mething with another philosopher, he passes the message across the 3-2 interface to his translator at layer 2 who uses the language that he has agreed to previously with the layer 2 translator at the other end. The transla- tors use their best common language, which may be English, and this agreed common language of the translators corresponds to the layer 2 protocol. The translator then gives the message to layer 1 for transmission. Engi- neers at layer 1 may use any channel they have agreed on in advance. This physical communications may use a telephone network, a computer net- work, or some other means. This engineer and the communications channel arranged by him correspond to the layer 1 protocol. When a message arrives in the Philippines, it is received from layer 1, translated by the translator (layer 2) at that end, and passed to the receiving philosopher. Let us now imagine that these English-speaking translators are replaced by others, for example, because of a lunch break. These new transla- tors notice that French is a better common language for them and they agree to use that. The service provided to layer 3 by layer 2 remains the same and the philosophers do not notice that the protocol of one lower layer is com- pletely changed. In the same way, engineers can change the communications channel in use and upper layers may not notice and do not even care how the communi- cation is arranged as long as the quality of service is acceptable. Note that each protocol layer is completely independent of the other layers, and higher layers do not have to concern themselves about how communications is actu- ally arranged by the lower layers, that is, what protocol they use as long as service provided remains the same. 6.3.3 Open Systems Interconnection (OSI) In the late 1970s the ISO began to work on a framework for a computer net- work architecture that is known as the OSI reference model. The purpose of this model was to eliminate incompatibilities among computer systems. In 1982, ISO published ISO 7498 as a d