OSI Reference Model

Hi All,

         Today let discuss something about OSI (Open System Interconnection) Model, which was created by  ISO (International Organization for Standardization), So that different manufacturer computers can communicate with each other by connecting them within network.

OSI Reference Model :-

            The OSI Model has 7 different layers, divided into 2 groups. The top 3 layers define how the applications within the end stations will communicate with each other & with users. The bottom 4 layers define how data is transmitted end to end,

1.    Application Layer: - It helps users to communicate with the computer. This layer comes into picture when it finds that user going to access the network soon. This layer acts as an interface between actual application program (which isn't at all part of layered structure, e.g. Internet Explorer (IE), Firefox, Google Chrome, Microsoft Office, etc.) & next layer down (i.e. Presentation layer) by providing ways for the application to send information down through the protocol stack. It’s also responsible for determining whether sufficient resources for the intended communication exist. Some programs which actually reside at this layer are FTP, TFTP, etc.

2.    Presentation Layer: - This layer is a translator i.e. it presents data to the application layer & is responsible for data translation & code formatting. It adapts the data into standard format which helps in successful data transmission, various tasks like data compression, decompression, encryption & decryption comes under this. 

3.    Session Layer: - This is responsible for Setting up, Managing, & then Tearing down the sessions between Presentation layer entities. It’s also provides dialog control between devices & nodes. It offers three different modes of communication: Simplex, Half Duplex, & Full Duplex. This layer basically keeps different application data separate from other application data.

4.    Transport Layer: - It provides end-to-end data transport service by establishing a logical connection between sending & receiving host on inter-network  This layer segments & reassembles data (from upper layers) into the same data stream. TCP (Transmission Control Protocol) & UDP (User Datagram Protocol) works at Transport Layer. This layer is responsible for providing mechanism for multiplexing upper layer applications, establishing sessions, & tearing down virtual circuits. It also hides details of any network dependent information from higher layers. This layer can be Connection-less or Connection-Oriented (Reliable Data Transfer).

5.    Network Layer: - It manages the network addressing. It tracks the location of the devices on the network & figure out the best way to route data packets. Device used in this layer are Routers & in Packet form data transmission occurs. 2 types of Packets are used – Data Packet & Route Update Packet.

Data Packet – It’s used to transport User Data & protocols used to support data traffic are called Routed Protocols, for e.g. IPv4, IPv6.

Route Update Packet – Used to update neighboring routers about the network connected. Protocols which send route update packets are called Routing Protocols, for e.g. RIP, RIPv2, EIGRP & OSPF. It helps to maintain routing tables on each router.

6.    Data Link Layer: - It’s a Hardware based Switching, It provides the physical transmission of the data & handles error notification (not correction), network topology, & flow control. It ensures that message delivered to the proper device on a LAN using Hardware address & translate message from network layers to Bits for Physical layer to transmit. It has 2 sub-layers,

      Media Access Control (MAC) 802.3 – It defines how packets are placed on the media. Error notification (not correction), ordered delivery of frames & optional flow control can also be used.

      Logical Link Control (LLC) 802.2 – In this, a host will receive the frame & looks in the LLC header, to find out the destination of the packet i.e. it is responsible for identifying Network layer protocols & then encapsulates them. It also provides flow control & sequencing of control bits. Switches & Bridges are used in this layer & uses Hardware (MAC) address to filter the network. 

7.    Physical Layer: - It specifies the electrical, mechanical, procedural, & functional requirement for Activating, Maintaining, & Deactivating a physical link between end systems. Hubs & Ethernet are used in this layer.

Below is the Summary of all OSI Layers Functioning,

Keep checking for new posts till than HaPPy Reading.

Comments are most Welcomed.

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