Daily Learning: Computer Networks – OSI Model

OSI stands for Open System Interconnection is a reference model that describes how information from a software application in one computer moves through a physical medium to the software application in another computer.

OSI consists of seven layers, and each layer performs a particular network function.

OSI model divides the whole task into seven smaller and manageable tasks. Each layer is assigned a particular task.

Each layer is self-contained, so that task assigned to each layer can be performed independently.

Below there is a table with all layers given with their individual task.

Layer 7ApplicationNetwork services provided to end-user applications such as a web browser.
Layer 6PresentationThe presentation layer formats or translates data for the application layer based on the syntax or semantics that the application accepts.
Layer 5SessionThe session layer controls synchronization of communication between different computers.
Layer 4TransportBreaks the data in chunk and Transfer the data between systems and hosts.
Layer 3NetworkDelivering frames to their intended destinations among based on the addresses contained inside the frame.
Layer 2Data LinkDirectly connected nodes are used to perform node-to-node data transfer where data is packaged into frames. Also corrects Error.
Layer 1PhysicalTransmitting electrically or optically raw data from sender to receiver

Below I am going to give some idea about important terminologies regarding OSI model –

  • MAC Address (Media Access Control) – It is built into all the network devices.
    • It is a 48 bit address represented in hexadecimal form.
    • Each hex character contains 4 bit.
    • There are 12 hex characters
    • E.G. – 00–40–05–60–7D–49
    • The first six digit represents the NIC manufactures. Last six digit refers to the unique ID of each NIC.
  • Frames – The data transfer in network happens in frames.
    • Different network have different frame size.
    • All NIC in a same network must have same frame size.
    • Generally the frame size is 1500 bytes

Data Transmission in Hub – When a specific NIC sends the data to a specific host in a network it just sends the data to the hub and the hub sends the data to all the connected host and the host with the matching MAC address will receive and others will discard it.

Data Transmission in Switch – Later we use switch which filter the MAC address and sends the data to only the destination address.

FCS – It is 4 byte long.

  • Data Link Layer parts –
    • LLC(Logical Link Control) – NIC that talks to the system’s operating system (usually via device drivers). The LLC handles multiple network protocols and provides flow control.
    • MAC Address – which is discussed above.
  • Network Layer –

Here it gives a logical address called IP address. It is a 32 bit address and represented as dotted decimal. There is a device called router which sends data though IP address.

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