7.2.9

• OSI model


  Open systems interconnection model - a conceptual framework used to describe the functions of a networking system
• OSI model


  • Based on the concept of splitting up a communication system into seven abstract layers, each one stacked upon the last
  • Each layer handles a specific job and communicates with the layers above and below itself
• OSI model layers

  • Application
  • Presentation
  • Session
  • Transport
  • Network
  • Data Link
  • Physical
• Application layer


  • The only layer that directly interacts with data from the user
  • Software applications like web browsers and email clients rely on the application layer to initiate communications
  • Responsible for the protocols and data manipulation that the software relies on to present meaningful data to the user
  • Application layer protocols include HTTP as well as SMTP
• Presentation layer


  • Responsible for translation, encryption and compression of data
  • Translates incoming data into a syntax that the application layer of the receiving device can understand
• Presentation layer operation when sending an email


  1. Translates incoming data from the email package into a syntax

  2. Compresses to get faster data transmission

  3. Uses encryption techniques to improve message security
• Session layer


  • Responsible for opening and closing communications between 2 devices
  • Ensures the session stays open long enough to transfer all the data being exchanged, and promptly closes in order to avoid wasting resources
  • Synchronises data transfer with checkpoints (e.g. every 5 mb) in case of a disconnect or crash - the session can be resumed
• Transport layer


  • Responsible for the end-to-end communication between two devices
  • Takes data from the session layer and breaks it up into chunks (segments) before sending it to layer 3- the network layer
  • Reassembles the segments into data the session layer can consume
  • Responsible for flow control and error control
• Network layer


  • Responsible for facilitating data transfer between two different networks
  • Breaks up segments from the transport layer into smaller units, called packets, on the sender's device, and reassembling these packets on the receiving device
  • Finds the best physical path for the data to reach its destination, this is known as routing
• Data link layer


  • Facilitates data transfer between 2 devices on the same network
  • Takes packets from the network layer and breaks them into smaller pieces called frames
  • Responsible for the flow control and error control for intra-network communications
• Physical layer


  • Includes the physical equipment involved in the data transfer, such as the cables and switches
  • Converts the data into a bit stream, which is a string of 1's and 0's
  • The physical layer of both devices must agree on a signal convention so that the 1's can be distinguished from the 0's on both networks
• OSI model


  Open systems interconnection model - a conceptual framework used to describe the functions of a networking system
• OSI model


  • Based on the concept of splitting up a communication system into seven abstract layers, each one stacked upon the last
  • Each layer handles a specific job and communicates with the layers above and below itself