Introduction to Industrial Automation

8.3.2   The ISO/OSI Model

In the field of communication networks, there has been a complete incompatibility in the 

various hardware and software manufacturers, as well as between the communication proto-

cols used in these networks. Thus, at the beginning of the communication network era, it was 

impossible to connect two computers or PLCs from different manufacturers on a common net-

work. The International Organization for Standardization (ISO) developed the Open Systems 

Interconnection model (OSI) in order to describe the communication process in the field of net-

works. The model has a key feature, that of a hierarchical seven-layer stratification. Each layer is 

directly linked with the previous one and has a definite relation with the rest. Each layer takes 

care of a very specific job, and then offers its services onto the next higher layer. The interface is 

so flexible that each manufacturer has the possibility to create its own communication protocol 

consistent with the OSI model. The ISO/OSI model is a reference tool for data communications, 

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  Introduction to Industrial Automation

and its seven layers are ranked from the lowest physical layer, data link, network, transport, and 

presentation to the highest application layer. In these layers, all the specifications of the commu-

nication networks are defined, from electrical details of bit transmission, the voltage that would 

represent the binary digit “1” and bit “0”, the time length of each bit-pulse, up to the applications 

concerning the user. Finally, the ISO/OSI model defines terms that networking professionals can 

use to compare basic functional relationships of different networks.