User's Manual

Elpro Technologies 450U-E Wireless Ethernet Modem & Device Server User Manual
42 www.cooperbussmann.com/wirelessresources Rev Version 1.4.0
Filter Example
Device B needs to communicate with Device E via modems C & D. The Filtering requires that Modem C has Device B in
its Whitelist , any traffic from Device E will be passed back into the
LAN via Modem C because the destination matches the IP for device B. This works because Device B is a Modbus
Master and it initiates all communications. If the communications was being initiated from each end, i.e. a non-polling
system you would need to put a filter list in each modem to allow the communications to be passed from each end.
With this filter configuration Device A will not be able to access Device E, as Device A is not present in the Whitelist in
Modem C.
ARP filters is also recommended as it would filter out broadcast ARP requests from other devices on the LAN which
would normally be sent over the radio. ARP (Address Resolution Protocol) is a communication protocol used by Ethernet
devices for associating MAC addresses and IP addresses and is a crucial part of normal network communications. When
a device on a LAN wishes to communicate with another device it needs to know the MAC Address. If the MAC address is
not already known or is in its lookup table it will broadcast an ARP request which subsequently would be passed over the
radio if the modems were setup in bridging mode. If this is a small network it may not matter however in larger systems
there can be a considerable amount of broadcast ARP traffic which if sent over the radio would compromise the reliability
of the wireless link.
It should be noted that adding ARP filters will only filter out ARP traffic and IP filters will only filter out
IP traffic.
Also if using an IP filter any Ethernet traffic that is not IP would be passed, this could include Netbios,
IPX, PPP, etc. These protocols could be more effectively filtered by using MAC filtering or configuring
the modems in a Router configuration instead of a Bridge.
If after configuring the modem with filteriing, you no longer have access to the modem, this probably
means the Computer IP or MAC address was not added to the filter list. SETUP mode must be
enabled to restore operation.
MAC Address Filter Configuration:
MAC addresses are uniquely assigned to each device and so can be used to permit or deny network access to specific
devices through the use of Blacklists and Whitelists.
In theory, MAC filtering allows a administrators to permit or deny network access to hosts associated with the MAC
address, though in practice there are methods to circumvent this form of access control through address modification.
The MAC filter entry will match only the source MAC address in the packet.
Note: It is important to add the MAC Address of the configuration PC when creating a Whitelist. If the
configuration PC is not on the Whitelist, it will be unable to communicate with the module for further
configuration.
Figure 48 - Filtering Example