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Distributed antenna systems and WLAN: A network management burden

Shamus McGillicuddy

As smartphones become the preferred mode of communications for enterprise users and consumers alike, network managers will find an increasing need to boost cellular coverage in office buildings and corporate campuses.

In fact, some network managers will find themselves managing at least two separate wireless networks in order to serve users adequately: a wireless LAN and an in-building cellular

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Last week, Yankee Group published research which revealed that 41% of consumers will choose a smartphone as their next mobile phone purchase and that 38% of all handsets sales will be smartphones by 2013. This growth in smartphone sales will lead to more bandwidth consumption as people increasingly use their phones for data services as well as voice.

Duke University, which has a Cisco-based 802.11n wireless LAN deployed throughout its campus, recognized that it also needed to build a distributed antenna system (DAS) to boost cellular service.

Bob Johnson, senior director of communications infrastructure at Duke, said the university had recognized that very few people were using landlines on campus anymore. Cellular communication was becoming critical, not only for faculty and staff with corporate cell phones and smartphones but also for students with personal devices. Duke recognized that when tragedies such as the 2007 Virginia Tech shooting happen, a school needs to be able to communicate with students and staff, Johnson said. In order to guarantee emergency communications, augmented cellular coverage is essential.

Duke had some existing InterReach Fusion in-building DAS systems from ADC already deployed in 20 of its 220 buildings, particularly in Duke University Hospital. Johnson considered adding more in-building systems, but the cost of deploying one in each building on campus was prohibitive. Instead, he decided on a hybrid approach, installing five FlexWave outdoor DAS radio heads to cover different areas of the campus. Now, he's evaluating the effect this has on cellular coverage before adding more.

"We did a survey of areas that were not covered very well by the carriers and identified spots where we felt we could put these outdoor distributed antenna systems and get the biggest bang for our buck," Johnson said.

Mobile carriers funded about 65% of the project, he said. "But as we required our system to be 'carrier agnostic,' Duke funded a portion of this as well."

With the DAS system and the school's Cisco-based 802.11n wireless LAN, Johnson's organization is now supporting two separate wireless networks on the campus.

"Obviously, we don't want to be in the position to have to support two networks over a long period of time," Johnson said. Networking professionals would love to see wireless LAN vendors produce access points that propagate cellular signals, he added.

Such converged access points aren't on the market today, but some enterprises do converge wireless LAN onto distributed antenna systems by placing access points in data centers or wiring closets and connecting them to DAS antennas. However, DAS vendors run into problems with 802.11n, the newest generation of wireless LAN technology.

"The idea of doing multiple wireless networks on the same DAS is not uncommon," said Michael Finneran, principal of dBrn Associates. "Cellular is going to be in there. Hospitals are putting pagers on there. The big problem they're facing now is the DAS guys don't seem to have mastered how to do 802.11n over DAS because you need a much smarter antenna head. [With 802.11n] you're using MIMO, so you're getting multiple independent signals coming in from each station, and basically you need to be able to distinguish between them. You have to have a processor and you have to have power, and you need to know which version of MIMO you're going to implement."

Ken Biba, founder and CTO of research firm Novarum, said some system integrators are creating converged wireless installations that combine in-building cellular systems with wireless LAN technology. He said Richardson, Texas-based InnerWireless sells converged wireless offerings that combine wireless LAN, in-building cellular and RFID technologies. It resells technology from ADC and Cisco to deliver these capabilities. And the company does claim to be able to converge 802.11n with cellular-based DAS technology.

"In enterprises where there are multiple kinds of wireless involved, not just Wi-Fi or just cellular, you really do have to look at a distributed antenna system," Biba said. "Rather than running multiple sets of wires, you have one common infrastructure."

Paul DeBeasi, senior analyst with Burton Group, said the fledgling femtocell industry might some day also be an option for consolidating wireless infrastructure. Femtocells are access points for cellular reception that can be installed in a home or office and backhaul across the Internet. Carriers are primarily aiming this technology at consumers in rural areas who want boosted cellular service in their homes. But DeBeasi said it is possible that femtocell manufacturers will integrate wireless LAN technology into the boxes as well.

"You could integrate a femtocell into Wi-Fi and have a single device in one enclosure and have an 802.11n radio and a femtocell cellular radio operating inside, and all the backhaul would be over the same wired infrastructure," DeBeasi said. "No one has anything like that today, and it would be difficult because the Wi-Fi portion of the wireless spectrum is typically controlled by the enterprise, and the cellular portion is controlled by Verizon or AT&T. Enterprises do not want to give up control of their wireless LAN. I think it would end up being a niche market because it's complicated."

Let us know what you think about the story; email: Shamus McGillicuddy, News Editor


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