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  1. 1. Unit03: The OSI Model
  2. 2. Overview • Open Systems Interconnection • Encapsulation • Physical • Data-link • Network • Transport • Session • Presentation • Application
  3. 3. Open Systems Interconnection • Proposed by the International Organization for Standardization (IOS) and released in 1984 • Decidedly structured set of seven protocol layers interconnecting as a stack • Developed to maintain the ability to connect diverse types of computers • Valuable as a teaching and development tool
  4. 4. OSI
  5. 5. Encapsulation The sending station converts data into increasingly smaller capsules until they are mere bits Puts those bits back together into increasingly larger capsules at the receiving station
  6. 6. Physical Layer • Transmission of raw data in the form of a stream of bits across physical media • Physical Layer conveys the bit stream data through electrical impulses, light, or radio signals • The connections that are supported at the physical layer are either multipoint or point-to-point
  7. 7. Physical Layer
  8. 8. Data Link Layer • It is the only layer with two sublayers: Media Access Control (MAC) and Logical Link Control (LLC). • The data-link layer supplies hardware addresses, identifies errors, and manages flow control. • Bit stream received from the physical layer is assembled into larger pieces, called frames • Able to insert control information into a frame header • Source and destination of the frame on that network segment
  9. 9. Data Link Layer • Media Access Control (MAC) – This sublayer defines how devices are able to gain access to the media – maintains the hardware address for the device • The three types of MACs are: – Contention – Polling – Token passing
  10. 10. Data Link Layer • MAC Address – also called the hardware address – MAC address is unique to each interface
  11. 11. Data Link Layer • Logical Link Control – second data-link sublayer – Responsible for logical link functions of single or multiple connections – LLC provides services to the higher-level network layer – The LLC primarily provides flow control and frame sequencing services
  12. 12. Network Layer • Providing an address for each network segment • Providing a logical address for each device that is separate from the MAC address • Routing and forwarding data • Selecting a route if there are multiple routes to the same network segment • Discovering the routes to other network segments • Error handling, congestion control, and packet sequencing
  13. 13. Network Layer
  14. 14. Network Layer • RIP – Distance Vector routing protocols periodically broadcast the entire routing table to neighboring routers • OSPF – These advanced protocols use a hello process to announce a new router on the network – the time it takes for the new routes to be added to the network is greatly reduced
  15. 15. Transport Layer • Provides for a transparent transfer of data between the sender and receiver nodes • Control information provides end-to-end recovery and flow control • Ensures that the data is completely transferred
  16. 16. Transport Layer • Connection Orientation – Connection-oriented • TCP • Reliable Transfer • use of acknowledgments (ACK) – Connectionless • UDP • Does not use ACK • Smaller headers and less time needed
  17. 17. Session Layer • Establishes a connection between applications and manages that connection until it terminates. • Also terminate a session in an orderly manner • Remote procedure calls (RPCs) function at the session layer • Example : Telnet and Citrix MetaFrame ICA sessions • Data is transmitted in a session in three ways – Simplex. – Half-duplex.. – Full-duplex.
  18. 18. Presentation Layer • Provides the way for data to be presented to the application • Encryption/decryption and data compression/expansion are handled at this layer • provide formatting data • Bit order • Byte order • Character code • File syntax.
  19. 19. Application Layer • Supports end-user processes • Specific to the application being used. • Application-layer protocols connect across the internetwork in three ways – Collaborative computing – Operating system call interception – Remote operation