2. A Closer Look at "Interoperability"

Much has been made in the press in the past month or so about "interoperability problems" among draft-11g products. From what I've seen, however, this may be partly a "whisper campaign" to spread FUD to slow down the competition, and partly incorrect conclusions due to the use of inadequate test methodology.

Although you may be inclined to think "functional issues" when you hear "interoperability problems", my testing with draft-11g devices has shown throughput variation to be the dominant interoperability issue. In some cases, however, the interoperability effect on throughput is so severe that it can easily be mistaken for functional problems if simple file transfer or other techniques that look only at average or total throughput are used.

To really understand what these products are doing, you need to look at throughput performance over time, with the data preferably in graphic form. A simple example that reinforces this point is shown in Figure 1.

Figure 1: Throughput comparison - Linksys vs. Buffalo Tech - original
(click on the image for a full-sized view)

This Chariot test result, which you've seen before in Part 1of this NTK, shows the throughput performance of the first shipping Buffalo Tech and Linksys draft-11g products. Although the numerical average 21.8Mbps result for the Linksys is as indicative of the product's performance as the throughput plot, that's certainly not the case for the Buffalo Tech test pair.

You can see by looking at the lower trace in the plot that the time and duration of a measurement will make a significant difference in the results obtained. And once you start doing tests of draft-11g and 11b products running simultaneously in a mixed environment, looking at throughput vs. time is essential to understanding how the products under test are interacting.

I've also found it helpful to separate interoperability into three categories:

draft-11g and "Legacy" 802.11b

draft-11g and draft-11g (Vendor Interoperability)

I'll look at each of these categories in the next sections.