CS601 - Data Communication - Lecture Handout 10

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Presentation Layer

  • This layer is concerned with Syntax and Semantics of info exchange between two systems

Presentation Layer

Functions of Presentation Layer

  • Translation
    • The processes (running programs) in two systems are usually exchanging info in the form of character strings, numbers and so on…..
    • The info should be changed to bit streams before being transmitted
    • Because different computers use different ENCODING SYSTEMS, presentation layer is responsible for interoperability b/w these different encoding methods
    • The presentation layer at the sender changes the info from its sender- dependent format to the common format
    • The presentation at the receiver changes info from common to the receiver dependent format
  • Encryption
    • To carry sensitive info , a system must be able to assure privacy
    • Encryption means that sender transforms original info to another form and sends the resulting message out over the network
    • Decryption reverses the original process to transform message back to its original form
  • Compression
    • Data compression reduces the number of bits to be transmitted
    • Data compression becomes particularly important in transmission of multimedia such as text, audio and video

Application Layer

  • Enables the user either human or software to access the network
  • It provides user interface and support for the services such as Electronic mail, Remote File access and Transfer, Shared Database Management and other services

Application Layer Figure

Application Layer Figure

  • In the figure, of many application services available, only three services are shown
    • X-400 (message Handling Services)
    • X-500 (Directory Services)
    • File Transfer, Access& Management (FTAM)
  • In this example user uses X-400 to send an e-mail message
  • No headers or trailers are added at this layer

Application Layer Functions

  • Network Virtual Terminal
    • NVT is a software version of a physical terminal and allows a user to log on to a remote host
    • To do so the application created emulation of terminal at the remote host
    • Users computer talks to the software terminal which in turn talks to the host *& vice versa
    • Remote host believes it is communication with one of its own terminals and allow you to log on
  • File Transfer, Access & Manage.(FTAM)
    • This application allows a user to access file on the remote computers to make changes or read data
    • The purpose of this access is to Retrieve files from a remote computer and to manage or control files in that remote computer
  • Mil Services
    • This application provides the basis for email forwarding and storag
  • Directory Services
    • Provides distributed database sources and access for global info about various objects and services

Summary of Layer Functions

Summary of Layer Functions

TCP/IP Protocol Suite

  • Transmission Control Protocol / Internetworking Protocol
  • Developed Prior to OSI Model
  • Widely used in the Internet Today

Layers in TCP/IP Protocol Suite

  • Physical (physical standards)
  • Data Link (N/w Interface)
  • Network (Interconnectivity)
  • Transport(Transport Functions)
  • Application(Session , Pres, app of OSI)

TCP/IP Protocol Suite

TCP IP Protocol Suite


  • The OSI Model
  • Functions of Layers
  • TCP/IP Protocol Suite

Reading Sections

Section 3.2, 3.3 “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan