Omron SYSMAC CJ1W-EIP21 - Manual (Page 56)

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Selecting the Network Devices Section 2-3
When selecting a switching hub, it is necessary to consider whether the
switching hub will be selected based on the kind and amount of communica-
tions that will be performed in the network or the kind of switching hub that you
want to use. Refer to the following precautions when selecting a switching
hub.
Refer to 10-2 Adjusting the Communications Load to estimate the communi-
cations load for tag data links.
Selecting the Switching Hub Based on the Types of Network Communications
Executing Tag Data Links
Only We recommend using an L2 switching hub without multicast filtering or an L2
switching hub with multicast filtering.
Using an L2 switching hub with multicast filtering prevents increased traffic
due to unnecessary multicast packets, so the tag data links can operate at
higher speed. If either of the following conditions exists, the amount traffic will
be the same for both kinds of L2 switching hubs (with or without multicast fil-
tering).
• The tag data links are set to share the same data with all nodes in the net-
work. (The multicast packets are transferred to all nodes in the network,
just like a broadcast.)
• The tag data link settings are all one-to-one (unicast) and multicast pack-
ets cannot be used.
If multicast filters are being used, settings must be made in the switching hub.
There must be enough multicast filters for all of the networks being used.
Executing Tag Data Links
and Message
Communications We recommend using an L2 switching hub with multicast filtering and L4 QoS.
By setting tag data links for higher-priority transmission, it is possible to pre-
vent problems such as transmission delays due to message communications
traffic and packet losses due to buffer overflow. Settings must be made in the
switching hub to enable this function and give higher priority to tag data link
packets.
Special settings must be made in the switching hub when using the multicast
filtering function and L4 QoS function.
Selecting the Switching Hub Based on the Hub’s Supported Functions
L2 Switching Hub without
Multicast Filtering We recommend this kind of switching hub when only tag data links are exe-
cuted and any of the following conditions is met.
• The tag data links are set to share the same data with all nodes in the net-
work. (The multicast packets are transferred to all nodes in the network,
just like a broadcast.)
• The tag data link settings are all one-to-one (unicast) and multicast pack-
ets cannot be used.
• There is little traffic in the tag data links.
No special settings are required for an L2 switching hub without multicast fil-
tering.
L2 Switching Hub with
Multicast Filtering We recommend this kind of switching hub when only tag data links are exe-
cuted and the following condition is met.
• There are many 1:N links (where N represents some number of nodes in
the network) in the tag data link settings, i.e., there are many multicast
packets used, or there is heavy traffic in the tag data links.
Special settings are required for an L2 switching hub with multicast filtering.
There must be enough multicast filters for the network being used.