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08.89
Layer 7:
Application Layer = Application function (information processing) e.g. File
Transfer, Remote Job Control
The application layer is the highest of the 7 layers of the OSI model and does not provide any
services to a higher layer with regard to the integration of open systems (OSI environment). It
rather provides the means for the application processes, so that these can access the OSI
environment. The application layer serves as a window between corresponding application
processes which are using the integration of (open) systems to exchange meaningful
information.
Each application process is represented to its peer by the application layer entity.
"Even when the same language is spoken and the same terms are used, a term can have
different meanings, when for example the same terms are used in a different environment. It
must therefore be ensured that the meaning of one or a combination of terms is identical for
both the transmitter and the receiver".
2.1.2
Access methods
Two access methods are today at the centre of both theory and practice:
The token principle and the collision principle (CSMA/CD, Ethernet). There are already several
variants of both procedures, however one variant of each has been standardized.
Process communication/access methods
Allocation systems/
token procedures
•
An authorization to send
(token) exists, which is held by
one station onlyat any time.
•
The authorization to transmit is
passed on to another station
after a given period of time.
•
Each station receives one
authorization to transmit
after a defined time interval.
© Siemens AG 1989 All Rights Reserved
SINUMERIK 840/880 (PJ)
Contention systems/
collision principle
(CSMA/CD, Ethernet)
•
Theoretically, all stations
in a network can transmit simultaneously.
•
In case of a collision,
this is recognized and the stations
repeat transmission after
different times.
•
High transmission capacity
keeps the number of
collisions low
•
A probability factor
can be specified with which a
message will be transmitted.
6FC5 197-0AB20-0BP0
2 Local Area Networks
2.1.1 The ISO seven layer model
2–7
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