Computer networks are designed in a highly structured way. Some of the structuring techniques are discuss here.
To reduce their complex design most networks are organized as a series of layers or levels, each one built upon its predecessor. The number of layers, name, content, and function of each layer differs from network to network.
In all networks, the purpose of each layer is to offer certain services to the higher layers, shielding those layers from details of how the offered services are actually implemented.
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Layer n on one machine carries on a conversation with layer n on another machine. The rules and conventions used in this conversation are collectively known as the Layer n protocol.
The entities comprising the corresponding layers on different machines are called peer processes. It is the peer processes that communicate using the protocol.
Each layer passes data and control information to the layer immediately below it until the lowest layer is reached. Below layer 1 is the physical medium through which the actual communication occurs.
Between each pair of adjacent layers there is an interface. The interface defines which primitive operations and services the lower layer offers to the upper one. For the designers of a network one of the most important considerations is defining clean interfaces between the layers.
Network Architecture is the set of layers and protocols. The specification of the architecture must contain enough information to allow an implementer to write the program or build the hardware for each layer so that it correctly obeys the appropriate protocol.
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