OSI Reference Model

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OSI Reference Model – OSI was developed by the International Organization for Standardization (ISO) and introduced around 1980. It is a layered architecture (consists of seven layers), which defines and explains how communication happens in between two or more network devices within the organization or the Internet. Each layer defines a set of functions in data communication.

Application Layer (Layer 7)

  • Application Layer is responsible for providing an interface for the users to interact with application services or Networking Services.
  • Ex: Web browser etc.
  • Identification of Services is done using Port Numbers.
  • Port is a logical communication Channel
  • Port number is a 16-bit identifier.
    • Total Ports                0 – 65535
    • Reserved Ports         1 – 1023
    • Unreserved Ports     1024 – 65535




Service Port #
FTP 21
Telnet 23

Presentation Layer (Layer 6)

  • Presentation Layer is responsible for defining a standard format for the data.
  • It deals with data presentation.
  • The major functions described at this layer are:
    • Encoding – Decoding
      • Ex: ASCII, EBCDIC (Text)
      • JPEG, GIF, TIFF (Graphics)
      • MIDI, WAV (Voice)
      • MPEG, DAT, AVI (Video)
    • Encryption – Decryption
      • Ex: DES, 3-DES, AES
    • Compression – Decompression
      • Ex: Predictor, Stacker, MPPC

So, in the OSI Reference Model, the other Layer is as Follows:-

Session Layer (Layer 5)

  • Session Layer is responsible for establishing, maintaining and terminating the sessions.
  • It deals with sessions or Interactions between the applications.
  • A session ID is used to identify a session or interaction
    • Ex: RPC, SQL, NFS

Transport Layer (Layer 4)

  • Transport Layer is responsible for end-to-end transportation of data between the applications
  • The major functions described at the Transport Layer are:
    • Identifying Service
    • Multiplexing & De-multiplexing
    • Segmentation
    • Sequencing & Reassembling
    • Error Correction
    • Flow Control
  • Identifying a Service: Services are identified at this layer with the help of port numbers.  The major protocols which take care of data transportation at the Transport layer are…TCP, UDP




Transmission Control Protocol

User Datagram Protocol

Connection-Oriented Connection Less
Reliable communication (With ACK) Unreliable communication (No ACK)
Slower data transportation Faster data transportation
Protocol number is 6 Protocol number is 17

So, in the OSI Reference Model, the other3rd  Layer is as Follows:-

Network Layer (Layer 3)

OSI Reference Model - Network Layer -

  • Network Layer is responsible for end-to-end transportation of data across multiple networks.
  • Logical addressing & path determination (Routing) are described at this layer.
  • The protocols works at Network layer are
    • Routed Protocols:
      • Routed protocols act as data carriers and define logical addressing
      • IP, IPX, AppleTalk… Etc
  • Routing Protocols:
    • Routing protocols perform path determination (Routing).
    • RIP, IGRP, EIGRP, OSPF & others
    • Devices works at Network Layer are Router, Multilayer switch etc.

Data-Link Layer (Layer 2)

  • Data-Link Layer is responsible for end-to-end delivery of data between the devices on a LAN network segment.
  • Data link layer comprises two sub-layers.
    • MAC (Media Access Control)
      • It deals with hardware addresses (MAC addresses).
      • MAC addresses are 12 digit Hexa-decimal identifiers used to identify the devices uniquely on the network segment.
      • It also provides ERROR DETECTION using CRC (Cyclic Redundancy Check) and   FRAMING (Encapsulation).
      • Ex: Ethernet, Token ring…etc
    • LLC (Logical Link Control)
      • It deals with Layer 3 (Network layer)
      • Devices works at Data-Link layer are Switch, Bridge, NIC card.

Physical Layer (Layer 1)

  • Physical Layer deals with the physical transmission of binary data on the given media (copper, fiber, wireless…).
  • It also deals with electrical, mechanical and functional specifications of the devices, media. Etc
  • The major functions described at this layer are.
    • Encoding/decoding: It is the process of converting binary data into signals based on the type of media.
      • Copper media: Electrical signals of different voltages
      • Fiber media: Light pulses of different wavelengths
      • Wireless media: Radio frequency waves
    • Mode of transmissions of signals: Signal Communication happens in three different modes
      • Simplex
      • Half-duplex
      • Full-duplex
  • Devices works at the physical layer are Hub, Modems, Repeater, and Transmission Media.
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