of the information exchanged between the two systems. This layer is a part of the operating system that converts the data from one format to another format. The presentation layer is responsible for translation, encryption and decryption.

Functions of Presentation Layer
1. Translation: Different computers use different encoding method, So the presentation layer responsibility to converts the data from sender-dependent format into a common format and changes the common format into receiver-dependent format at the receiving end.

2. Encryption/Decryption: Encryption is needed to maintain privacy in communication. Layer 6 is responsible for adding the encryption on the sender’s end as well as decryption on the receiver’s end.

3. Compression: Presentation layer is also responsible for compressing the data it receives from the layer 7 before delivering it to layer 5. It reduces the number of bits to be transmitted, it helps to improve the speed and efficiency of communication.

Session Layer (Layer 5)

When two devices, computers or servers want to communicate with each other, they need to create a session, and this is done by session layer. Session layer responsible for opening and closing communication between the two devices. This layer ensures that the session stays open long enough to transfer all the data being exchanged. Authentication and authorization happen at this layer.

Functions of Session Layer
1. Synchronization: The best thing of the session layer is, it synchronizes data transfer with checkpoint. Means if you are transferring 100MB data in which only 60MB was transferred and the session was disconnected. When you reconnect the session, it resumed from the last checkpoint.

Transport Layer (Layer 4)

Transport Layer is responsible for end-to-end communication between the two devices or host. This includes taking data from the layer 5 and breaking it up into pieces known as segments before sending it to network layer. The transport layer on the receiving device is responsible for reassembling the segments into data and send it to the session layer. The main two protocols used in transport layer are Transmission Control Protocol (TCP) & User Datagram Protocol (UDP).

Functions of Transport Layer
1. Error Control: The transport layer performs error control on the receiving system by ensuring that the data received is complete. Sender transport layer ensures that message reach at the destination without any error. After receiving complete data receiver send acknowledgement to the sender.
2. Flow Control: Flow control determines an optimal speed of transmission to ensure that a sender with a fast connection doesn’t overwhelm a receiver with a slow connection.

Network Layer (Layer 3)

Network layer that manages device addressing, tracks the location of devices on the network. This layer takes segments from transport layer and convert it into smaller unit called packets. Each packet has a specific address and the network layer ensures that the data is sent to its destination address. On receiver end network layer responsible for reassembling these packets.

Functions of Network Layer
1. Addressing: Network layer labels the source and destination address to the header of the packet. Addressing is used to identify the device on the internet.
2. Routing: network layer also finds the best physical path out of multiple path for the data to reach its destination; this process is known as routing.

Data Link Layer (Layer 2)

The data link layer is very similar to the network layer, except the data link layer facilitates data transfer between two devices on the same network. data link layer takes packets from the network layer and breaks them into smaller pieces called frames and forward it to physical layer. Data link layer has two sublayer—Logical Link Control (LLC) layer, which identifies network protocols, performs error checking and synchronizes frames, and Media Access Control (MAC) layer which uses MAC addresses to connect devices and define permissions to transmit and receive data.

Functions of Data Link Layer
1. Access Control: When two or more devices are connected to the same communication channel, then the MAC helps to determine which device has control over the channel at a given time.
2. Flow and Error control.

Physical Layer (Layer 1)

Physical layer is the lowest layer of OSI model. which represents the electrical and physical representation of the system. This can include everything from the cable type, radio frequency link (as in an 802.11 wireless systems), This is also the layer where the data gets converted into a bit stream, which is a string of 1s and 0s. 

Functions of Physical Layer
1. Bit rate control: Physical layer also defines the transmission rate i.e. the number of bits sent per second.
2. Topology: It defines the way how network devices are arranged. i.e. bus, star or mesh topology.
3. Data Transmission: It defines the transmission mode whether it is simplex, half-duplex or full-duplex mode between the two devices on the network.

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