In a perfect world, any Wi-Fi Alliance-certified 802.11 client device would perform consistently with any alliance-certified access point of the same network type. However, the alliance certifies products for interoperability only; it makes no statements or judgments about how well any given client/AP pair performs. So at this juncture, say experts, there will be some variability in the performance of different client devices.

Asking a prospective system vendor about how client-to-AP associations and roaming happen (particularly if you are looking to use Wi-Fi for voice) is not a bad idea. In the majority of systems available today, it is the client that makes the decision about which AP it connects to and when it’s time to roam to a different AP. The exceptions are Meru Networks and Extricom, which, instead, move the decisions to the centralized controller in an effort to get greater and more consistent overall network performance.

But different network interface cards (NIC) tend to behave differently from on another, depending on the NIC vendor and driver configuration. That’s because the 802.11 standard doesn’t specify a technique for how client-to-AP decisions should be made.

Most of the clients that make the decisions themselves use signal strength as their primary metric and attempt to communicate with the strongest AP signal. And most monitor the signal-to-noise ratio of received frames and the data rate to determine when to roam to a new AP. When the signal-to-noise ratio is low and the data rate is slow, that’s when most clients seek another AP.

Many of these clients put off moving as long as possible so as not to interrupt communications in process. The tendency to stay put is known as client “stickiness.”

Some NICs allow you to configure a NIC’s stickiness level in the card’s drivers. However, keep in mind that what’s optimal for a given client device and user may not be optimal for the overall network. For example, if all NICs are aggressive, contention on the network might grow.

So a top-down plan for determining just how sticky you want your clients is in order, rather than configuring endpoints, willy-nilly, for varying levels of stickiness. Also, check with your vendor about any controller software capabilities for managing the impact of diverse client behavior on the WLAN. (Compare WLAN Management products)

Joanie Wexler is an independent networking technology writer/editor in Silicon Valley.