The OSI Reference Model, in its simplest terms is a seven layered design designed to make software development, product evolution, modular engineering and multi vendor interoperability easier

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Introduction

Developed by the International Standards Organisation (ISO) in the 1980s and often described simply as The Stack, The OSI Reference Model, in its simplest terms is a seven layered design designed to make software development, product evolution, modular engineering and multi vendor interoperability easier. This enables the solutions offered by one layer to be updated without adversely affecting the other layers thus making it easier for both application and hardware developers alike. By the time the OSI Reference Model had been fully developed The Defence Advanced Research Project Agency had continued to develop what has now become the de facto standard for the internet, the four layered TCP/IP Protocol Stack. TCP/IP attempts to create a heterogeneous network with open protocols that are independent of operating system and architectural difference. TCP/IP protocols are available to everyone, and are developed and changed by consensus. Everyone is free to develop products to meet these open protocol specifications.

The Department of  Defences development of The TCP/IP Protocol Stack halted further development of the OSI Reference Model and indeed other vendors such as Netware, AppleTalk, Xerox and 3com developed there own standards. Although based on the OSI Reference Model, the concepts of protocol hierarchy are somewhat different.

This report will focus on the different protocol layers and their functions within the OSI Reference Model and the TCP/IP Protocol Stack, some comparisons with other vendor specific stacks will also be made.

Report

“The real key to OSI is not the details of functions at each layer, but rather an understanding of why the layers have been separated as they have been; those layers represent sets of consistent concepts and functions related to data handling. The various layers treat data very differently (different interpretations).  It is important to differentiate between network-oriented functions, and those associated with a GUI or other user interface”.

                                                                                                                                        John Drabik, Arcturus Networks

                                                                                                                                        Apr 15, 2003

Layers & Protocols

The OSI Reference model is a set of seven layers that define the different stages that Date (know as Data Encapsulation) needs to travel to get from one network to another (think of it as a set of ladders that needs to be descended or ascended at both sender and receiver). Although it has seven layers this can be further broken down into two parts, the upper layers or Application set (top three Layers) which is closest to the end user and the lower layers or Network set (bottom four layers) responsible for data transport and placing data on the medium (the wires for example).  Whilst closely mirroring The OSI Model, the TCP/IP Protocol Stack has only four layers, again further broken into the Application set and Network set. “Xerox developed a five layered stack, The Xerox Network System (XNS) that was also adopted by most of the early LAN companies including Novell, Ungermann-Bass and 3 Com”.

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                                                                                                                                        Internetworking Technologies Handbook

Irrespective of which Layered Model we look at, each layer has a task or group of tasks assigned to it and whilst applications and protocols rely on other protocols across the stack, each layer can only communicate either one layer up or one layer down and with its respective protocol or layer at both ends. As the Data travels down the Layers from the sender it is encapsulated within protocols pertaining to that particular layer, the same happens at the receiving end and this is known as de-encapsulation. Breaking down the layers offers us a ...

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