Another scaling issue is that once clients are issued IP addresses, they may end up keeping them far longer than needed, so they can't be reissued to newly arriving clients on the same subnet. In some cases, addresses can be exhausted.
At the University of Tennessee, these address leases are limited to two hours, at Brandeis, to just 30 minutes. Both institutions make use of Aruba's virtual LAN pooling, which associates a pool of addresses to a given VLAN. It's an efficient and effective tool, says Turner, but network administrators still "have to think carefully about this."
Carnegie Mellon currently has a flat WLAN, essentially configured as one campuswide subnet, with one large pool of addresses. The university is planning to carefully segment the network, probably into several geographical zones, once the 11n rollout is completed. Then, address exhaustion could become an issue if not properly managed and monitored, says Scott Ambrose, CMU's manager of network design and development.
Ambrose plans to collect a mass of statistics on such things as average number of devices on the network and peak numbers of users, and the locations of the access points they associated with. All that data will go into planning the size and number of zones, and how to allocate the available IP addresses to each one.
Scaling for multimedia
Multimedia use is surging, and 802.11n is expected to make it surge still more. All these universities are configuring their wireless LANS for multicast support, to minimize bandwidth demands where possible. Users in effect tune into a single multicast stream (analogous to viewing broadcast TV) rather than each one receiving his or her own separate, unicast stream. "You have to look at your application and ask 'what am I trying to serve here?' " Noblet says. "That will dictate whether you have to make use of a unicast or multicast transport."
"We enabled multicast everywhere we can," says the University of Tennessee's Hanset. But with that, to further improve performance, the university also disabled the slowest WLAN data transfer rates, of 1M and 2Mbps. "So every broadcast packet is sent at 5.5Mbps," he says.
Test everything, says Carnegie's McCarriar. "Support for multicasting [by vendors] is all over the map," he cautions.