Friday, Jun 23rd

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OSI Layers - Building blocks of Networking

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Whenever we think about computer networking and its implementation, the first thing to come to our mind is the OSI (Open Structure/System Interconnection) model. It is the basic structure of computer networking. OSI is a layered structure of 7 layers. Each layer has its own functionality. Here we will understand the purpose of each layer briefly. Below is the diagram, which shows how layers on one machine interact with other side on a network on a different machine, which can also be a Server.

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OSI Protocol Stack

The OSI Stack of Layers

From top to down

  1. Application Layer: This layer is the top most & nearest to the user. Everything at this layer is application specific. It represents services which directly support user application. Data is called Message at this layer. Lots of protocols run over application layer e.g. FTP, SNMP, Telnet, DNS, DHCP, Remote login, email. Internet Explorer, Mozila, Google Chrome are few famous applications, which we use for internet.OSI Model
  2. Presentation Layer: To communicate with someone, we have to know about his language & need to change our data in that other party's understandable language. This layer also works in a similar way; present the data in the required form i.e. it converts the data from application to network format & vice-versa. This layer is also helpful for protocol conversion, encryption/decryption, data compression etc. It is also called syntax layer, because it changes the syntax of data according to the application or network. Message comes from Application layer.
  3. Session Layer: For every communication, we need to create a connection with particular party, need to maintain that connection & in the end, disconnect. This layer work is to establish a connection, maintain & then successfully terminate the connection between separate applications on the network. It helps in identification of the other end so that only the designated party can participate in communication.
  4. Transport Layer: To deliver any packet to anywhere, we use some good transport medium, which keep & deliver the packet safe & secure. Like that in OSI layer, transport layer is one of the main layers. It is responsible for end-to-end reliable data transfer, error recovery & flow control. TCP (Transmission Control Protocol) connection-oriented & UDP (User Datagram Protocol) connection-less, are 2 types of protocol, which works on this layer. We will discuss about these protocols in separate session. It divides the data stream into chunks or packets, which assemble on the other side of communication link.
  5. Network Layer: Transport layer needs some path to follow to deliver the packet to destination. That is the functionality of this layer; find the path to deliver the packet to its destination with the same quality of service requested by upper layer. Switching & Routing is the main functionality of this layer. This layer also converts the IP address (logical address) to MAC address (Physical address) & vice-versa. Layer 3 switches works on Network layer. In L3 switch, we can mention IP address to ports. Each packet is stamping on each node between the end users, which helps into count the hop for the packet.
  6. Data Link Layer: Second layer of the OSI layer is Data Link Layer, which works on Hardware (MAC) address. Data packets are divided into frames on this layer and transfer over the physical layer. The acknowledgment of successful delivery, from the receiver for frames is also handled by this layer. This layer concerned about the frame delivery over the same network only.
  7. Physical Layer: Physical layer is the actual hardware i.e. Network Interface Card (NIC), which transfer the data. NIC card is connected directly with other NIC card with some medium, usually twisted/coaxial/fiber-optical wire. Data transmission on physical layer is in raw bit form, which is producible by electrical/mechanical.

Every layer has its own importance. Whenever a packet originates from a machine, it passes from all 7 layers (Application to Physical) of the OSI model. Every layer insertes its header, according to its functionality, in the packet & forwards it to next layer. On the receiving end, when the packet passes all the 7 layers in reverse order i.e. Physical to Application, the header from of each layer is removed on the corresponding layer and we get the same packet on Application layer, which we sent from other end.

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