Wi-Fi technology is steadily expanding: IEEE 802.11a and 802.11g are now firmly entrenched alongside 802.11b, and WPA (Wi-Fi Protected Access) security is now accepted as a standard. Yet Wi-Fi compatibility between devices is still at an all-time low.

That's what the folks at Wi-Fi Alliance tell me, at least, and they are best positioned to know because they test product interoperability between Wi-Fi access points, wireless cards, and software.

The failure rate on 802.11b products alone, which have been well established in the market for at least three years, is about 25 percent, according to Dennis Eaton, the Wi-Fi Alliance chairman.

Believe it or not - I say with tongue firmly planted in cheek - there are actually some companies that put products on the market before the Alliance lab gets its hands on it. To be fair, it should be noted that the Alliance charges between US$5,000 and $12,000 for each product it tests. But this is a small increment on the development and marketing costs of a product. And all of those fees go directly to its five independent test labs worldwide. The Alliance's payroll and expenses are paid for solely through membership dues.

The quickie solution might be for IT managers to only issue RFPs (requests for proposal) that specify products which carry the Alliance lab's seal of approval. Unfortunately, that isn't always possible, or practical.

I asked Eaton to identify some of the areas that should alert you to a potential problem. Among them, he noted that there is a large number of Wi-Fi chip sets. Buying new access points, even from your usual vendor, doesn't guarantee interoperability with your current air cards because the latest model may be using a chip set from a different manufacturer.

The obvious advice here is to check which chipset is in your access points, but the Wi-Fi chipset market is in flux. Leading WLAN chip-makers include Broadcom, Atheros, and Intersil, which makes chips for the enterprise leader, Cisco. Intersil, however, has been bought by GlobespanVirata, along with another Wi-Fi chip-maker, Conexant. Other vendors include Intel, along with Agere, Airnet, Atmel, Cambridge Silicon Radio, Marvell, RF Micro and Texas Instruments.

Cost pressures - here's another believe it or not - actually force vendors to cut corners, which may lead to lower-quality products that don't stay on frequency as well as others. The 802.11g chip set is supposed to fall back to 802.11b if there are no other 802.11g devices available. That mechanism, what Eaton calls the "fall-back algorithm," is not implemented well by all vendors.

Security is complicated, and it takes a lot of work to implement it right. If not done correctly, WPA and WEP (Wired Equivalent Privacy) may work on your device but won't interoperate with other devices.

Problems of performance
How else do all of these interoperability problems manifest themselves? The symptoms are quite similar. Performance and throughput may be compromised. If a user roams between subnets or moves even one foot in another direction and associates with another access point, the session may be dropped. Power-save protocol failures may severely reduce a notebook's battery life, or sometimes the radio in the air card will shut down and won't wake up.

Yes, Eaton tells me, all Wi-Fi devices are built to certain industry specs and standards. But like any recipe, those standards are subject to interpretation. And the specifications are still evolving quickly.

Next year, things promise to get even more complicated when AES (Advanced Encryption Standard) 802.11i is ratified and implemented in devices - although AES is already there in some products.

Also in 2004, products will start to meet the 802.11h standard, designed to help 802.11b systems appeal to European radio regulators, by moving frequencies around to avoid military radar, and by limiting transmission power to just enough to close the link with the other node.

While you can never be sure of 100-percent compatibility, you can improve your chances by asking the right questions. Unfortunately those questions are complicated and keep getting more so.