As part of our ongoing efforts to bring you practical information on the development of wireless local area networks, we decided to revisit Donna Ballast’s previously published observations on the importance of conducting a site survey. Cabling Installation & Maintenance will continue to cover wireless technologies, including site surveys and standards, in the coming months. Visit www.cable-install.com to view the Webcast, “Wireless LANs and the wires behind them,” which addresses site surveys as well as TIA’s TSB-162 and other WLAN issues.-Ed.
A wireless infrastructure can simultaneously support cellular voice, 802.11 voice and data, pager, and video services offered by both enterprise and commercial wireless service providers.
Current wisdom offers two different schools of thought on the best way to design an 802.11 voice and data network-radio frequency (RF) modeling based on a site survey, and a grid-based structured cabling plan.
RF modeling
Even when using omnidirectional antennas, radio waves don’t really travel the same distance in all directions. Things like walls, windows, elevator and utility shafts, and even people, get in the way. Each of these obstructions attenuates the radio waves differently, causing irregular and sometimes unpredictable antenna radiation patterns. Hence, you need to survey your “LANdscape” for wireless applications.
The ultimate goal of an RF site survey is to determine the number and placement of wireless access points (WAPs) to offer ample coverage throughout the site, and to detect any interference-competing RF signals in the same band as your WLAN equipment. Sources of this interference include not only things like microwaves and cordless telephones, but also other WLAN equipment on other networks in the same building.
How complicated this is to perform depends on the size of the WLAN. A small three-room doctor’s office may not require anything beyond locating an available port on the switch and plugging it in. If the WAP runs into RF interference from another nearby, then it is very likely that simply selecting a different channel will mitigate the problem. But a hospital would require an extensive RF site survey.
Basic site survey steps
Step one: Locate or draw a set of floor plans. Then walk the site to verify the floor plans are accurate. Make notes on the floor plans describing any potential barriers that may affect the propagation of RF signals. Also note where the users will likely collect and how far they are likely to roam within this area.
Step two: Using the floor plan and your notes, and what you know about the operational range of the WLAN products you are using, determine the preliminary WAP locations, as well as antenna type and orientation. At this point, this is, at best, an educated guess. You will need to plan for some propagation overlap among adjacent WAPs, but not too close. Channel assignments for WAPs will need to be far enough apart to avoid interference with each other.
Step three: Temporarily install a WAP at each preliminary location. No need to connect the WAP to the cabling system; the tests are only going to pin the WAP. No data cable, however, also means no line powering of the WAP, so local powering will be required. Remember to bring along a few extension cords.
And now you test. Many WLAN vendors provide free RF site survey tools that identify the associated WAP, signal strength, signal-to-noise ratio, and connection speed. You can load this software on a laptop or personal digital assistant (PDA) and test the coverage of each preliminary WAP location. Or, you could use a handheld site survey tool. But these are definitely not free.
Survey the site by walking varying distances away from the WAP. Note the readings at different points as you move to the outer boundaries of the WAP coverage area. In a multi-floor building, you will need to perform the same tests on the floor above and below the WAP.
Step four: Based on your test results, relocate and retest until you cover your intended area.
Step five: Install the cabling; connect the WAPs, and you are ready for the users.
But what if the building is still in the blue-line stage? You cannot slowly walk the floors of the facility with a laptop or PDA in hand. Now what?
TIA’s Telecommunications Systems Bulletin, TSB-162 Telecommunications Cabling Guidelines for Wireless Access Points, addresses an unshielded twisted-pair cabling plan for WLAN.
A word of caution, however, to those of you who design centralized cabling systems that tend to push the channel-length limits: these are not your normal TIA/EIA-568-B.1 links.
The wireless (cabling) link length is limited to a maximum of 262 feet. This limitation is to allow for a maximum 39-foot (12-meter) solid-conductor patch cord between the telecommunications outlet in the center of each grid cell and the WAP at the end of the patch cord.
Grid shape factoring
It is the grid’s shape that actually determines the density of the outlet locations to be installed. TIA has chosen to form its grid using squares. ISO uses hexagons. Despite the fact that it is simpler to draw a grid of squares than a honeycomb of hexagons, using squares creates additional overlapping between the grid cells. This tighter package of the circles will translate to more cables being required to cover a given area.
OK, so now the building construction is complete. You have a lot of “wire for wireless” in the facility. Now it is time to go back to the basics (see “Basic site survey steps,” above).
Yes, regardless of how much cabling you have installed, you will have to locate the WAPs and the antennas to bring the magic of “wireless LAN” to life.
A point to ponder
As I was thinking about this column, a phrase from a Kevin Costner film came to me. Field of Dreams is a film about Shoeless Joe Jackson and the 1919 Black Sox scandal-an appropriate reference with the Chicago White Sox winning the World Series in 2005. In the movie, eight players from the notorious 1919 team came back as “ghost” players to redeem themselves and play baseball together once more. Throughout the film, Costner’s character is told, “If you build it, they will come!”
OK, but what happens if you don’t “build it”? If the “it” is a wireless network, then “they” (the users) will very likely build it themselves-leaving you to manage “their” exception. Don’t think it can happen? Well, did you think the Red Sox and White Sox would win back-to-back World Series championships?
DONNA BALLAST is BICSI’s standards representative, and a BICSI registered communications distribution designer (RCDD). Send your questions to Donna via e-mail: [email protected]. Chief Editor PATRICK McLAUGHLIN also contributed to this article.