OSI Model


  • Stands for Open Systems Interconnection model
  • 7-Layer Model
  • Standardizes the communication functions of a telecommunication or computing system
  • Conceptual model enables diverse communication systems to communicate using standard protocols

7. Application

  • Protocol Data Unit (PDU): Data
  • Human-computer interaction layer, where applications can access the network services
  • Examples: Web browsers (Chrome, Safari, IE), Apps: Skype, Zoom, Telnet, FTP

Functions of the layer:

  • File transfer, access, and management (FTAM)
  • Mail services
  • Directory services

6. Presentation

  • Protocol Data Unit (PDU): Data
  • Translates of application format to network format, and network format to application format
  • Ensures that data is in usable format

Functions of the layer:

  • Encryption
  • Compression
  • Encoding

5. Session

  • Protocol Data Unit (PDU): Data
  • Maintains connections and is responsible for controlling ports and sessions

Functions of the layer:

  • Dialog control
  • Synchronization

4. Transport

  • Protocol Data Unit (PDU): Segment, Datagram
  • Main responsibility is to transfer the data completely
  • Transmits data using transmission protocols: TCP and UDP

Transmission Control Protocol (TCP)

  • Standard protocol used for communication over the internet
  • Establishes and maintains a connection between hosts
  • Data is broken into segments, segments arrive at destination using different routes and are reordered
  • Receipt acknowledgement

User Datagram Protocol (UDP)

  • Receiver does not send any acknowledgement when the packet is received
  • Sender does not wait for any acknowledgement
  • Faster transmission than TCP, mostly used for Media streaming
  • Unreliable due to potential packet loss

Functions of the layer:

  • Service-point addressing - transmitting data from one computer to another, and then to the correct process - via port
  • Segmentation and reassembly
  • Connection control
    • Connection-oriented service
      • Connection oriented protocol makes a connection
      • Checks whether message is received or not
      • Sends again if an error occurs
    • Connection-less service
      • Does not guarantees a message delivery
  • Flow Control
  • Error Control

3. Network

  • Protocol Data Unit (PDU): Packet
  • Decides which physical path the data will take
  • Determines the best path to move data from source to the destination based on the network conditions, the priority of service, and other factors
  • Routers are the layer 3 devices, they are specified in this layer and used to provide the routing services within an internetwork

Functions of the layer:

  • Internetworking
  • Addressing
  • Routing
  • Packetizing
    • Network layer receives the packets from upper layers and converts them into packets
    • This process is achieved by Internet Protocol (IP)
  • Protocol Data Unit (PDU): Frame
  • Defines the format of data on the network

Functions of the layer:

  • Framing
    • Adds header and trailer to each frame
  • Physical Addressing
  • Flow Control
    • Technique through which the constant data rate is maintained on both the sides so that no data get corrupted
  • Error Control
    • Calculated value CRC (Cyclic Redundancy Check) that is placed to the Data link layer’s trailer
  • Access Control

1. Physical

  • Protocol Data Unit (PDU): Bit, Symbol
  • Transmits raw bit stream over the physical medium

Functions of the layer:

  • Line Configuration
  • Data Transmission
    • Defines transmission mode between the two devices on the network, i.e. simplex, half-duplex or full-duplex mode
  • Topology
  • Signals
    • Type of the signal used for transmitting the information