User's Manual

28 Document Number: 0100SM1401 Issue: 10-14
Compression
In determining whether to use compression, the type of application and the latency requirements may need to be
considered. Compression will typically reduce the size of data packets being sent over the radio channel, if the original data
is compressible, at the expense of slightly (ie: a few milliseconds) longer latency. SCADA traffic such as DNP3 or Modbus are
well suited to compression. Applying compression to data that is highly random (ie: encrypted data) should be avoided as it is
unlikely to be compressible. Compression is enabled by default.
Ethernet Filtering
Ethernet filtering provides an easy to configure Layer 2 filtering mechanism, which can help prevent unnecessary Ethernet traffic
and increasing channel loading. There are various different addressing methodologies that can be filtered, which include:
Unicast:
Unicast is an addressing methodology that delivers messages to a single network destination
identified by a unique address.
Multicast:
Multicast is an addressing methodology that delivers messages to a group of destination addresses
simultaneously in a single transmission. Spanning tree messages are an example of multicast
messages.
Broadcast:
Broadcast is an addressing methodology that delivers messages to every device on a network. The
broadcast address of a device is calculated from the subnet mask. If all devices within a network use a
common network mask, the broadcast address will also be common.
Although typical SCADA applications only require Unicast & ARP data, the filtering mechanism provides the option to allow:
•All Ethernet traffic
•Unicast & ARP only (ARP is primarily used by networks to identify which physical devices own which IP addresses).
•Unicast only (Only used when a MAC address table is statically assigned).
•Or allow traffic from a single MAC address only.
Although Spanning tree messages are multicast messages, they are also filtered out unless the user is allowing all Ethernet
traffic to pass. This also helps to prevent unnecessary channel loading.
Part D – Feature Detail