Wi-Fi and DASs: A good mix or oil and water?

* The yays and nays of piggybacking Wi-Fi onto your DAS

Enterprises frequently install equipment to boost indoor cellular service reception. While you will need a separate RF signal source for each licensed carrier's traffic you wish to amplify, the trend is for a single distributed antenna system (DAS) to support multiple broadband wireless signals - including Wi-Fi - in a single infrastructure. There are both pros and cons to adding Wi-Fi to your DAS.

On the pro side, DASs can be expensive, depending on your size and carrier clout. So the more mileage you can get out of the DAS, the more cost-justifiable it becomes.

One set of antennas installed all over the building accommodates all of your broadband signals. Generally, if using a DAS, you purchase your Wi-Fi access points (AP) without antennas, stick them in the wiring closet on each floor, and cable them directly to the local DAS equipment. Wi-Fi traffic rides alongside the cellular traffic across coax or copper cabling on each floor and up and down fiber risers between floors. It also shares the distributed antennas with the cellular traffic to send and receive signals.

This might be cost effective from an antenna standpoint and attractive from an AP-management standpoint. Still, there are some potential issues to pay attention to:

* DASs are installed primarily to improve cellular service. So the site survey is generally conducted for cellular optimization with Wi-Fi as an afterthought. If the antennas don’t serve your Wi-Fi coverage needs, you might still need to put some APs out in the ceiling.

* Because DASs are optimized for maximum cellular coverage, they might be omnidirectional. But there may be places where different types of antennas are more Wi-Fi-appropriate. For example, you might want to use Yagi antennas for focusing the signal energy down a long narrow space, such as a hallway, and to keep it from seeping through to another office building or to outdoors.

* It’s not clear at this point how DASs accommodate 802.11n, which uses at least three antennas on each AP. If your 11n traffic is looking for three antennas and you have just one, you are likely to forfeit the throughput benefits of 11n.

Copyright © 2008 IDG Communications, Inc.

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