Udp With Broadcast And Multicast - Siemens S7-300 Configuration Manual

5.7.2

UDP with broadcast and multicast

Application
With UDP, the frames are received without acknowledgment because the UDP protocol has
no provision for acknowledgments. This is intentional so that a higher speed at lower
network load is achieved. This has advantages particularly when using multicast. If, for
example, frames are sent to 100 partners, 100 acknowledgments (1 per partner) would
arrive at the same time at the sender.
When you select the connection partner, you have the following two extra options on UDP
connections:
● Connection to all broadcast nodes
● Connection to all multicast nodes
When do you use multicast instead of broadcast?
To allow the simultaneous transfer 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 need to be sent to groups of partner stations.
Why does an S7 CP prevent reception on broadcast connections?
It is often necessary for one station to send frames to a number of partner stations. It is
important that the frames are sent at the same time and arrive at practically the same time.
In such situations, the use of broadcast may well be suitable. A broadcast message is
received by all nodes in the network.
A typical application is the sending of broadcast frames to find a MAC address for an IP
address (ARP request).
Configuring and commissioning S7 CPs for Industrial Ethernet
Configuration Manual, 09/2013, C79000-G8976-C182-13
If you select "broadcast / all broadcast nodes" as the partner, you specify that UDP
frames are sent to all reachable broadcast nodes.
Note
When using broadcast, you can only send with S7 CPs; reception is not possible (see
below)!
If you select "multicast / 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.
Configuring communications connections
5.7 Configuring UDP connection properties
107