A distributed antenna system (DAS) is a way to deal with isolated spots of poor coverage inside a large building by installing a network of relatively small antennas throughout the building to serve as repeaters.
The antennas are physically connected to a central controller which is connected to the wireless carrier network’s base station. Because these systems operate on RF spectrum licensed to wireless carriers, an enterprise cannot undertake a DAS deployment on its own without involving at least one carrier.
Distributed antenna systems can be passive or active. A passive DAS grabs cellphone signals from antennas on the roof and runs them through leaky feeder cables throughout the building. In this approach, the signal leakage distributes the signal. In an active system, the signal is passed from roof antennas through fiber cables. Along the way, systems boost and amplify signals as needed.
Deployment is the most expensive stage of a DAS project because installing antennas and stringing fiber optical or coaxial cable between antenna modules and the controller are all very labor intensive processes. Generally, the carrier bears the costs of installing the system as well as maintenance expenses and many times, the carrier will only agree to take on these costs if the deployment fits within their network plans, covers a large number of subscribers or fills a significant gap in service. To keep the cost down, a DAS may be shared by multiple carriers.
Distributed antenna systems are transparent to mobile devices, providing both voice and data services to mobile devices just like any tower on a cellular network. Densely populated indoor spaces such as shopping malls, medical centers and high-rise buildings are all good candidates for DAS deployments.
While building their new stadium, the NFL’s Arizona Cardinals turned to a distributed antenna system (DAS) to simplify the various wireless networks they had to maintain while making sure the system was future-proof and robust enough to carry the thousands of calls fans would make on their cell phones during Super Bowl XLII.
Distributed antenna systems consolidate all the wireless connections a building will need — such as cellular, emergency bands and Wi-Fi — into a few centralized locations and then route the signals from those connections through a single wireless antenna infrastructure that is deployed throughout the building.
Mark Feller, vice president of technology for the Arizona Cardinals, said the franchise began investigating DAS well before stadium construction began.
“We knew we would need to accommodate a wide variety of wireless services, so we started looking at DAS four or five years ago,” Feller said. “It wasn’t a very common thing.”
But the alternative — maintaining separate wireless systems and implanting antennas for each cellular company, for Wi-Fi, for two-way paging networks – was a daunting one, particularly if every new technology required a new antenna deployment. Eventually, the Cardinals went with Mobile Access’ Universal Wireless Network.
“In concept, [DAS] has been around for a long time, but the idea is to implement as simple a technology as possible and move signals indoors from the outside,” said Craig Mathias, principal of Farpoint Group, who thinks the technology is primed for growth.
This means that to add a new wireless service, for example, only one wireless connection needs to be added at the head end for the entire building to get the benefits, as opposed to installing new antennas throughout the building for each wireless network.
DAS gets heavy consideration during the construction of large enterprise headquarters or public buildings, particularly when those buildings have poor cellular signal strength and will need to build out multiple types of wireless networks.
From the networking end, Mathias said, the biggest changes were the physical installation and maintenance, with APs all located in closets as opposed to installed in the ceiling or on walls. Most wireless management tasks would remain unchanged.
Most DAS systems are fully compatible with a wide variety of wireless vendors, and many of the costs associated with deploying a wireless LAN remain: The same number of access points must be deployed, for example, and they will still interact with a central controller in the same way. But instead of being installed in ceilings or walls, they are set in central closets that then disperse the Wi-Fi signal through the antennas of the DAS.
“It’s really a great system to manage,” Feller said. “Our intentions all along were to use Cisco products, and their system works hand in glove with Cisco access points and the Cisco management system.”
Mathias said not every organization with a DAS would want to integrate its Wi-Fi network into it. In situations where the physical security of an AP is not a major issue, it may be simpler and quicker to continue to deploy APs on top of cubicles, for example, rather than maintain them in secure closets.
Florida Hospital, another Mobile Access DAS customer, has decided against integrating its Cisco wireless LAN, partially because that LAN is already widely deployed.
“Does it make enough sense for us to jerk out existing, functioning Wi-Fi components? No,” said Todd Frantz, associate chief technology officer for the hospital. “We’re not about to start pulling out access points from the wall and start pumping them through the DAS for the intellectual purity of it.”
As Florida Hospital continues to deploy the DAS at its various campuses, they continue to keep an eye on the potential benefits of integration, Frantz said. “There are some places where it seems to make some really good sense, especially when you’re doing newer construction,” he said.
Mathias said the ultimate driver for DAS adoption was spotty cellular access. But networking professionals should still be aware of the technology, particularly in settings where physical security is paramount. The centralized architecture of a DAS helps limit contractor access to sensitive areas.
The two biggest concerns in implementation are finding local service providers that will provide cellular access to the DAS and making sure that current wireless LAN infrastructure is compatible with the DAS system chosen.
The former problem could actually be the trickier of the two.
“One of the things I tell other customers is to engage with cell customers early and often,” Frantz said, adding that his hospital now has a good working relationship with local providers.
“These systems are wonderful to get installed, and now you realize you have to convince the cell phone companies to connect, which is an art unto itself.”
