Access Methods - Siemens SIMATIC NET System Manual

1.3.7

Access Methods

Switching mechanisms
PROFINET is based on Fast Ethernet with 100 Mbps or Gigabit Ethernet with 1000 Mbps
and switch mechanisms and has further developed this technology.
Compared with other methods this has the advantage that each node can send at any time
since it always has a free point-to-point connection to the next switch. The connection is
bidirectional. The nodes can send and receive in both directions at the same time (full
duplex).
Switches in SIMATIC meet the real-time properties in PROFINET with two transmission
techniques "Store and Forward" and "Cut through".
"Store and Forward"
With the transmission technique "Store and Forward", the device stores the frames and then
enters them in a waiting queue. The frames are now forwarded selectively to the port that
has access to the addressed node.
If the device supports the international standard IEEE 802.1Q the frames are sorted into
different queues according to their priority. First, the frames with the highest priority are
processed.
Advantage
With the transmission technique "Store and Forward", the frames are checked that they are
correct and valid. This prevents bad or damaged frames being distributed through the
network.
Cut through
With the transmission technique "Cut Through" the entire frame is not stored temporarily in a
buffer but forwarded directly to the destination port as soon as the first 6 bytes (destination
address) have been read and the destination port identified.
The times that the data packet requires to pass through the switch are minimal and not
dependent on the frame length. The data is only stored temporarily using the store and
forward mechanism according to its priority when the section between the target part and the
port of the next switch is in use.
With PROFINET switches Cut Through is implemented by using ERTEC ASICs, for example
in the IRT switches of SCALANCE.
See also
Switches and switched LANs (Page 50)
Industrial Ethernet
System Manual, 09/2019, C79000-G8976-C242-10
Basics of communication with Industrial Ethernet
1.3 Technologies of Industrial Ethernet
45