Siemens Simantic Net NCM for Industrial Ethernet S7 - Manual (Page 134)

Configuring Communication Connections
134
SIMATIC NET NCM S7 for Industrial Ethernet
C79000-G8976-C129–06
4.7.3 UDP with Broadcast and Multicast
Application
When you select the connection partner, you have the following two extra options
on UDP connections:
S Connection to all Broadcast Nodes
If you select the “all broadcast nodes”, you specify that UDP frames are sent to
all obtainable broadcast nodes.
Notice
When using broadcast, you can only send with S7-CPs; reception is not possible
(see below)!
S
Connection to all Multicast Nodes
If you select “all multicast nodes” as the partner, you specify that UDP frames
are sent to all nodes of a multicast group and that multicast frames can be
received.
Multicast is a special, configurable connection option that is supported by
Industrial Ethernet CPs only on UDP connections.
The frames are sent without acknowledgment because the UDP protocol has no
provision for acknowledgments. This is designed to avoid floods of data resulting
from the acknowledgments. If, for example, frames are sent to 100 partners, 100
acknowledgments (1 per partner) will arrive at the same time. The sending module
would not be able to evaluate such a flurry of data.
When to Use Multicast instead of Broadcast ?
To allow the simultaneous transmission of a frame to a number of partners, the
connection option Multicast for UDP connections was introduced.
In contrast to the broadcast connection option, it is also possible to receive frames
sent to several nodes in the multicast circle on this connection type.
By specifying a particular group of recipients (multicast circle), load on recipients
for which the message is not intended is prevented. Multicast therefore represents
a better solution than broadcast when frames are sent to groups of partner
stations.