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Network Reference Models (Network Architectures)

The two most important reference modes are listed below


i) The OSI reference model and

ii) The TCP/IP reference model




i)        The OSI reference model


An ISO standard that covers all aspects of networks communications is the Open System Interconnection (OSI) model. An open system is a model that allows any two different systems to communicate regardless of their underlying architecture. Vendor specific protocols close off communication between different systems without requiring changes to the logic of the underlying hardware and software. An OSI model is a layered framework for the design of network systems that allows for communication across all types of computer systems. The purpose of each layer is to offer certain services to the higher layers. Layer n on one machine (source) carries on a conversation with layer n on another machine (destination).  The rules and conventions used in this conversation are collectively known as the layer n protocol. Basically, a protocol is an agreement between the two machines as how communication link should be established, maintained and released.
The users of computer network are located over a wide physical range i.e. all over the world. Therefore, to ensure that nationwide and worldwide data communication systems can be developed and are compatible to each other and international group of standards has been developed. These standards will fit into a framework which has been developed by the International Organization of Standardization (ISO). The OSI model is not a protocol rather it is a model of underlying designing a network architecture which is flexible, robust and interoperable.

ii)      The TCP/IP reference model


The TCP/IP reference model which was used earlier by ARPANET and then it is being used in the Internet. TCP/IP is a short form of transmission control protocol and interned protocol. ARPANET was a research network sponsored by the US department of Defense. It included many universities and government installations using the leased telephone lines. Later on, the satellites and radio networks were added to it. This inclusion could not be handled by the existing protocols at that time. So, new reference architecture was needed. This new architecture is known as TCP/IP reference model due to the use of the two protocols TCP and IP. While designing the new model, certain goals were to be achieved. Some of them were as follows:
i)           First design goal was to have an ability to connect multiple networks together in a seamless way.
ii)          Another goal was the network should be able to survive loss of subnet hardware with existing conversation not being broken.
iii)        Next, a flexible architecture was needed to deal successfully with the divergent requirements of various applications.


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