Distributed Antenna Systems: Remoting the AP

A distributed antenna system (DAS) is a vehicle for extending the coverage of primarily outdoor wireless services like cellular, private radio systems, public-safety radios, and the like. In-building penetration varies, of course, with frequency and power, but no radio signal originating (or terminating) outdoors is going to completely cover a large indoor space well, not even the almost-auctioned 700 MHz. stuff, which will regardless likely be the best yet in commercial use in the US. A DAS can function as a simple repeater, or can also be a complete infrastructure for indoor and outdoor wireless services of all forms - including signals intended to originate and terminate entirely within the building, like wireless LANs.

Wait a minute - does that make sense? If access points can already be distributed around a building, such installation being intended by their essential design, why would anyone connect them to a DAS? And if the DAS itself involves the distribution of antennas around a building (but you already knew that), then why would one substitute one form of installation for another? There doesn't appear to be any cost savings involved; installing either involves primarily labor and a little bit of low-cost wire. Looks like a bad idea.

And, I think in most cases it's unnecessary. I like the idea of putting APs in as cheaply as possible. Short of bolting them down for physical security (and that's really the only security that's at stake with today's APs), leaving APs essentially out in the open and simply plugging them into the nearest PoE RJ-45 should be all that's required in most cases. Easy installation, easy maintenance, easy monitoring, easy upgrades, easy everything.

But there are times when the DAS approach to WLAN provisioning can make sense. If one is paranoid about leaving critical infrastructure out in the open, then centralize the APs in a secured closet, and connect them to the DAS. DAS antennas are usually installed in the ceiling, so there can be a significant labor component to putting them in. But amortizing this expense across the cellular or other services that can be provisioned on this single infrastructure helps the economics somewhat here.

And there are some cases where the DAS approach may be the only one makes sense. Consider just three examples. In health care, some areas, like operating rooms, are sterile and must remain so. Equipment maintenance and the disruption of new installations must be kept to a minimum. So, install a DAS when the hospital is built or otherwise undergoing a major renovation where at least a few areas will be out of commission for a while, and off you go. Ditto for laboratory areas, in healthcare or not, where any disruption to operations or contamination with dirt of any form would be unacceptable.

Finally, there are high-security areas where non-cleared people just need to be kept out. I know, I know, why would anyone be using wireless in a high-security application? It happens. Encryption and authentication specifications beyond the already-very-effective AES and 802.1X exist, like FIPS 140-2. And encryption standards for classified communications are also in place (although, to be fair, I've not seen any classified use of commercial WLANs; there may in fact be some; I've just not seen it). Anyway, there are spaces where all but cleared staff need to avoid, and a DAS carrying WLAN traffic can work well in these.

So the DAS may see significant application beyond simply bringing cellular inside. And other applications, like building management, energy management, physical and other security, and, yes, provisioning WLANs, just might be the icing on the cake, eventually providing a significant boost to the market for DASes.

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