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Designing 'iPad WLANs' poses new, renewed challenges

Numbers, use patterns, channels complicate Wi-Fi networks

By , Network World
March 26, 2012 12:07 AM ET

Network World - 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.

The hospital decided in 2010 to deploy 3,000 iPad tablets to doctors, internists, and pharmacists in support of strategic patient care applications. All data was going to be streamed to the iPads, and the Apple tablets only communicated over Wi-Fi. "Therefore, the [wireless] network became quite important," says CIO Dale Potter. "We invested a mound of money in the network [in summer 2011]."

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The hospital picked Aruba Networks to design and deploy a Wi-Fi network that currently numbers nearly 1,600 802.11n access points. "No one could answer me when I asked 'what will happen when we scale to 3,000 iPads by year-end, and eventually to about the same number of other iOS devices?'" Potter recalls.

Aruba CEO Dominic Orr called in the Aruba Customer Engineering (ACE) team, a small, elite consulting group that among other things handles the toughest, most baffling challenges that the vendor encounters in enterprise deployments.

Based on their experience over the past couple of years with enterprise WLANS, ACE has identified a range of issues that these networks face with the surge in iOS and Android mobile devices, according to Chuck Lukaszewski, senior director of Aruba's professional services group, and the ACE team. Currently, iPhones and iPads overall remain the most popular brands of new smartphones and tablets in the enterprise.

"We're seeing extremely rapid increases in the aggregate number of mobile devices on customer networks, across all sizes of organizations," says Lukaszewski. Especially in higher education, numbers are doubling or tripling from year to year.

The issues confronting IT groups include changes in RF design and the appropriate level of access point density; poor device roaming; and new and different loads on the WLAN's control path - the demand for authentication services, for IP addresses, license management, multicast and broadcast traffic spikes.

RF design, density

For years, an industry rule of thumb was to deploy one access point for every 3,600 to 5,000 square feet for data networks, and every 2,500 square feet for VoIP. ACE routinely now goes with the higher density for mobile device deployments.

There are two key benefits, Lukaszewski says. One is better Wi-Fi signal levels. Virtually all smartphones and tablets with 802.11n radios support only one data stream, and use one antenna. As a result they cannot make use of techniques like Maximal Ratio Combining (MRC), which can exploit multiple streams and antennas to boost the signal to noise ratio on the radio link.

Many mobile Wi-Fi radios are actually quite powerful in terms of transmit power. The iPhone 4S and iPad, and many other devcies, use Broadcom's BCM4329 chipset, which "easily rivals or exceeds the power of many laptop chipsets," Lukaszewski says. The "weakness" lies in the receiver's more limited capacity to process inbound frames and the inability to fall back on MRC to compensate if the signal is impaired. "They're more vulnerable to frame loss than a multi-antenna device," he says. "So keeping the signal levels high compensates for this."

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