Configure access points for capacity, not access
Noblet urges network administrators to configure access points for performance (or capacity), rather than for access. He's found some access points are configured without any limits on the number of client associations. If a large group of users coalesce around an access point, they'll find slow associations or none at all. "What it's really about is understanding the throughput performance of a particular data stream," Noblet says.
But everyone agrees that capacity planning at the level of the access point is more art than science. "When I speak on this topic, I always emphasize that we, the IT professionals, not the vendors are the ones who best understand the user and application scenarios we'll be dealing with in our deployments," says Dan McCarriar, assistant director of network services at Carnegie Mellon University (CMU).
CMU is about halfway through an 802.11n deployment using gear from two vendors, Aruba for academic areas and Xirrus for residence halls. Xirrus packs a WLAN controller along with four, eight or 16 Wi-Fi radios into a single oversized "smoke detector" like package, called an array, with sectorized antennas to prevent interference. The result lets CMU plug a single array into a high-density area, without having to do complex microcell planning and administration.
Keeping up with DHCP
In some cases, DHCP servers can't keep up with a flood of clients. "We're definitely seeing this," says Turner. But CMU's DHCP servers are able to keep pace. The key is designing the centralized IT infrastructure for these kinds of services, which are used by both wired and wireless clients, so it can scale quickly and easily.
In the future, Turner plans to create a more seamless mobile experience across the campus by tying location and mobility services into DHCP. "The DHCP server is not aware that someone has disconnected," he says. "We might be able to do something between the central WLAN controller and DHCP so we’re not holding addresses for people who are never coming back."
The University of Tennessee at Knoxville has run into a slightly different DHCP problem, says Hanset, from the school's network services group: Some returning student notebook PCs or "rogue" access points in dorms act as DHCP servers themselves, serving out useless DHCP leases to requesting clients. The school blocks these hosts at switch ports or the Aruba WLAN controller.