• What Is OSI Model?

    OSI Model 

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    OSI Model

    OSI stands for Open Systems Interconnection. The (OSI) model is a conceptual model created by the International Organization for Standardization which enables diverse communication systems to communicate using standard protocols. 

    Most vendors involved in telecommunications make an attempt to describe their products and services to the OSI model. 


     The OSI model doesn't perform any functions in the networking process. It divides network communication into seven layers. In this model, layers 1-4 are considered the lower layers, and mostly concern themselves with moving data around. Layers 5-7 called the upper layers, contain application-level data. It's Basically 7 layers of Networking.

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    Layers

    The Seven Layers of the OSI Model:-

    Physical Layer (Layer 1)

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    Physical Layer in OSI

    • The physical layer contains information in the form of bits.
    • It is the lowest layer of the OSI model.
    • This layer includes the physical equipment involved in the data transfer, such as the cables and switches.
    • Fast EthernetRS232, and ATM are protocols with physical layer components.
    Functions of a Physical layer:
    • Bit synchronization:  This clock controls both sender and receiver thus providing synchronization at bit level.
    • Data Transmission: It defines the transmission mode whether it is simplex, half-duplex or full-duplex mode between the two devices on the network.
    • Physical Topologies: Physical layer specifies how the different, devices/nodes are arranged in a network i.e. bus, star or mesh topology.

    Data Link Layer (DLL) (Layer 2)
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    Data Link Layer in OSI

    • It defines the format of the data on the network.
    • This layer sets up links across the physical network, putting packets into network frames.
    • This is the layer on which the Switches operate on.

    The Data Link layer is divided into two sub-layers :
    • Logical Link Control (LLC)
    • Media Access Control (MAC)
    Logical Link Control Layer
    • It also provides flow control.
    • The packet received from Network the layer is further divided into frames depending on the frame size of NIC(NetworkInterface Card).
    Media Access Control Layer
    • It is used for transferring the packets over the network.
    • The Receiver’s MAC address is obtained by placing an ARP(Address Resolution Protocol) request onto the wire asking “Who has that IP address?” and the destination host will reply with its MAC address.
    If You Read About What is MAC Address? Click On Link and Read Out.

    Functions of the Data Link layer are :
    • Framing: Framing is a function of the data link layer. The Data link layer adds the header and trailer to the frame.
    • Flow Control: Flow control is the main functionality of the Data-link layer.  flow control coordinates that amount of data that can be sent before receiving an acknowledgment.
    • Physical Addressing: The data link layer adds a header to the frame that contains a destination address.
    • Error control: Error control in which it detects and retransmits damaged or lost frames.

    Network Layer (Layer 3)
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    Network Layer in OSI
    • Layer 3 provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node.
    • The sender & receiver’s IP address is placed in the header by the network layer.
    • The Data Link layer is responsible for routing and forwarding the packets.

    Functions of The Network Layer:

    • Routing: The network layer protocols determine which route is suitable from source to destination.
    • Addressing: A Network layer adds the source and destination address to the header of the frame. Addressing is used to identify the device on the internet.

    Transport Layer (Layer 4)

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    Transport Layer in OSI
    • Layer 4, provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control.
    • The transport layer also provides the acknowledgment of the successful data transmission and re-transmits the data if an error is found.
    • The main function of the transport layer is to transfer the data completely.

    The two protocols used in this layer are:-

    • Transmission Control Protocol (TCP)
    • User Datagram Protocol (UDP)
    Functions of Transport Layer:
    • Flow control: The transport layer also responsible for flow control but it is performed end-to-end.
    • Error control: The sender transport layer ensures that message reach at the destination without any error.
    IF You Read About What is IP Address? Click On Link and Read Out.

    Session Layer (Layer 5)

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    Session Layer in OSI
    • This is the layer responsible for opening and closing communication between the two devices.
    • The session layer is used to establish, maintain and synchronizes the interaction between communicating devices.

     For example, if a 100 megabyte file is being transferred, the session layer could set a checkpoint every 5 megabytes. In the case of a disconnect or a crash after 52 megabytes have been transferred, the session could be resumed from the last checkpoint, meaning only 50 more megabytes of data need to be transferred. Without the checkpoints, the entire transfer would have to begin again from scratch.

    Functions of Session layer: 
    • Dialog Controller: The session layer allows two systems to start communication with each other in half-duplex or full-duplex.
    • Synchronization: Session layer adds some checkpoints when transmitting the data in a sequence. 

    Presentation Layer (Layer 6)
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    Presentation Layer in OSI
    • This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa.
    • The presentation layer is also known as the syntax layer.
    • Finally, the presentation layer is also responsible for compressing data it receives from the application layer before delivering it to layer 5.
    Functions of the Presentation layer: 
    • Translation : It converts the data from sender-dependent format into a common format and changes the common format into receiver-dependent format at the receiving end. For example, ASCII to EBCDIC. 
    • Encryption: Encryption is needed to maintain privacy. 
    • Compression: Reduces the number of bits that need to be transmitted on the network.

    Application Layer (Layer 7)
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    Application Layer in OSI
    • Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified.
    • It handles issues such as network transparency, resource allocation, etc.
    • This layer provides the network services to the end-users.
    • Application layer protocols include HTTP as well as SMTP (Simple Mail Transfer Protocol is one of the protocols that enable email communications).
    Functions of Application layer:
    •  File transfer, access, and management (FTAM): The application layer allows a user to access the files in a remote Technique.
    • Mail services: An application layer provides the facility for email forwarding and storage.



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