Weidmuller WI-MOD-945-E: 900Mhz Wireless Ethernet & Device Server v2.16 - Manual (Page 20)

Weidmuller Wireless Ethernet Modem & Device Server WI-MOD-945-E User Manual
Page 20 www.weidmuller.com Rev 2.136
CHAPTER 3 - OPERATION
3.0 - Start-up
“Access Point” Start-up
When an Access Point (AP) unit starts up it will immediately begin transmitting periodic messages, called 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.
“Client” Start-up
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, Encryption method, etc. 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 AP to which they are connected.
When a Client transfers data to another Client, it first transmits the data to the AP, which then forwards the data to the
destined Client.
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 light, however data cannot be passed over the wireless network.
A maximum of 127 Clients may be linked to an Access Point.
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 the AP is out-of-range and the link is dropped. Whenever a
Client is not connected to an AP, it will cyclically scan all available channels for a suitable AP.
Roaming Clients
Clients can roam within a system however if the link to the Access Point fails or the radio signal level becomes too weak it
will scan for beacons and connect to an Access Point (provided the SSID and any Encryption methods, keys, etc. are