Lesson 2: OSI Reference Model

The Layered Model | Physical & Data Link Layers | Network Layer | Transport, Session, Presentation, and Application Layers

Session Layer

  - Network File System (NFS)
  - Structured Query Language (SQL)
  - Remote-Procedure Call (RPC)
  - X Window System
  - AppleTalk Session Protocol (ASP)
  - DEC Session Control Protocol (SCP)

The session layer establishes, manages, and terminates sessions among applications. This layer is primarily concerned with coordinating applications as they interact on different hosts. Some popular session layer protocols are listed here, Network File Systems (NFS), Structured Query Language or SQL, X Window Systems; even AppleTalk Session Protocol is part of the session layer.

Presentation Layer

The presentation layer is primarily concerned with the format of the data. Data and text can be formatted as ASCII files, as EBCDIC files or can even be Encrypted. Sound may become a Midi file. Video files can be formatted as MPEG video files or QuickTime files. Graphics and visual images can be formatted as PICT, TIFF, JPEG, or even GIF files. So that is really what happens at the presentation layer.

Application Layer

The application layer is the highest level of the seven layer model. Computer applications that you use on your desktop everyday, applications like word processing, presentation graphics, spreadsheets files, and database management, all sit above the application layer. Network applications and internetwork applications allow you, as the user, to move computer application files through the network and through the internetwork.

Examples:-

COMPUTER APPLICATIONS

  - Word Processor
  - Presentation Graphics
  - Spreadsheet
  - Database
  - Design/Manufacturing
  - Project Planning
  - Others

NETWORK APPLICATIONS

  - Electronic Mail
  - File Transfer
  - Remote Access
  - Client-Server Process
  - Information Location
  - Network Management
  - Others

INTERNETWORK APPLICATIONS

  - Electronic Data Interchange
  - World Wide Web
  - E-Mail Gateways
  - Special-Interest Bulletin Boards
  - Financial Transaction Services
  - Internet Navigation Utilities
  - Conferencing (Voice, Video, Data)
  - Others

 - SUMMARY - 

  - OSI reference model describes building blocks of functions for program-to-program     communications between similar or dissimilar hosts

  - Layers 4–7 (host layers) provide accurate data delivery between computers

  - Layers 1–3 (media layers) control physical delivery of data over the network

The OSI reference model describes what must transpire for program to program communications to occur between even dissimilar computer systems. Each layer is responsible to provide information and pointers to the next higher layer in the OSI Reference Model.
The Application Layer (which is the highest layer in the OSI model) makes available network services to actual software application programs.
The presentation layer is responsible for formatting and converting data and ensuring that the data is presentable for one application through the network to another application.
The session layer is responsible for coordinating communication interactions between applications. The reliable transport layer is responsible for segmenting and multiplexing information, keeping straight all the various applications you might be using on your desktop, the synchronization of the connection, flow control, error recovery as well as reliability through the process of windowing. The network layer is responsible for addressing and path determination.
The link layer provides reliable transit of data across a physical link. And finally the physical layer is concerned with binary transmission.

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