OSI Reference Model-Networking 101

OSI_model | Networking | Sanket R Jain

OSI reference Model is a logical model not physical model. This model provides a guidelines for developers that would help them to develop applications that run on networks. OSI also provides a framework for creating various networking standards and guidelines. There are in all 7 layers in OSI model.

OSI_model | Networking | Sanket R JainThe upper 3 layers provides with the understanding on how applications on user end would communicate with each other.

OSI_upper_Layers | Networking | Sanket R JainThe lower 4 layers provides with information on how data is been handled, transported along the networks.

OSI_Lower_Layer | Networking | Sanket R Jain Application Layer

This layer is layer 7 in OSI model. This layer marks the spot where user actually communicate with computers and also comes into play when there is an access to network.Thus, this layer acts as an interface between the actual application program and layer beneath in order to pass on the information. The following are the some of the protocols that work at the application layer are:-

  • HTTP

  • FTP

  • DNS

  • SMTP


Presentation Layer

Purpose of this layer can incurred from the name itself. The main purpose of this layer to present the data in the right format to the Application layer along with data translation and code formatting. This layer can also be considered as providing coding and conversion services. This layer also performs some other functions like data compression, decompression, encryption and decryption.

Session Layer

All communication management of user with other host is been by session layer. Communication management includes creating a session, dismantling a session and keeping each data separate for each application. Session layer also maintains the communication modes between client and server like simplex, half- duplex, full duplex mode. Simplex communication includes only one way communication, half duplex is two way communication but can exist in direction at a time. Full duplex is normal real world communication where both users can send and receive message.

Transport Layer

Transport layer segments and reassembles the data into single data stream. Data received from upper layers are been combine into single data stream. This layer also provides end to end transport services to the data and also established connection between host for communication. The most common protocols TCP and UDP work at this layer.¬† TCP provides with reliable connection where as UDP doesn’t which means Transport layer be either connection oriented connectionless oriented. The application developer selects any of the two protocol to be use in the application for communication. This layer also provides mechanism for multiplexing upper layer application ,establishing sessions and can also hide details related to network from the higher layers.

Network Layer

The main function of network layer is track the networking devices and provide the best path to move data. Devices that work at this layer router and some layer 3 switches.So the working starts in this way, packet is received at router and destination network address of packet is been checked. If the address is been address to a particular router then packet is been sent through the route through an interface the destined router, if not then destination address is been looked up in the routing table and then forwarded to the destined router through an interface. And if network address is not found then packet is drop. Networkl layer deals with two type of packets:-

  1. Data Packets:- They are used to transport user data through inter network and protocols supporting this are IPv4 IPv6
  2. Network Packets:- These packets are use to send updates to the neighbouring router about the network connected to all routers within the inter network and protocol supporting this are RIP, RIPv2,EIGRP, OSPF.

Data Link Layer

This layer provides with the functionality of physical transmission of data, handles error notification, network topology and flow control. In a nutshell this layer makes sure that is been transmitted to proper device on LAN and also translates message from network layer into bits for physical layer. The data is been format into small data frames and customized header is been added in front of the frame with destination and source hardware (MAC) address.

data_frame_data_link_layer | Networking | Sanket R Jain

The header and trailer which is been added by data link layer is been truncated at the receiving end and only packet which contains the information is been given to upper later.

Note:- Data is just been encapsulated by the header which contains source and destination address and Trailer which contains checksum which is used to check whether the packet sent is error free or not and also tamper free.

Data link layer is has two sub layers:-

Logical Link Layer (LLC):- This layer is responsible for identifying network layer protocol and then encapsulating them and also this layer tells data link layer what to do with packet when it is been received. This layer also provides sequencing of bit and flow control.

data_link_layer_Sub_Layers \ Networking } Sanket R JainMedia Access Control (MAC):- This layer defines how layer is been placed in the media. Contention media is placed on the logic of “First come First Serve” since each packet shares the same bandwidth. Physical as well as logical topologies are been defined in this layer. Other functionality like line discipline, error notification, optional control flow, ordered delivery of frames are also done in this layer.


Physical Layer

This the bottom most layer in OSI model in which actual data bits are been transmitted over the network.Data transmitted and received in 1’s and 0’s. This layer communicates with different type of communication media and each media represents these bits in different way. Some represent them as audio tone and some represent at state transitions. This is an important layer it identifies¬†DTE- Data terminal Equipment and DCE- Data communication Terminal. DCE is the usually located at service provider and DTE is the attached device. Services offered to DTE are been accessed by modem or Channel service unit / Data service unit (CSU/DSU).

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