Open System Interconnection (OSI) Model

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Open System Interconnection (OSI) Model
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Objectives

• Explain the OSI reference model, which sets
  standards for LAN and WAN communications
• Discuss communication between OSI stacks when two
  computers are linked through a network
• Apply the OSI model to realistic networking
  situations

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OSI Reference Model

• Foundation that brings continuity to LAN and WAN
  communications
• Product of two standards organizations
• ISO
• ANSI
• Developed in 1974
• Set of communication guidelines for hardware and
  software design

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OSI Guidelines Specify

• How network devices contact each other how
  devices using different protocols communicate
• How a network device knows when to transmit and
  not transmit data
• How physical network network devices are arranged
  and connected

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OSI Guidelines Specify

• Methods to ensure that network transmissions are
  received correctly
• How network devices maintain a consistent rate of
  data flow
• How electronic data is represented on network
  media

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OSI Layers
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OSI Layers

• Bottom layers
• Support for physical connectivity, frame
  formation, encoding, and signal transmission
• Middle layers
• Establish and maintain a communication session
  between two network nodes
• Monitor for error conditions
• Uppermost layers
• Application/software support for encrypting data
  and assuring interpretation/presentation of data

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Physical Layer Functions

• Provides transfer medium (eg, cable)
• Translates data into a transmission signal
• Sends signal along the transfer medium
• Includes physical layout of network
• Monitors for transmission errors
• Determines voltage levels for data signal
  transmissions and to synchronize transmissions
• Determines signal type (eg, digital or analog)

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Analog Signals
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Digital Signals
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Data Link Layer Functions

• Constructs data frames
• Creates CRC information checks for errors
• Retransmits data if there is an error
• Initiates communications link makes sure it is
  not interrupted (ensures node-to-node physical
  reliability)
• Examines device addresses
• Acknowledges receipt of a frame

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Data Link Layer

• Data link frame contains fields consisting of
  address and control information
• Two important sublayers
• Logical link control (LLC)
• Media access control (MAC)
• Connectionless service versus connection-oriented
  service

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Network Layer Functions

• Determines network path for routing packets
• Helps reduce network congestion
• Establishes virtual circuits
• Routes packets to other networks, resequencing
  packet transmissions when needed
• Translates between protocols

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Transport Layer Functions

• Ensures reliability of packet transmissions
• Ensures data is sent and received in the same
  order
• Sends acknowledgement when packet is received
• Monitors for packet transmission errors and
  resends bad packets
• Breaks large data units into smaller ones and
  reconstructs them at the receiving end for
  networks using different protocols

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Session Layer Functions

• Establishes and maintains communications link
• Determines which node transmits at any point in
  time
• Disconnects when communication session is over
• Translates node addresses

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Presentation Layer Functions

• Translates data to a format the receiving node
  understands (eg, from EBCDIC to ASCII)
• Performs data encryption
• Performs data compression

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Application Layer Functions

• Enables sharing remote drivers and printers
• Handles e-mail messages
• Provides file transfer services
• Provides file management services
• Provides terminal emulation services

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Communicating Between Stacks

• OSI model provides standards for
• Communicating on a LAN
• Communicating between LANs
• Internetworking between LANs and WANs and between
  WANs and WANs

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Peer Protocols
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Applying the OSI Model