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• The Internet as a whole is a collection of different layers functionally stacked upon each other. One such popular (and now ancient) model is the OSI Model.
• OSI Model stands for Open Systems Interconnection Model
• It was Developed in 1984 by the International Standards Organization (ISO).
• It is a standard of various standards that work with each other in synchronization to provide countinuous data transmission.
• The OSI system is divided into smaller parts called layers and it is considered to be the first layered model of communication.
• Before the advent of such layered models, every functions of the layers was written down in the application itself by the programmers.
• Whereas in a layered model like the OSI model, each layer is assigned some specific tasks that defines some key functions.
• These assigned tasks are completed in their respective layers with the help of standards (called as protocols)
• Each layer has one or more protocols to complete one or more tasks
• Each layer provides a set of functions to the layer above and in turn depends on set of functions of the layer below it.
• For example, on the receivers' end, data link layer gets the bits (data) from the physical layer, converts it into data frames and passes it onto the network layer.
• Also, each layer communicates with its peer layer on another side (denoted by the dotted lined in the diagram below) by sending control messages back and forth.
• For example, the transport layer of system A (transmitter) sends control messages to the Transport layer of system B (receiver).
• Transmits and receives bits through the network at the electrical level (through a wired medium).
• Defines protocols (standards) for sending and receiving data over the network devices.
• It acts as an interface between network layer and physical devices like hubs and switches.
• The physical layer standards defines optical, electric and mechanical characteristics of signals like voltage levels, voltage fluctuations, physical data rates, transmission distances and physical connections.
• Cables, Connectors and Hubs reside over the physical layer.
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• Takes a stream of bits from the physical layer and passes it onto the to the upper layers.
• It then converts the raw bits into data frames.
• It also performs the work of Error Detection and Correction.
• After the error correction process, it transmits each frame separately to the next layer (network layer)
• Devices like Switches reside over the network layer.
• Converts the frames that it received from the data link layer into packets.
• This layer is responsible for assigning logical address (IP address) to the devices.
• Network layer also translates logical network address to their physical address (e.g., Device name to MAC address).
• It determines the routes for sending the data efficiently over the network.
• It also takes care of network problems such as data congestion and data lost.
• Network layer converts the data frames received by the data link layer into packets.
• It also updates the higher layers with the details of how the data reaches its destination.
• Device like Routers reside ove the network layer.
• Converts the packets that it received from the network layer into segments.
• It also assures that the segments are now ready for transmission over the network.
• Provides error-checking to guarantee error-free data delivery, with no losses or duplications of segments.
• It also provides acknowledgment of successful transmissions.
• Requests retransmission if some packets don’t arrive error-free or don't arrive at all.
• Transport layer is equipped with features like flow control and error-handling of data.
• Devices like Firewalls reside ove the network layer.
• Initiates, maintains and ends sessions across the network.
• For example, if you close the data connection of your system, you actually end the session. Sessions can also be started and terminated automatically by the session layer.
• Responsible for identification of the devices so only the authorized people can participate in the session.
• Provides synchronization services by planning check points in the data stream.
• If session fails, only data after the most recent checkpoint needs be transmitted.
• Manages who can transmit data at a certain time and for how long.
• The most important function of the presentation layes is of formatting data.
• Different formats from all sources are made into a common uniform format that the rest of the OSI layers can understand.
• Responsible for protocol conversion and data encryption/decryption.
• Used for applications that user sees and directly works on.
• Allows access to network services that support applications.
• It includes application like web browsers (firefox, chrome, etc.).
• Directly represents the services that directly support user applications (e.g., file transfer and email).
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