Open Systems Interconnection Model

                      Open Systems Interconnection model

0SI (Open System Interconnection) model depicts that how applications can communicate over a network.

Whenever we connect two or more computers to share information, we need to create a data network This data network uses several protocols during the data exchange process. OSI Layer model explains this process and used protocols in simple way by dividing it's in seven layers.

Why it was developed?

Before 1980, developing protocols was the responsibility of hardware manufactures.

They developed proprietary protocols to support their own products.

With proprietary protocols we were not able use mix equipment from different manufactures on same network to run a network we had to buy all devices from the same company. This entire method makes collaboration between different types of computers nightmare.

To sort out this issue, open standard protocols were developed.

Open standards are industry-wide protocol that are not tied with a specific manufacturer company. With open standard protocols, we can use equipment from any vendor to build our network

As long as the equipment implements the standard protocols.

With open standard protocols, networks can consist of many different computing platforms running entirely different software.

Protocols are implemented on a network in several ways. Some are integrated into the operating system. Others are device drivers, such as the driver for a particular network interface adapter supplied by its manufacturer.

Many organizations are involved in developing protocols for networking

·         American National Standards Institute (ANSI).

·         Institute of Electrical and Electronics Engineers (IEEE:

·         International Organization for Standardization (ISO).

·         Internet Engineering Task Force (IETF).

·         World Wide Web Consortium (W3C).

Layers of OSI model: -

Application Layer:

Application layer provides platform to send and receive data over the network All applications and utilities that communicate with network fall in this layer.

For examples:

·         Browsers: - Google Chrome, Mozilla Firefox, Internet Explorer etc.

·         Email clients: - Outlook Express, Mozilla Thunderbird etc.

·         FTP clients: - FileZilla, sFTP

Application layer protocols that we should know: -

·         SNMP (Simple Network Management Protocol) — Used to control the connected networking devices.

·         TFTP (Trivial File Transfer Protocol) — Used to transfer the files rapidly.

·         DNS (Domain Naming System)— Used to translate the name with IP address and vice versa.

·         DHCP (Dynamic Host Configuration Protocol) — Used to assign IP address and DNS information automatically to hosts.

·         Telnet— used to connect remote devices.

·         HTTP (Hypertext Transfer Protocol) — Used to browse web pages.

·         FTP (File Transfer Protocol) — Used to reliably sends/retrieves files.

·         SMTP (Simple Mail Transfer Protocol) — Used to sends email.

·         POP3 (Post Office Protocol v.3) — Used to retrieves email.

·         NTP (Network Time Protocol)— Used to synchronizes clocks.

Presentation Layer: -

·         Presentation layer prepares the data.

·         It takes data from application layer and marks it with formatting code.

·         such as .doc, .jpg, .txt, .avi etc.

·         These file extensions make it easy to realize that particular file is formatted with particular type of application.

·         With formatting presentation layer also deals with compression and encapsulation.

·         It compresses (on sending computer) and decompresses (on receiving computer) the data file.

·         This layer can also encapsulate the data.

Session Layer: -

·         Session layer deals with connections.

·         It establishes, manages, and terminates sessions between two communicating nodes.

·         This layer provides its services to the presentation layer.

·         Session layer also synchronizes dialogue between the presentation layers of the two hosts and manages their data exchange.

·         For example: -  web servers may have many users communicating with server at a given time. Therefore, keeping track of which user communicates on which path is important and session layer handle this responsibility accurately.

Transport Layer: -

·         It sets up and maintains the connection between two devices.

·         It multiplexes connections that allow multiple applications to simultaneously send and receive data.

·         Data transmission method can be use connection oriented or connection less.

·         For unreliable data delivery its use connection less method.

·         Connection less method uses UDP protocol.

·         For reliable data delivery its use connection oriented method.

·         Connection oriented method uses TCP protocol.

·         Its layer is responsible for end-to-end message delivery with acknowledgments

·         When Implemented a reliable connection, sequence numbers and acknowledgments (ACKs) are used.

·         Reliable connection controls flow through the uses of windowing or acknowledgements.

·         It receives data from the Session layer, convert the data into segments and passes it on to the Network layer.

·         It's also provides Segmentation, Connection management Reliable and unreliable,

·         data delivery, Flow control and Connection multiplexing.

Network Layer: -

·         Network layer is responsible for providing logical address known as IP address.

·         Router works on this layer.

·         Network layer receive segment from transport layer and wrap it with IP header that is known as datagram.

·         Main functions of this layer are following: -

·         Define IP address

·         It decides by which route data should take.

·         It acts as a network controller. It manages the Subnet traffic.

Data Link Layer: -

·         Defining the Media Access Control (MAC) or hardware addresses.

·         Data link layer is responsible for synchronizes the information which is to be transmitted over the physical layer.

·         Data link layer receive packet from network layer and wrap it with layer two Head'

·         Transmitting and receiving data frames sequentially is done by this layer.

·         This layer sends and expects acknowledgements for frames

·         Re-sending of non-acknowledgement frames is also handled by this layer.

·         This layer establishes a logical layer between two nodes.

Physical Layer: -

·         It is the lowest layer of the OSI Model.

·         Physical layer deals with communication media.

·         This layer receive frame from data link layer and convert them in bits.

·         It loads these bits on actual communication media.

·         Depending on media type these bit values are converted in single.

·         It activates, maintains and deactivates the physical connection.

·         It is responsible for transmission and reception of the unstructured raw data over physical media.

·         Voltages and data rates is defined in this layer.

·         Data encoding is also done in this layer.

·         It transmits the digital/analog bits into electrical signal or optical signals.

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