Networking Technologies

Chapter 10: The OSI Session Layer

 

 

Objectives:

Chapter 10 is the fifth chapter about OSI model layers. The objectives important to this chapter are on page 10-1:

  1. Understanding the overall purpose of this layer.
  2. Understanding the three dialog control methods..
  3. Understanding the three elements of session administration.

Concepts:

The overview of the chapter on page 10-2 shows that the Session Layer is concerned with communications between service requesters and providers. A comparison is made to telephone operators and directory services that help the communication process.

The topics and methods for this layer are on page 10-2:

  • Session - This layer has 2 topics and is associated with Packets
    • Dialog Control - simplex, half-duplex and duplex
    • Session Administration - connection establishment, data transfer and connection release

The first topic, Dialog Control, concerns three ways a dialog can be conducted:

  • Simplex - this is communication in one direction, more like a monolog than a dialog. It is compared to a public speech or a television transmission.
  • Half-Duplex - this is a dialog that can flow both directions, but only one direction at a time. After one side transmits, the channel has to be "reversed" for the other side to transmit. It is compared to CB or Ham radio, using only one frequency at a time and taking turns.
  • Full-Duplex - this is a dialog in which both sides can transmit and receive at the same time. It is compared to a telephone conversation, in which both sides have a live earphone and microphone.

The second topic, Session Administration, concerns three methods:

  • Connection Establishment - there are five subtasks in this method:
    • verify user names and passwords
    • establish an ID number for the connection
    • agree on what services are required, and for how long
    • determining who transmits first
    • coordinate acknowledgments and retransmissions
  • Data Transfer - there are three subtasks for this method
    • data transfer
    • ACK and NAK
    • resuming interrupted communications
  • Connection Release - this is an agreement on how to know when to hang up. Without it, one party or the other may try to resume the connection.