Technical data

www.westermo.com Theoretical and general applications 53
OSI model
In order for systems to communicate with each other a structured framework is nec-
essary that makes it possible to connect together solutions from individual suppliers.
This was the reason for the creation of the OSI-model (Open System Interconnec-
tion). The OSI-model was developed by ISO and explains how the communication
between any two systems works. As the name implies the purpose is to make systems
open and with that supplier independent. Company specific systems make it impossi-
ble to communicate with equipment manufactured by other companies; these disad-
vantages are eliminated when you use a standardised protocol. Note that this is a
model and not a protocol, its purpose is to explain and design networks that are flexi-
ble, robust and above all open.
Structure of the OSI-model
In 1983 the International Standards Organization for (ISO) developed a model, OSI,
(Open System Interconnection Reference Model) for just this purpose. This defines all
parts, structures and functions needed for communication and arranges these on
7 layers or levels, in order according to the different phases of the communication
process.
Simplified, you can say that each layer (except the application layer) works so that it
communicates with the adjacent layer. Further information, a header, is added to allow
communication between the layers. This is necessary so that the underlying layer can
interpret and manage the data. When the data reaches the receiver, each layer
removes the added information (header) that the particular layer needs. The informa-
tion is then sent on to the nearest layer above. When the information finally reaches
the uppermost layer, all the extra information has been removed. Consequently, each
layer communicates with the corresponding layer on the other computer.
Using the European V.24 standard as an example, this is a logical specification that is
specified by the physical layer. It only defines the task of the lines: control, data and pos-
sible transmission rates. Hence the V.24 standard is supplemented with an electronic
specification known as V.28, which is also a subset of the physical layer.
V.24 and V.28 have their counterpart in the American standard RS-232, which specifies
the physical as well as the electrical interface.
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