We're trying to determine a WLAN direction... (i.e. 11a or 11g.) However, as some companies are building dual radio devices - isn't it likely that we will see APs with 11g and 11a in them thus supporting all client cards?
Dual-band devices are definitely on the rise. The real question is whether to use dual-band APs or dual-band cards or both.
Dual-band cards are currently available for 802.11 A+B, but 802.11 A+B+G cards have been announced and will probably be shipping by the time you read this. The advantage of dual-band cards is that stations can adapt to use the nearest AP - somewhat analogous to the way that dual-band phones roam seamlessly between higher-speed PCS and lower-speed DAMPS cellular networks. In a heterogeneous wireless world where users may encounter 802.11a at work, 802.11b at Starbucks and 802.11g at home, dual-band cards give users maximum flexibility to work within any network. Caveats include higher prices, increased power consumption, and space for 32-bit cardbus adapters, but I think these limitations will fade as technology matures.
Dual-band APs are available for 802.11 A+B now, and it won't be long before APs ship with integrated A+B+G or have N slots that support any combo of single-band cards. As you point out, dual-band APs have the advantage of handling any kind of station. However, A and B/G footprints are not usually the same because attenuation is greater at 5 GHz than 2.4 GHz. Optimium 802.11a AP placement may differ from 802.11b AP placement. Because A has more channels than B, it is also possible to put more A devices in one spot. As a result, you may prefer to build two independent WLANs, deploying B/G APs everywhere for blanket coverage plus A in key spots that demand high-density / high-bandwidth coverage. You could build an A+B+G network out of dual-band APs (particularly those with swappable cards), but you could also do so with a combination of single-band APs.
Note: 802.11g devices are required to interoperate with 802.11b, so any 802.11g AP or card is really a single-band 2.4 GHz B+G device.
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