1Chapter OSI

• OSI


  Open Systems Interconnection (ISO standard that covers all aspects of network communication)

  
• Open system


  • Allows any two different systems to communicate regardless of their underlying architecture
  • Purpose is to open communication between different systems without requiring changes to the logic of the underlying hardware and software
• Reference model


  Conceptual framework for understanding relationships
• The purpose of the OSI reference model is to guide vendors and developers so the digital communication products and software programs they create will interoperate, and to facilitate clear comparisons among communications tools

  
• OSI model


  • Consists of seven separate but related layers, each of which defines a segment of the process of moving information across a network
  • Understanding the fundamentals of the OSI model provides a solid basis for exploration of data communication

  
• Communication between two endpoints in a telecommunication network

  1. Data flows down through the layers in the source computer

  2. Across the network

  3. Then up through the layers in the receiving computer

  
• The seven ordered layers

  • Physical (layer 1)
  • Data link (layer 2)
  • Network (layer 3)
  • Transport (layer 4)
  • Session (layer 5)
  • Presentation (layer 6)
  • Application (layer 7)
• Three subgroups of the seven layers

  • Network support layers (1, 2, 3)
  • User support layers (5, 6, 7)
  • Transport layer (4)
• Entities


  Active protocol elements in each layer, typically implemented by means of a software process
• Peer entities


  Entities in the same layer on different computers
• Communication between layers within a single machine

  Each layer calls upon the services of the layer just below it
• Communication between layers across machines

  Layer x on one machine communicates with layer x on another machine using agreed-upon protocols
• Peer-to-peer processes


  The processes on each machine that communicate at a given layer
• Communication at the physical layer


  Machine A sends a stream of bits to machine B
• Communication at the higher layers


  Moves down through the layers on machine A, over to machine B, and then back up through the layers
• Interface data unit (IDU)


  Passed from layer-n entity to layer-(n-1) entity, consisting of a protocol data unit (PDU) and some interface control information (ICI)
• Protocol data unit (PDU)


  The data that the layer-n entity wishes to pass across the network to its peer entity, consisting of the layer-n header and the data that layer n received from layer (n+1)

  
• Service data unit (SDU)

  The layer-n PDU becomes the layer-(n-1) SDU, because it is the data unit that will be serviced by layer n
• Data encapsulation


  The sending and receiving of data from a source device to the destination device using networking protocols, where data is encapsulated with protocol information at each layer of the OSI reference model

  
• Upon reaching its destination, the signal passes into layer 1 and is transformed back into bits, then moves back up through the OSI layers where headers and trailers are removed at each layer</b>
• Physical layer


  • Concerned with the transmission and reception of the unstructured raw bit stream over a physical medium
  • Defines the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers

  
• Functions of the physical layer

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

  
• Data link layer


  • Transforms the physical layer, a raw transmission facility, to a reliable link and is responsible for node-to-node delivery
  • Makes the physical layer appear error free to the upper layer (network layer)

  
• Responsibilities of the data link layer

  • Framing
  • Physical addressing
  • Flow control

  
• Full-duplex (or duplex) mode

  Two devices can send and receive at the same time
• Data Link Layer


  1. Transforms the physical layer to a reliable link

  2. Responsible for node-to-node delivery

  3. Makes the physical layer appear error free to the upper layer (network layer)

  
• Specific responsibilities of the data link layer

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

  
• Network Layer


  1. Responsible for the source-to-destination delivery of a packet across multiple networks

  2. Ensures each packet gets from its point of origin to its final destination
• Specific responsibilities of the network layer

  • Logical addressing
  • Routing
• Transport Layer


  1. Ensures messages are delivered error-free, in sequence, and with no losses or duplications

  2. Relieves higher layer protocols from any concern with the transfer of data
• Services provided by the transport layer

  • Service-point addressing
  • Segmentation and reassembly
  • Connection control
  • Flow control
  • Error control

  
• Session Layer


  1. Establishes, maintains, and synchronizes the interaction between communicating systems

  2. Allows two application processes on different machines to establish, use and terminate a connection
• Specific responsibilities of the session layer

  • Dialog control
  • Synchronization
• Presentation Layer


  Concerned with the syntax and semantics of the information exchanged between two systems
• Specific responsibilities of the presentation layer

  • Translation
  • Encryption
  • Compression
• Application Layer


  1. Enables the user, whether human or software, to access the network

  2. Provides user interfaces and support for services
• Application services


  • X.400 (message-handling services)
  • X.500 (directory services)
  • File transfer, access, and management (FTAM)
• Specific services provided by the application layer

  • Network virtual terminal
  • File transfer, access, and management (FTAM)
  • Directory services
  • Mail services