Upper Layer

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Upper Layer

• Here, youll learn about
• The Session layer
• The Presentation layer
• The Application layer
• Some protocols in the upper layer

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Session Layer
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Session Layer

• Provides a set of standard cues for applications
  to use in structuring their dialogues
• Applications insert the cues into the data stream
  as appropriate

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Session Layer

• Cues act as universal control signals for the
  applications to
• Control which end has the right to transmit
  (interaction management)
• Delimit the start and end of some processing task
  (activity management)
• Confirm progress on the processing of data that
  has been transferred (synchronization)
• Advice of exception conditions

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Session Layer Services to Presentation Layer

• Connection
• A session connection is created when requested by
  a presentation entity
• Session services are used by presentation
  entities to regulate their dialogue, and to
  ensure an orderly message exchange on the session
  connection
• Session connection exists until it is released by
  either presentation or session entitites

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Session Layer Services to Presentation Layer

• Addresses
• The initiating presentation entity designates the
  destination presentation entity by a session
  address

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Presentation Layer
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Presentation Layer

• Two open systems need to select common syntaxes
  for the transfer of information between them
• Presentation layer acts on behalf of the
  Application entities to determine the syntaxes to
  be used on a connection and selects encoding
  schemes to represent the information exchanged as
  a string of bits transmitted over the Session
  service

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Presentation Layer

• Aim is to separate data representation from the
  applications
• Application development is simplified
• Applications become more portable
• Bit transformations, if required, are internal to
  the Presentation layer
• Operational flexibility can be improved, since
  appropriate transformations can be selected by
  the Presentation layer at connect time
• Transformations can, in principle, be generated
  automatically

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Application Layer
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Application Layer

• Provides a range of system independent services
  to real users or user programs
• Builds on the functions of the six lower layers
  to provide new services of particular use in the
  construction of distributed systems
• Hides much of the complexity of the lower layers
  by taking responsibility for the coordination of
  the joint operation of distributed applications
  in peer systems

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Application Process

• The complete realization of an application within
  a single end system
• Two parts
• The user application
• The application entity

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User Application

• A local system dependent part that interfaces to
  the user the operating system and to local
  resources such as a filestore and peripherals,
  and that uses the application services provided
  by an application entity for its OSI communication

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Application Entity

• A system independent part that provides the
  standardized functionality of the Application
  layer to the user application

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Application Layer Standards

• MHS Message Handling Systems for electronic
  mail
• Directory an electronic database of names,
  addresses and other communication information
• FTAM File Transfer, Access and Management for
  file handling
• VT Virtual Terminal for vendor independent
  communication between character mode terminals
  and applications

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Application Layer Standards

• Systems Management for control and management
  of network equipment and components
• DTP Distributed Transaction Processing for
  linking online transaction processing system
• JTM Job Transfer and Manipulation
• MMS Manufacturing Message Specification

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Overall Operation of the Upper Layers

• Focus on end user services and the problems of
  distributed systems
• Work very closely together All of the session
  services are passed directly through the
  Presentation layer for use by applications
• Software for the upper layers are often packaged
  so that the three layers are implemented in one
  product without exposed interfaces to the Session
  or Presentation layers

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Upper Layer Devices

• Gateways translates one type of network data
  into another
• E-mail gateway

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Upper Layer Protocols

• SMTP (simple mail transfer protocol)
• HTTP (hypertext transfer protocol)
• FTP (file transfer protocol)
• NFS (network file system)
• Telnet

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SMTP

• Works at the application layer
• Routes e-mail messages
• Does not provide a user interface
• Uses TCP/IP to route e-mail messages across the
  internetwork

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HTTP

• Used for communication between a web server and a
  web browser

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FTP

• Functions at the top 3 layers
• At the session layer
• Provides session administration, handling
  connection establishment, file transfer and
  connection release
• At the presentation layer
• Concerned with translation using a
  machine-independent file syntax
• At the application layer
• Supplies network services file service and
  collaborative service
• Peer-to-peer protocol

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NFS

• An application layer protocol
• Provides file service and remote operation
  service
• Allows remote file systems to appear as though
  they were part of the local machines file system

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Telnet

• Use for remote terminal emulation
• Provides connectivity between dissimilar OS
• Functions at the top 3 layers

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Telnet

• At the session layer
• Provides dialog control, session administration,
  handling connection establishment, file transfer,
  and connection release
• At the presentation layer
• Concerned with translation using byte order and
  character codes
• At the application layer
• Supplies the service functions for remote
  operation