Interview Questions

What is Roaming?

WLAN testing FAQ

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What is Roaming?

The ability of a wireless client to communicate through the wireless LAN is based on its association with an Access Point (AP) which serves as the gateway for a connection to other wireless or wired devices. As long as the signal quality received by the wireless client remains unchanged, the client will continue to communicate through the same AP. As the client moves away from the AP, or as changes occur in the physical medium between the client and the AP (e.g. an office environment in which a door is being closed or a file cabinet is being moved) and the signal quality degrades, the AP steps down the transmission speed (also known as rate adaptation) to maintain the connection. The client can measure the signal quality from all the APs servicing its area by utilizing the Beacon Frames generated by the APs. At some point the quality of the link will degrade significantly and the client will be forced to use a different AP for continued connection. This process is called ROAMING. When a wireless client moves to a different AP, re-authentication is required by the AP.
The 802.11i security standard, ratified in June 2004, adds a few provisions that allow for seamless transition between APs without the need for complete re-authentication, while maintaining a secure connection.
The first provision, known as Pair-wise Master Key (PMK) Caching, allows the client to store a master key with the AP, and in the case the client roams away from the AP, and back again, that same key will be used by the AP, making it unnecessary to re-authenticate.
A second provision allows for advance authentication, or “pre-authentication”. This enables a client to authenticate with one AP, and use a wired network to convey the authentication to another AP, thus pre-authenticating with the second AP before actually coming into its range.

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