The OSI Model

• A seven layer model that is widely studied. (Page 43)
• You should spend some time looking at this model and comparing it to the TCP/IP model.
  • The physical layer is the same as the TCP/IP model
  • The Data Link layer provides reliable connection over the physical link
  • The Network layer provides transmission across networks.
  • The Transport layer provides reliable transfer with flow control and error recovery
  • The Session layer provides control structure and manages connections.
  • The Presentation layer deals with syntax and data representation.
  • The Application layer requires less work due to the previous layers.
• Take a look at page 44.
• TCP probably dominates due to it being the first developed, and was well working by the time the OSI model was developed.

  Standardization within a protocol architecture

  • By standardizing the protocol architecture complete layers may be replaced without change to layers above and below.
  • This is most evident at the lower layers.
    • We replace media/lan protocols all of the time.
  • We also are doing this with IPv4 vs IPv6
  • The OSI model defines 3 requirements:
    • Protocol Specification: two entities communicate at the same level.
      • They might actually be using different "systems" but they can communicate because they use the same protocol.
    • Service Definitions: The services provided at a given layer are precisely defined.
      • Again, no matter how this layer is implemented, layers that depend on it don't need to change.
    • Addressing:Using a service access point (or port) allows for multiple users at each layer.
  • Primitives within the OSI model.
    • A Request allows the user of a service at a higher layer to request a service from the next layer down.
    • An Indication allows a service to provide feedback.
    • A Response Allows a service user to acknowledge or complete a procedure.
    • A Confirm from service to user.
  • Look at the picture on page 47.