Basic Principles of Serial Data Transmission
2.2 Transmission Procedure with a Point-to-Point Connection
Point-to-point connection CP 440 Installation and Parameter Assignment
26 Manual, 09/2007, A5E00057742-03
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, baud rate
2. Data-link layer
– Security procedure for the transmission
– Access modes
3. Network layer
– Network connections
– Specifies the addresses for communication between two partners.
4. Transport layer
– Error-recognition procedure
– Debugging
– Handshaking
5. Session layer
– Establishing communication
– Communication control
– 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.
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