Siemens SIMATIC CP 440 Manual page 25

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 as follows:
1. Physical layer
– Physical conditions for communication, e.g. transmission medium,
transmission rate
2. Data link layer
– Security procedure for the transmission
– Access modes
3. Network layer
– Network connections
– Definition of the addressing for communication between two partners
4. Transport layer
– Error-detection procedure
– Debugging
– Handshaking
5. Session layer
– Establishing communication
– Data exchange management
– Terminating communication
6. Presentation layer
– Conversion of the standard form of data representation of the
communication system into a device-specific form (data interpretation rules)
7. Application layer
– Defining 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 layer) to the lowest (no. 1 – physical layer), while the
receiving partner processes the protocols in the reverse order, i.e. starting with
layer 1.
Not all protocols have to take all 7 layers into account. If the sending and receiving
partners both use the same protocol, layer 6 can be omitted.
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02
Basic Principles of Serial Data Transmission
2-7