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Cooper Bussmann 245U-E Wireless Ethernet Modem & Device Server User Manual
Rev Version 2.19
a bridge priority of 32768. The intention is to reduce traffic that the 245U-E must handle by placing it at the branch
level in the network tree. As a branch, the 245U-E need only pass traffic to devices that are its leaf nodes.
There is some overhead in maintaining a network utilizing the Spanning Tree Algorithm. Users wishing to increase
their throughput at the expense of redundancy should disable Spanning Tree. The Spanning Tree Protocol can be
configured on the Repeaters configuration page.
3.6 Compatibility
3 Address Mode and 4 Address Mode
Addresses in a Wireless Ethernet Data Frame
There are two Wi-Fi communication modes—3 Address mode (Layer 3 Bridge) and 4 Address mode. The modes
have a slightly different way of addressing the data frames to other devices on the network. In 3 Address mode,
the addressing is made up of a destination address (DA), a source address (SA), and a BSSID (MAC address of the
access point). In 4 Address mode, the addressing is made of a destination address (DA), a source address (SA), a
radio transmit address (RTA), and a radio receive address (RRA).
Packets Between Access Point and Client
Communications between access points and client generally uses 3 Address mode because there are only
three addresses within the communication path, as indicated in the path from the laptop to Station #1 in
Figure 30. If the laptop wishes to communicate to Station #1, the DA will be Station #1, the SA will be the laptop,
and the BSSID will be the access point’s MAC address—therefore, 3 Address mode is used.
The example in Figure 30 shows that the laptop may need to communicate to the Ethernet device connected to
Station #2. You can see that there is now a fourth address (laptop, access point, Station #2, and Ethernet device)
that cannot be addressed using the 3 Address mode of communications. The example shows the access point and
the station as both being 245U-E modules that will communicate, since both support 4 Address mode. However,
there are some instances where a third party access point may not be able to communicate using 4 Address mode
to a client (Sta).
There are only a couple of ways around this situation. You could change Station #2 to a router, but this may mean
that Station #2 and the Ethernet device will use a different network address scheme, which may be inconvenient
and will require more setup. You could configure the access point to communicate using 4 Address Mode, but this
is only possible if the station and the access point support 4 Address Mode (some third party access points do not
support this mode).
240U-E Compatibility
The 245U-E-G modules can communicate with ELPRO 240U-E Ethernet modems, but only in certain modes. The
following table shows compatible configurations. If communicating with ELPRO 240U-E Ethernet modems, the
240U-Es need to be configured with WDS (4 Address mode), not the default 3 Address mode, and the 240U-E
access points cannot be auto-connect to any WDS access points.
