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February 2010
Chapter Three OPERATION
3.1
Start-up
“Access Point” Start-up
An Access Point (AP) unit starts and immediately begins transmitting periodic messages, called
beacons, on the configured channel. Beacons include capability 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 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 Link LED. For an Access Point, the Link LED will be
OFF while no links have been established. Once one or more links have been established, the Link
LED is ON. 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 may not be passed over the wireless
network.
A maximum of 255 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 beacons from an Access Point to determine whether the link is still present. If the
Client can no longer receive beacons from the AP, the AP is considered to be out-of-range and the
