01 May 2004
No single network protocol can
The demands placed on data transmission within a company network will depend on the source and type of data handled.
At the enterprise level, normal office programs, such as text processing, spreadsheets, databases, and e-mail, will be in use together with applications, such as SAP. The prime use of the network is to allow clients to call up or store data from a central server. Thus it is not in constant use, but has to handle relatively large amounts of data that are not time critical.
At the field, I/O, and control level, the opposite is true: the network is constantly in use, the response times and data packets must be relatively short, and communication must be deterministic.
In manufacturing, where there is a high degree of sequential control, high transmission speeds and very short signals are at a premium. In contrast, in processes where temperature, level, and the like serve as control, the speed is less critical, but the signal must contain more status information.
The plant management level must handle both types of data. Archiving, visualization, and maintenance programs are in regular contact with the control level, scanning and mapping selected process data. There is also the need to change device configurations online.
On the other hand there are often exchanges with the enterprise level as for computer-aided design data, computer numeric control programs, recipes, and spreadsheets with plant data. There is also the same demand for client/server services as on the enterprise level.
This variety of applications means that no single network protocol can operate efficiently at all levels. The protocol defines the principal network characteristics such as the speed of data transmission, length of data packets, degree of data security, and the method by which data is distributed.
Thus in a brewery, the beer production will use a protocol suitable for process control, whereas the bottling and packing facility will use one more suited to automated manufacturing. Information from both systems will be required at the plant management and enterprise levels.
To promote efficient data exchange, there is a need for standardization. This ensures that the devices on a particular network work together and that there is transparency between the various levels of the communications hierarchy.
The central pillar of all network standards is the International Standards Organization–Open Systems Interconnection (ISO-OSI) model.
Source: Instrument Engineers Handbook, ISA Press and CRC Press