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CS601 - Data Communication - Lecture Handout 09

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Related Content: CS601 - VU Lectures, Handouts, PPT Slides, Assignments, Quizzes, Papers & Books of Data Communication

Transport Layer

  • Responsible for Source-to-Destination delivery of Entire Message
  • Network Layer oversees source-to-destination delivery of the entire packets but it does not recognize any relationship b/w those packets
  • Network layer treats each packet independently
  • Transport Layer ensures that whole message arrives at the destination intact

Transport Layer

Functions of Transport Layer

  • Service Point Addressing
    • Computers run several programs at the same time
    • Source-to-Destination delivery means delivery not only from only from one computer to the other but also from a specific process on one computer to a specific process on the other
    • Transport layer header includes a type of address called Service Point Address or PORT Address
    • Network layer each packet to the correct computer while Transport layer gets entire message to the correct process on that comuter
  • Segmentation and Reassembly
    • Message is divided into transmittable segments
    • Each segment contains a sequence number
    • These sequence no.s enable Transport layer at the receiving m/c to reassemble message correctly at the destination and to identify and replace lost packets
  • Connection Control
    • Transport layer can be either connection-less or connection-oriented
    • Connectionless
      • Treats each segment as an independent packet and delivers it to the transport layer of the destination m/c
    • Connection-Oriented
      A connection is established first with Transport layer before delivering the packet.
    • After all data is Tx. , the connection is disconnected
  • Flow Control
    • Like Data link layer, Transport layer is also responsible for Flow control
    • Flow control is performed end-to-end rather than across a single link
  • Error Control
    • Like data link layer, Transport layer is responsible for the Error Control
    • Error control is performed end-to-end
    • This layer makes sure that entire message reaches Rx Transport layer w/o error
    • Error can be a result of Lost, damaged or duplicated data and usually Re Tx is done

Example Figure

Example Figure

Session Layer

  • Session layer is the Network Dialog Controller
  • Establishes, Maintains, and Synchronizes the interaction between communicating systems

Session Layer

Function of Session Layer

  • Dialog Control
    • Session layer allows two systems to enter into a dialog.
    • It allows communication between two processes to take place either in half duplex or full duplex mode
  • Synchronization
    • Session layer allows a process to add check points (synchronization points) in a stream of data
    • If a system is sending a file of 2000 pages, it is advisable to insert check points after every 100 page to ensure that each 100 page unit is received and acknowledged independently
    • In this case, if a crash happens during the transmission of page 523 , retransmission at page 501
    • Page 1-500 need not be retransmitted

Presentation Layer

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

Presentation Layer

Summary

  • The OSI Model
  • Functions of Layers

Reading Sections

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