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08.89
In order to exchange information, each of them hires a translator. To understand each other,
the translators must agree on a common language, e.g. English. The translators constitute
layer 2.
In order to transmit the information, the translators deliver their texts to the respective post
office.
To send the texts, the employees of the post offices agree on a common transmission method
(e.g. telex or fax). They constitute layer 1.
The procedure is as follows:
Scientist 1 (layer 3) hands the message written in Japanese to his translator (layer 2). The
translator translates the text into English and delivers it to the post office (layer 1). The post
office in Japan transmits the text by fax to the respective post office in Spain. The translator in
Spain (layer 2) receives the fax from the post office employee and translates it from English
into Spanish. Scientist 2 (layer 3) receives a Spanish text from his translator which contains
the thoughts of his Japanese colleague.
Irrespective of the common language the translators have chosen (e.g. French instead of
English) or the type of transmission selected by the post (e.g. telex instead of fax) the
scientists receive the message in a language they understand.
This example shows that individual layers can change their agreements (protocols) without
affecting the other layers. This is the main reason for using the layer structure for networks.
Scientist 1
(Japanese)
Translator 1
(Japanese/English)
Post office 1
(fax)
3 layer model
© Siemens AG 1989 All Rights Reserved
SINUMERIK 840/880 (PJ)
layer 3
layer 2
(English/Spanish)
layer 1
6FC5 197-0AB20-0BP0
2 Local Area Networks
2.1.1 The ISO seven layer model
Scientist 2
(Spanish)
Translator 2
Post office 2
(fax)
2–3
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