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Designing 'iPad WLANs' poses new, renewed challenges

John Cox | March 26, 2012
Complications that the influx of Apple iPads and iPhones bring to enterprise Wi-Fi networks and wireless LAN administrators are illustrated vividly at The Ottawa Hospital in Ontario.

Second, the greater number of access points can handle the growing client population. Enterprise users increasingly have two or more mobile devices per person. "If we have more devices in the same space [than planned], I need additional APs to support them," Lukaszewski says. Aruba and most other WLAN vendors have various techniques to distribute devices across other access points.

The real limiting factor is "how much data you can actually put on a wireless channel," Lukaszewski says. "If you slice this 'pie' too thin, they won't get the throughput they need...Customers are used to seeing '300Mbps' on access point product literature. But these mobile devices don't run close to that."

Mobile devices today typically will have an 802.11n radio that supports one data stream and has one antenna, and can only use the conventional 20-MHz wide channel (802.11n gains a lot of throughput by bonding two of these into a "fatter" 40 MHz channel, and using two or more antennas). "That means the highest data rate they can achieve is 65Mbps, the maximum single-stream rate without bonding two channels together," Lukaszewski says. And actual throughput will be even less: in the area of 40Mbps.

"If I can get 40Mbps of throughput on the access point, and I want an average of 1Mbps per device, I can only have 40 devices" on one access point, Lukaszewski says.

Channel challenges

Just when enterprises have finally begun aggressively embracing the open 5-GHz band, first netbooks, and then smartphones and other portable devices, arrived with radios that run only in the 2.4 GHz band. Besides being crowded with devices, the 2.4 GHz band has only three non-overlapping Wi-Fi channels (tablets, like the iPad, run also on the 5 GHz band, which can have up to 21). With lots of access points in a small area, avoiding channel overlap can be tricky.

"These devices are technologically taking us back about two years from a spectrum management perspective while at the same time we're adding many, many more devices into the environment," Lukaszewski says.

Aruba now routinely recommends that in high-use areas like lecture halls, stadiums and the like that bonded 40-MHz channels simply be disabled, and more access points be packed in.

Roaming

Roaming between access points for mobile devices is still a problem, though not as bad as it was just a few years ago. "In the past, early mobile devices had very poor roaming algorithms," Lukaszewski says. Once connected to an access point, many devices simply clung to it, even when the user had moved to another location with a nearby access point that had had a much better signal.

"Apple devices used to be very challenged in this regard though they've made some improvements," Lukaszewski says. "Their driver behavior is better but it's still nowhere near what we see in, for example, voice-over-IP Wi-Fi devices."

 

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