Thursday, September 18, 2008

Lecture 5


 

  • OSI reference model
    • Acronym: open systems interconnection
    • Model was the first attempt at standardizing a network
    • Made by IOS
      • Acronym: International Organization of Standardization
    • Partitions networking into seven layers
    • Layers are used to
      • Reduce the complexity of design
      • Analogous to the concept of functions: layer (n-1) provides a service layer to n keeping its internal details hidden from layer n
      • Applications can be developed at the top most layer without worrying about the intrinsic details in the lower layer
    • OSI MODEL
      • Application Layer
        • Provides frequently requested services
        • Request for a service is made by following a protocol
        • Uses the service offered by the presentation layer by passing an application protocol data unit (APDU) to the presentation layer
          • Example: to access a www document, http protocol s used by the web browser
        • other application layer protocols include FTP , SMTP, TELNET
      • Presentation
        • Concerned with syntax and semantics of the information transmitted
        • Makes common data structures compatible on different machines
        • It is the function of the presentation layer to ensure that the transmitted bits are properly mapped to the correct alphabet
        • Allows higher level data structures to be defined
        • Communicates with the session layer using a presentation protocol data unit PPDU
        • Especially useful for banks and hospitals
      • Session
        • Allows users to establish sessions between them
        • Sessions are defined based on the requirements for users and may vary from half duplex to full duplex and inclusion or omission of synchronization point
        • Services include
          • Dialog control
            • Tracking whose turn is to transmit
          • Token management
            • Preventing parties from attempting the same operation at the same time
          • Synchronization
            • Check pointing long transmission by including synchronization points
          • Communicates with the transport layer with a session protocol data unit SPDU comprised of PPDU and Session header
      • Transport
        • Responsible for end to end ransfer of data from a session entity in source to its peer session entity at destination
        • Accepts SPDU from session layer, labels and source and destination addresses segments the data if needed and passes segments to the network layer
        • Kinds of services include:
          • Reliable connection – oriented: error free transmission of data in sequence to its destination
          • Unreliable connectionless: no guarantee of being error-free or as a matter of fact, even delivering
          • Communicates with the network layer using TPDU
      • Network
        • Provides for transfer o data in packets
        • Deals with routing and congestion
          • Routing implies not the actual route but the procedure used for selecting the route
      • Data link layer
        • Provides for transfer of frames across transmission line
        • Packets are further compose as frames with framing information on the boundaries
        • Does checksum on each frame allowing error detection
        • Also includes medium access control sublayer than allows for LAN connectivity
      • Physical
        • Performs actual transmission of bits over some communication channels
        • Wire / cable/ optical fibre / air
        • Design issues are largely electrical, mechanical, timing interfaces, and physical medium
    • Critique of OSI
      • Bad timing
        • Came too late. The competing TCP/IP was already widely in use by the time OSI was standardize
      • Bad technology
        • Choice of seven layers was more political then technical. The two layers (session and presentation) are nearly empty while two layers (data link and network) are overfull and complex
      • Bad implementation
        • Initial implementations were huge and slow. Though the products improved later but the initial impression lingered on
      • Bad politics
        • OSI was widely perceived as a European product while many people thought of TCP/IP as an extension of Unix
      • Bad government policy

Government support of OSI was thought of as an attempt to 'shove a technically inferior product down the throat of poor researchers;

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