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Networking
Distributed Antenna Systems
11/1/2007
DAS Boot!
Move to distributed antenna systems and give your campus a unified system for public safety wireless, cellular, and WiFi coverage.
THE RESIDENTIAL SERVICES group is lobbying to
dump its aging hard-wired phone system because students
don't use it. The town and campus public safety officials are
demanding that their portable two-way radios operate well
not only outdoors, but within campus buildings. Your students
are now expecting text messaging and WiFi service to
work everywhere on campus. Faculty members are grumbling
that their chosen wireless provider's coverage is good
at home, but poor on campus. And the CFO wants to know
why her BlackBerry doesn't work in the tunnel between the
main administration building and the annex. Welcome to the
wide world of wireless wants and wishes.
Thankfully, it is possible to satisfy such diverse requests for unwired service. The technology required? A distributed antenna system, or DAS.
Was Ist DAS?
In essence, a DAS can connect to a variety of wireless services and then rebroadcast those signals throughout the areas in which the DAS is installed. The sources can include cellular service from multiple wireless carriers, public safety radio frequencies, and WiFi. If your campus has a medical clinic or hospital, the special medical wireless systems it uses also can be supported.
To better grasp how a DAS operates, it helps to know some of the methods by which wireless signals are propagated. Nearly all of us know what a cell tower looks like; well, each of those towers carries antennas for one or more macrocells (multiple carriers when there are “layers” of antenna arrays). But a microcell covering a limited area may have its antenna or antennas placed on a tower or pole, or mounted on or in a building. Wireless carriers use microcells to add capacity in areas with a high density of mobile wireless device users. A picocell has an even smaller coverage area. A WiFi access point can be thought of as one type of picocell. In a DAS setup, any or all of these technologies may come into play.
There are also multiple alternative methods for obtaining the necessary signals from the carriers, including T1s or fiber cabling directly connected to wireless carriers' networks. However, the wireless signal from an existing wireless tower (macrocell) is one of the most common sources of obtaining the service “injection.” The signal of a public safety radio system (e.g., from local or campus police) also would be taken from the air via the nearest repeater for that system.
Now that we know we're “inserting” signals, the next logical step is to think about what we're doing with them. Every DAS has a “head end” into which these source signals are combined for distribution.
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