270 • ETHERNET
Network communication
Modular I/O System
ETHERNET TCP/IP
5.2 Network
communication
5.2.1
Channel access method
In the ETHERNET Standard, the fieldbus node accesses the bus using
CSMA/CD (Carrier Sense Multiple Access/ Collision Detection).
•
Carrier Sense:
The transmitter senses the bus.
•
Multiple Access:
Several transmitters can access the bus.
•
Collision Detection: A collision is detected.
Each station can send a message once it has established that the transmission
medium is free. If collisions of data packets occur due to several stations
transmitting simultaneously, CSMA/CD ensures that these are detected and the
data transmission is repeated.
However, this does not make data transmission reliable enough for industrial
requirements. To ensure that communication and data transmission via
ETHERNET is reliable, various communication protocols are required.
5.2.2 Communication
protocols
In a WAGO ETHERNET fieldbus coupler, several important communication
protocols are implemented in addition to the ETHERNET Standard:
>
IP, ICMP and ARP on the third ISO/OSI layer (network layer)
>
TCP and UDP on the fourth ISO/OSI layer (transport layer).
Moreover, the fieldbus coupler contains several additional user protocols,
which use these protocols for transmitting and receiving data.
The following diagram is intended to explain how the protocols and their
functions are layered and their data packets nested in each other for transmis-
sion. A detailed description of the tasks and addressing schemes of these pro-
tocols is contained in the following chapter.
Fig. 5-8: Communication protocols
G012907e