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Recommended Facility Documentation

Any engineer surveying and installing a WLAN system needs facility documentation, the collection of which should be part of the initial design stage. Facility documentation enables the engineers defining the WLAN, or trying to survey and install it, to identify areas of concern, areas of coverage, user densities, and even types of antennas to consider, before ever walking onto the site. After the survey has been completed, the facility documentation becomes a critical part of the overall documentation and should be kept current as to any changes and maintained with the rest of the network documentation for future reference. The facility documentation should include (but is not to limited to) the following:

• Site map (or floor plans)

• Building construction details

• Building contents inventory

• User-area and user-density information

• Problem-area (for WLAN) information

Site Map

Before beginning any site survey, obtain a good site map or floor plan of the site. In some cases, the site map might not really be a floor plan, but more of a site layout, including building layout and contents as well any outdoor areas that will be covered.

This site layout document will become part of the final survey and network documentation. As such, having a soft copy of this document is very helpful so that it can be copied and distributed as necessary to the survey engineers, installation team, and network support staff. Extra copies of the site map should be available to make notes on during the actual survey and installation steps, as well as during any presurvey discussions.

Prior to the survey, you can use the site map document to define desired coverage areas; identify where coverage is not needed; and define user locations and densities, problems areas, network closets, cable runs, and plenum areas. Basically, this document becomes the physical schematic of the wireless network (see Figure 8-1).

Building Construction

RF to react in completely different ways. Figure 8-2 shows examples of various building materials.

A multifloor building might use precast, reinforced concrete for flooring. Although this type of construction might create some attenuation problems for RF, the effect on RF penetration is significantly less than it would be with a floor of poured concrete over a steel pan. Although some RF may get through in this latter example, the steel pan provides a very good RF shield between floors (see Figure 8-3).

Walls can be similarly deceiving. In most industrial buildings, it is common to use steel studs with drywall or plasterboard over them. The drywall and plasterboard cause only a slight attenuation of RF signals, and the placement of the steel studs has little effect at all. Other walls might be concrete block, with or without steel reinforcement, which cause only limited attenuation of RF. However, precast concrete, typically using steel reinforcement, is a different story. The amount of steel used for reinforcement inside the concrete will cause the RF attenuation to vary from one building to another.

Although drywall and plaster usually minimally affect RF, the material behind the wall can pose problems. Consider a real example from a health-care facility. The RF energy was having a hard time getting into several offices. Further questioning of maintenance personnel at the facility and reviewing some older building documents revealed that this area had been remodeled recently. Before the area was used as offices, it was the radiology department. The x-ray room had been turned into offices. And, as typical with x-ray rooms, the walls were shielded to prevent x-ray energy from leaking out of the room. The walls were not removed, just covered over; therefore, the RF could not get into the offices.

In some buildings, the walls might be made from a form of reinforced wire mesh, with a plaster-type material spread across it (often called stucco). The mesh can work much like an RF screen, causing a severe level of signal loss or RF attenuation.

Steel outside walls, or steel walls separating parts of a building, can detrimentally affect RF coverage (because the wall might not just restrict RF penetration, it might also create a large number of multipath signals). This is common in industrial facilities, where a building has undergone one or more additions. What was once the outside wall might now be a partition between the old and new sections of the building, causing both multipath signals and an RF shield between building sections.

Be sure to research this information before or during the survey and document your findings on the site map.

Building Contents

One often-overlooked area of concern is the building contents. Those with minimal WLAN and RF experience sometimes underestimate the effect that building contents can have on a WLAN.

Figure 8-4 shows several examples of problems that can occur in a typical office environment. Areas such as file rooms and storage rooms are often filled with steel cabinets, creating a very large RF shield for RF entering that room, or even passing through it to other areas of the facility. Although most would assume an area filled with cubicles should have minimal effect on RF, it might in fact create a challenge for RF coverage. The number of cubical partitions, the amount of steel in the partitions and desks, and the size and make-up of the bookshelves can affect RF range.

Another area that is very difficult to cover is a library or documentation area. Shelves full of books are shelves full of paper, and most paper has a high level of attenuation to WLAN frequencies. It is very common for WLANs to use directional antennas, focusing the RF energy down the aisles of the books. (See Figure 8-5.) Because of similar shelving, warehouses and even some retail stores also use directional antennas in this way.