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 let you 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 wireless LAN (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 an RF site survey 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 WAPs run into RF interference from another nearby, then it is very likely that simply selecting a different channel will mitigate the problem. A hospital, on the other hand, 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 that 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 an educated guess at best. 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 ping 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 there is not yet a site to survey? If the building is still in the blue-line stage, you cannot slowly walk the floors of the facility with laptop or PDA in hand. Now what? Well, this would still be part of “structured cabling,” so let us check with TIA.
Grid-based structured cabling
TIA TR-42, the TIA/EIA Engineering Committee responsible for User Premises Telecommunications Cabling Infrastructure, does not currently address “wiring for wireless” in its family of standards. But that is changing.
TR-42 is presently working on a Telecommunications Systems Bulletin, or TSB, entitled “Telecommunications Cabling Guidelines for Wireless Access Points,” addressing an unshielded twisted-pair (UTP) cabling plan for WLAN.
So far, the TSB is shaping up to be a “North American” version of ISO/IEC Technical Report 24707:2004(E) Information Technology-Customer Premises Cabling for Wireless Access Points, which was published in July.
A word of caution 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 distance 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.
At this very early stage in the TSB’s development, it appears that the group may be more certain about what they will not address than what they will address.
The current draft specifically states that it does not address the influence of building construction materials (such as concrete, sheetrock, wood, and steel); or, the effects of different building configurations (closed, semi-closed, or open space); or, what happens when building furnishings (cabinets, partitions, furniture) are added or moved within a building. Also on the “beyond the scope” list is co-existence of different RF applications. Read that as “anything other than 802.11.”
My review of TIA’s current draft left me asking, “Why are they doing this, when there is already an ISO Technical Report published that addresses this exact same topic?”
So, I spoke with Masood Shariff of SYSTIMAX Solutions (a CommScope company), who is the TR-42-appointed project coordinator for this TSB. I asked him my “why?” question. Shariff explained that his TIA group is using the ISO Technical Report as its base document, but that it plans to add details that are not in the ISO Technical Report.
So far, the only evidence of “additional details” is in the form of a series of graphics depicting various mounting scenarios for the WAPs and the antennas, which include “Typical Ceiling Mount Installation,” “Typical Wall Mount Installation,” and “Typical In-the-Grid Ceiling Mount Installation.”
Noticeably absent are any situations using optical-fiber cabling or a telecommunications enclosure (TE), or any indication that you can simply locate the WAPs in the telecommunications room (TR) or TE and install longer antenna cables. If you are a manufacturer of any of these pieces of equipment, and would like this scenario to change, I suggest you contact TIA about participating in this effort.
And then there's the grid
The draft instructs the designer to overlay a 55 x 55-foot square grid over the building and to install at least one UTP cable to the center of each square if less than 20 users are expected, and install additional cables if more users are expected.
While both the ISO and the TIA documents are using a 39-foot (12-meter) circle for depicting where the WAP can be placed using the telecommunications outlet in the center of the circle, it is the grid shape that actually determines the density of the outlet locations to be installed.
TIA has chosen to use squares to form its grid. 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,” page 8).
Yes, regardless of how much cabling you have installed, you still have to locate the WAPs and the antennas to bring the magic of “wireless LAN” to life.
DONNA BALLASTis BICSI’s standards representative, and a BICSI registered communications distribution designer (RCDD). Send your questions to Donna via e-mail: [email protected]
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. In the movie, eight players from the notorious 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.