OSI Reference Model

• Layering


  Decomposing the problem into more manageable components or layers
• Layering


  • Provides more modular design
  • Easy to troubleshoot
• Protocols

  Set of rules that governs data communication
• Protocols in each layer govern the activities of data communication
• Protocols take care of IP address, MAC address and port address before sending data
• Layered architectures


  • OSI reference model
  • TCP/IP model
• OSI (Open System Interconnection)

  Model for understanding and designing a network architecture that is flexible, robust and interoperable
• OSI model is a guideline, not a protocol, developed by ISO
• Purpose of OSI model

  To facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software
• TCP/IP (Transmission Control Protocol and Internet Protocol)


  Protocol suite developed prior to OSI model, with a hierarchical structure of interactive modules providing specific functionalities
• TCP/IP layers do not exactly match OSI model layers
• OSI reference model


  A guideline for data communication between two different systems
• Layers of the OSI reference model

  • Application layer
  • Presentation layer
  • Session layer
  • Transport layer
  • Network layer
  • Data link layer
  • Physical layer
• Data communication between two systems

  1. Application layer generates data

  2. Presentation layer modifies/converts data format

  3. Session layer carries out session activities

  4. Transport layer adds transport layer information

  5. Network layer adds network layer information

  6. Data link layer adds data link layer information

  7. Physical layer converts data to zeros and ones and sends over the physical medium
• Application layer


  Enables the user to access network resources
• Services offered by application layer


  • File transfer and access management
  • Mail service
  • Directory services
• Presentation layer


  Concerned with the syntax and semantics of the information exchange between systems
• Services offered by presentation layer

  • Translation
  • Encryption
  • Compression
• Session layer


  Establishes, maintains and synchronizes the interaction among communicating devices
• Services offered by session layer

  • Dialogue control
  • Synchronization
• The OSI reference model has 7 layers: application, presentation, session, transport, network, data link, and physical
• Services offered by application layer


  • File transfer and access management
  • Mail services
  • Directory services
• Presentation layer


  Concerned with the syntax and semantics of the information exchanged between two systems
• Services offered by presentation layer

  • Translation
  • Encryption
  • Compression
• Session layer


  Establishes, maintains, and synchronizes the interaction among the communicating devices
• Services offered by session layer

  • Dialog control
• Transport layer


  Responsible for process to process delivery of the entire message
• Services offered by transport layer

  • Port addressing
  • Segmentation and reassembly
  • Connection control
  • End-to-end flow control
  • Error control
• Network layer


  Responsible for delivery of data from the original source to the destination network
• Services offered by network layer

  • Logical addressing
  • Routing
• Data link layer


  Responsible for moving the data or frames
• Services offered by data link layer

  • Framing
  • Physical addressing
  • Flow control
  • Error control
  • Access control
• Physical layer


  Responsible for transmitting the bits over a medium and provides electrical and mechanical specifications
• Services offered by physical layer


  • Physical characteristics of the media
  • Representation of bits
  • Data rate
  • Synchronization of bits
  • Line configuration
  • Physical topology
  • Transmission mode