The first computer networks were designed with the hardware as the main concern and the software as an afterthought.This strategy no longer works. Network software is now highly structured.
To reduce their design complexity, most networks are organized as a series or hierarchy of layers or levels.
To reduce their design complexity, most networks are organized as a series or hierarchy of layers or levels.
The number of layers, the name of each layer, the contents of each layer, and the function of each layer differ from network to network.
Layer n on one machine communicates with layer n on another machine on the network using an some rules known as the layer n protocol.
A protocol is an agreement between the communicating parties on how the communication is to proceed.
The entities comprising the corresponding layers on two communicating machines over the network are called peers.
In realty, no data is transferred from layer n on any two machines. Instead, each data and control information is passed to the layer below.
Additional information including protocol control information may be appended by each layer to data as it travels from higher to lower layers in the form of layer headers.
Below layer 1 is the physical medium through which actual communication occur over communication channels.
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 layer.
The set of layers and associated protocols is called network architecture.
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