User manual

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Cooper Bussmann 245U-E Wireless Ethernet Modem & Device Server User Manual
19Rev Version 2.19
ChapTEr 3 - OpEraTION
3.0 Startup
Access Point Startup (245U-E-G)
When an access point unit starts up, it checks if the channel selection is set to “Auto.” If it is, the access point
will scan all available channels, pick the quietest then begin transmitting periodic messages, called beacons.
Ifthe access point is configured with a fixed channel, it will immediately begin sending beacons on the configured
channel. Beacons include information that a client may examine in order to identify if the access point is suitable
for link establishment. Clients will only attempt to establish a link with an access point whose beacon indicates a
matching SSID. Access points do not initiate link establishment.
Access Point Startup (245U-E-A)
If the modem is configured to use Dynamic Frequency Selection (DFS), it will behave slightly differently than
the 245U-E-G because it needs to comply with DFS regulations. When an access point starts up, it will scan all
available channels from the selected groups and then select the quietest, similar to the 245U-E-G. It will then
go into a scan mode for 60 seconds where it listens for any radar signals. If a radar signal is detected, it will flag
the channel as being unavailable (the channel will stay unavailable for 30 minutes) and then pick another random
channel and repeat the same scanning process until a radar-free channel becomes available.
Client Startup
When a client powers up, it scans for beacons from access points. While a link is not established, the client
cyclically scans all available channels for a suitable access point. The client will attempt to establish a link with an
access point only if it has matching SSID and encryption method, and other compatible capabilities as indicated by
the beacon. If more than one suitable access point is discovered, the client will attempt to establish a link with the
access point that has the strongest radio signal.
Link Establishment
Once a client identifies a suitable access point for link establishment it attempts to establish a link using a two-
step process—authentication, and association. During authentication, the client and access point check if their
configurations permit them to establish a link. Once the client has been authenticated, it will then request an
association to establish a link.
Status of the wireless link is indicated via the TX/LINK LED. For an access point, the TX/LINK LED will be off while
no links have been established. Once one or more links have been established, the TX/LINK LED is on green. For
a client, the Link LED will reflect the connection status to an access point. Link status is also displayed on the
Connectivity page of the Web interface.
After the link is established, data may be transferred in both directions. The access point will act as a master unit
and will control the flow of data to the clients linked to it. Clients can only transmit data to the access point to
which they are connected. When a client transfers data to another client, it first transmits the data to the access
point, which then forwards the data to the destined client. A maximum of 127 clients may be linked to an access
point.
NOTE The presence of a link does not mean that the connected unit is authorized to communicate over
radio. If the encryption keys are incorrect between units in the same system or a dissimilar encryption
scheme is configured, the LINK LED will turn on, but data cannot be passed over the wireless network.
How a Link Connection is Lost
The access point refreshes the link status with a client every time a message is received from that client. If nothing
is received from a client for a period of 120 seconds, the access point sends a “link-check” message. If there is no
response to the link-check, a de-authenticate message is sent and the link is dropped.
A client monitors beacon messages from an access point to determine whether the link is still present. If the client
can no longer receive beacons from the access point it assumes that the access point is out of range and the link