Siemens Simatic S7-300 CP 341 User Manual page 34

Basic Principles of Serial Data Transmission
2.2 Transmission Procedure with a Point-to-Point Connection
ISO 7-layer reference model
The reference model defines the external behavior of the communication partners. Each
protocol layer, except for the lowest one, is embedded in the next one down.
The individual layers are defined as follows:
1. Physical layer
– Physical requirements for data transmission, e.g., transmission medium, baud rate
2. Data link layer
– Security procedure for the data transmission
– Access method
3. Network layer
– Definition of communication paths
– Specification of the addressing for the data transmission between two communication
4. Transport layer
– Error detection procedure
– Corrective actions
– Handshaking
5. Session layer
– Setup of data transmission
– Execution
– Release of data transmission
6. Presentation layer
– Implementation of the standardized communication system display type in a device-
7. Application layer
– Specification of the communication task and the functions it requires
Processing the protocols
The sending communication partner runs through the protocols from the highest layer
(No. 7 - application-oriented) to the lowest layer (No. 1, physical specifications) while the
receiving communication partner processes the protocols in the reverse order, i.e., starting
with layer 1.
Not every protocol has to take all seven layers into account. If the sending and receiving
communication partner speak the same language, layer 6 is omitted.
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partners
specific form (data interpretation guidelines)
CP 341 Point-to-Point Communication, Installation and Parameter Assignment
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Manual, 09/2008, A5E02191071-01