The second scenario carries a bit more baggage and applies to those deployments experiencing medium to heavy usage of 802.11a. Syracuse University, a dual-radio, dual-band deployment, has about 30 percent of its clients using 802.11a radios. According to Xirrus, some of its education customers are evenly split between 802.11a and 802.11b/g. If an 802.11a-heavy organization swapped out its legacy dual-radio APs with new ones operating 2.4-GHz 802.11b/g and 5-GHz 802.11n, the 802.11a clients would default to 802.11b/g; this would result in a massive influx of usage, thereby reducing overall performance. No vendor has indicated that it will be offering a tri-radio AP (2.4 GHz for 802.11b/g, 5 GHz for 802.11a and 5 GHz for 802.11n) for these situations, so the most obvious solution would be a separate overlay of single-radio 802.11n APs operating in the 5-GHz range. Benefits include a single-radio 802.11n AP, which can be powered with existing 802.3af-based PoE and a lower unit cost. The legacy dual-radio APs can continue to operate indefinitely and can be phased out, given enough time.
Vendor differences in Wi-Fi architecture continue to become more nuanced. While the centralized model for enterprise Wi-Fi has been well received over the last few years, there's been a shift back to a more distributed architecture. As a result, it's now more important than ever to understand the placement of the management, control and data planes. It's my bet that the increased speeds of 802.11n combined with the growing pervasiveness of wireless will highlight the potential bottlenecks of a centralized architecture, if not in performance then in price.
The management plane controls the APs and any other WLAN elements. At a minimum, it includes applying a configuration on the AP and retrieving its status. Historically, this was one of problems of scaling the traditional fat AP deployment, and companies such as AirWave, Bluesocket, Vernier Networks and WaveLink stepped in to fill that void. But wireless switch vendors eventually subsumed them in market share.
The control plane oversees central security (the exchange of security keys between APs and controllers), roaming and RF control, among other things. Although the control plane is important, failure does not mean that clients are knocked off, but existing clients may lose the ability to roam and new clients may not be able to join.
The data plane is made up of end-user data targeted at other devices (such as servers) or locations (the Internet). It also involves QoS tagging, security enforcement (access control lists and firewalls) and packet forwarding. The data plane makes up the bulk of the traffic in a moderate to busy wireless network, but during idle times it may be dwarfed by a wireless network's 802.11n management traffic (not to be confused with the management plane).
