Models and Protocols

It is easy to confuse the OSI and Internet models with the OSI and Internet protocols. A model is a set of guidelines on how one should go about designing a network protocol. For example, it can say 'use a physical layer which will deal with voltages, frequencies, etc.'. The model does not say 'use copper wire and voltages of 5 V representing 1 bit'. That is a specific protocol implementation.

A model can have many implementations that fit it. For example, consider the following network: two plastic cups joined by a piece of string. The physical layer is the cups and string; the network layer is empty; the transport layer is saying 'over' at the end of each voice packet; the application layer is whatever we are talking about. This is a network implementation that fits the Internet model.