Legacy Equipment And 802.11n

By Frank Bulk

Organizations apprehensive about 802.11n have pointed to the lack of IEEE ratification and interoperability with legacy devices as two significant concerns. While the first concern won't likely be satisfactorily addressed for another year, thankfully the IEEE has made interoperability a key component of the IEEE 802.11n draft. But it's one thing for engineers to agree to something on paper; the rubber doesn't really hit the road until the devices are installed into production systems and put into everyday use.

Without completely stealing my colleagues' thunder, let me just say that Network Computing RealWorld labs already identified an issue between a leading Wi-Fi client vendor and equally estimable AP vendor. It was discovered when comparing the coverage and performance between the client vendor's 802.11n client and their legacy client. Essentially, the 802.11n client and 802.11n AP stopped passing traffic between each other, even though both sides perceived they were associated. Fortunately, the AP vendor was able to reproduce the problem and provide us a patch.

One of my colleagues made this point: What does it mean for these products to have Wi-Fi Alliance 802.11n draft 2.0 certification if a test that 'basic' fails? It wasn't as if it was a sophisticated or artificially construed test requiring lots of measuring equipment. If an interoperability issue was found with a simple test, how many more flaws will be found but in only more complex scenarios, and how many more will go on undiscovered, leading to underperforming wireless networks and esoteric issues that the help desk will attribute to "user error" or the "black science of RF"?

I'm tempted to point my finger at the Wi-Fi Alliance, but I have to admit that they're somewhat off the hook. The alliance has never promised perfect interoperability, and it never will. This vendor-driven body doesn't test products against each element of the relevant IEEE standard and is just as much a marketing arm for the vendors and tool to instill confidence into consumers' buying decisions. So the Wi-Fi Alliance has been careful to limit its interoperability claims and promises.

"The test bed for Wi-Fi CERTIFIED 802.11n draft 2.0 incorporates legacy devices for verifying backwards interoperability of the device under test. Legacy devices, 802.11a/b/g, also can be tested in the 802.11n draft 2.0 test bed to verify forward compatibility with 802.11n. The forward compatibility testing isn't required and can be performed at the vendor's discretion."

Basically, the Wi-Fi Alliance will do some backwards interoperability testing, but it's not exhaustive and will not be re-testing all the previously certified legacy gear against the new 802.11n equipment. In fact, it's not even promising to use all the testbed equipment it used for its legacy (802.11a, b, or g) certifications in the 802.11n draft 2.0 certification tests.

Even though I wish the Wi-Fi Alliance performed more thorough and transparent testing, it's unlikely to happen. The alliance always has been about basic interoperability testing, and the most enterprise buyers could hope for is a higher-grade, perhaps "enterprise-grade," certification. In the meantime, it will be up to customers in the field and vendors in their QA labs to discover the incompatibilities and deal with them as they arise.