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2 Local Area Networks
2.1 Bus connection
Since the bus cable can be run throughout the facility, only short branch cables are necessary.
The cable costs are thus reduced, and the expenditure for extending the system remains
linear.
Basically, each connected station can communicate with another one over the common bus
cable. However, since the bus is a common resource for all stations, its use has to be
controlled by an access method in order to prevent one station from interfering with the
operation of another one.
Process communication bus topology
Advantages of bus systems:
•
Low cable expenses
•
Cost-saving even with few nodes connected
•
Extension costs are linear
•
Any traffic between all units is possible simultaneously
•
Bus is a common resource access method
2.1.1
The ISO seven layer model
Various specifications have to be observed for the communication between systems. Both
partners must have compatible physical interfaces, they must speak the same "language", and
they must "mean" the same thing when they say something.
To ensure this between the peers on a local area network, ISO (International Standardisation
Organization) has defined a hierarchical layer model.
In this model, a layer is defined as a program or a process that communicates with the
corresponding process of its peer on the same layer .
The layers function as follows:
A layer receives a job from the next higher layer. It processes the job with its own resources
and then passes a job to the next lower level. This means that to handle the job the layer uses
its own resources as well as the services made available by the next lower layer.
Example: 3 layer model
A "humanized" example of a 3 layer model may help to explain the layer structure:
Two scientists would like to exchange their findings. Scientist 1 lives in Japan and speaks
Japanese, scientist 2 lives in Spain and speaks Spanish. The two scientists represent layer 3.
2–2
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