Open Systems Interconnection Reference Model OSI

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Open Systems Interconnection Reference Model (OSI)

• After this presentation you should
• know the history of the OSI Reference Model
• understand the role of the OSI Reference Model
• know the different layers of the model and what
  they are used for
• understand that other models exist

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OSI Reference Model

• Definition
• A model that network professionals can use to
  develop and administer networking systems.
• Purpose
• To allow heterogeneous applications, data, and
  hardware to be able to function as networks.

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OSI Reference Model

• Describes
• A general architecture for data network design
  and interconnection.
• Intention
• To provide functional guideline for permitting
  the standardization of network protocols.

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OSI Reference Model

• History
• not initially accepted
• manufactures had their own networking standards
• allowed others to enter then world of networking
• specified network standards for compatibility
• became the international standard for networking

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OSI Reference Model

• History
• based on a proposal developed by the
  International Standards Organization (ISO)
• deals with connecting open systems - systems that
  are open for communication with other systems
• adopted in 1983
• Open Systems were born

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OSI Reference Model

• Allows the physical transmission of bit stream
  voltages and currents, frames data, inserts
  addresses, takes care of error control between
  nodes on the network, establishes paths between
  source and destination, makes sure reliable
  delivery of the frames is made between computers
  and the process-to-process communications is
  performed. In addition, it takes care of
  end-user services, data compression and
  encryption.

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Layering

• The OSI model is defined by layers. The
  principles that were applied to arrive at the
  seven layers are as follows
• 1. A layer should be created where a different
  level of abstraction is needed.
• 2. Each layer should perform a well defined
  function.

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Layering - contd

• 3. The function of each layer should be chosen
  with an eye toward defining internationally
  standardized protocols.
• 4. The layer boundaries should be chosen to
  minimize the information flow across the
  interfaces.
• 5. The number of layers should be large enough
  that distinct functions need not be thrown
  together in the same layer out of necessity, and
  small enough that the architecture does not
  become unwieldy.

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Layering - contd

• Basic Ideas
• protocols running between entities at layer N
  implement functions which are provided as
  services to layer N1
• communication at layer N is accomplished by
  making use of the services provided at layer N-1
• by building upon services offered by lower
  layers, the higher layers can provide increasing
  levels of power and functionality

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OSI Reference Model

• most commonly used model for defining network
  layers
• originally developed for mainframe networks
• defines 7 layers
• Physical Data Link Network Transport
• Session Presentation Application Layers

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OSI Reference Model
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Physical Layer

• concerned primarily with transmitting data bits
  (0s and 1s) over a communication circuit
• defines the rules by which the signals are
  transmitted
• voltages of electricity
• timing factors data rates
• full duplex or half duplex transmission
• connector cable standards
• maximum transmission distances

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Data Link Layer

• manages the basic transmission circuit
  established in layer 1 and transforms it into a
  circuit that is free of transmission errors
• frames the information
• performs error detection, correction and
  retransmission of data

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Data Link Layer

• Divided into two sublayers
• MAC - Media access control
• LLC - Logical link control

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Network Layer

• performs addressing and routing
• provides connectivity and path selection
• encapsulates the data with a header
• source logical address
• destination logical address

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Transport Layer

• establishes, maintains and terminates logical
  connections for the transfer of data
• generates the address of the end user
• breaks up large packets
• eliminates duplicate packets
• multiplexes several streams of messages onto one
  physical circuit

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Session Layer

• initiates, maintains and terminates each logical
  session between end users
• the session starts when there is two way
  communication
• manages and structures each session
• session accounting is handled in this layer

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Presentation Layer

• formats the data for presentation
• accommodates the different interfaces on each
  terminal or computer type
• makes sure the data is readable from one
  application to another regardless of the system

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Presentation Layer

• Common data representation formats, or the use of
  standard image, sound, and video formats, enable
  the interchange of application data between
  different types of computer systems.
• Conversion schemes are used to exchange
  information with systems by using different text
  and data representations, such as EBCDIC and
  ASCII.
• Standard data compression schemes enable data
  that is compressed at the source device to be
  properly decompressed at the destination.
• Standard data encryption schemes enable data
  encrypted at the source device to be properly
  deciphered at the destination.

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Presentation Layer

• Presentation-layer implementations are not
  typically associated with a particular protocol
  stack
• Some well-known standards for video include
• QuickTime
• Motion Picture Experts Group (MPEG)
• Graphics Interchange Format (GIF)
• Joint Photographic Experts Group (JPEG)
• Tagged Image File Format (TIFF)

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Application Layer

• end users access to the network
• includes
• identifying communication partners
• determining resource availability
• synchronizing communication
• provides for e-mail access
• provides a method for file transfer
• provides interface to all application programs

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Application Layer

• TCP/IP applications
• Telnet - Terminal emulation
• FTP - File Transfer Protocol
• SMTP - Simple Mail Transport Protocol
• OSI applications are protocols
• FTAM - File Transfer, Access and Management
• VTP - Virtual Terminal Protocol
• CMIP - Common Management Information Protocol

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Benefits of OSI

• allocation of tasks
• open systems
• off-loading of applications
• easy upgrades
• provides a simple way of defining functions

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Other Models

• SNA
• TCP/IP Internet Model