Discovering Site-Specific Requirements for 802.11 Wireless Network Site Surveying and Installation
This chapter covers the following topics:
Recommended Facility Documentation
Limitations Affecting Equipment Installations
Using Cookie Cutter Designs
Often when a decision is made to install a WLAN, the decision makers do not realize that WLAN installation sites vary widely. Those who decide to move to wireless networks understand their environment and assume that most other WLAN systems and sites are much the same as theirs, and therefore the WLAN installation should be similar (and easy). Even in something as similar as a chain of retail stores, minor variations such as stock and shelving arrangements can cause coverage to vary. In reality, unless the two sites are built from the same architectural plans and populated with users and contents in a similar way, the two seemingly similar sites can be as different as night and day. Before a survey or installation is scheduled, each site needs to be evaluated separately.
This chapter addresses site-specific issues relevant to WLAN survey and installation tasks. This chapter examines topics such as site layouts, facility contents, building and construc-tions variations, different environmental conditions at the site, and other site-specific requirements. This chapter also discusses the need for accurate floor plans, building and construction specifics, an inventory of building contents, and an awareness of customer issues related to specific installation limitations.
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 ena-bles 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
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).
Figure 8-1 Typical Site Map
Building construction can vary widely from site to site. Materials and construction techniques in San Francisco differ significantly from those used in New York City, London, or Cairo. Differences in building construction, even though sites might look similar, can cause RF to react in completely different ways. Figure 8-2 shows examples of various building materials.
Figure 8-2 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.
Figure 8-3 Steel-Pan Floor Construction
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.
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.
Figure 8-4 Office Issues
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, ware-houses and even some retail stores also use directional antennas in this way.
Figure 8-5 Directional Antennas for Library Coverage
Kitchens and break-rooms usually contain microwave ovens. Microwaves are also found around many health-care and industrial facilities for purposes other than heating food. Although microwaves pose no problems for 5-GHz WLANs, they can be problematic for 2.4-GHz WLANs. The typical microwave oven uses the same frequencies as a 2.4-GHz WLAN. (This is because 2.4 GHz is the resonant frequency of water, and when 2.4-GHz energy strikes water molecules, it is absorbs the energy and causes the molecules to vibrate, creating friction and heat.) Locating a 2.4-GHz access point (AP) close to a microwave can cause undue interference and result in poor RF communications. Take care to keep these APs (and clients when possible) at least 10 feet away from any standard microwave oven. It is therefore recommended to note the location of any such devices on the site map. Also be aware that industrial microwave ovens sometimes have a much higher power than those found in the home or office, possibly creating even more interference. Testing should in-clude RF coverage verification while any microwaves in the local vicinity are in full operation.
In one case, a health-care facility was having trouble with one particular AP that was dropping all associations intermittently. Close inspection of the facility turned up a microwave oven in an area that was not part of the RF-covered area, but was located in a lab adjacent to the AP-covered area. The problem was that the oven was located on the other side of the wall (made from drywall) from the AP, with a total of about 5 feet (and two pieces of drywall) separation. This is why it is important to understand the entire site, including areas where coverage may not be needed. Figure 8-6 shows a site map with potential problem areas labeled.
Figure 8-6 Hidden Site Problems
Locations such as emergency rooms and cardiac care in hospitals use sensitive equipment such as electrocardiographs (EKGs) and other monitoring systems. Although these devices are not generally a problem with WLANs, take care to locate the radio gear near the devices and to verify that the RF in the area does not cause any interference. One common problem occurs with older plotters and printers. The RF energy can cause slight variations in the print and plotter driver mechanisms, resulting in "glitches" in the patient printouts.
As discussed in Chapter 6, "Preparing for a Site Survey," all cordless and wireless devices should be inventoried. Phones, speaker, cameras, cordless mice, cordless keyboards, baby monitors, and virtually anything that might be RF related should be noted.
In a warehouse, retail environment, or even an office building, a change of contents can greatly affect the coverage of an AP. Inventory levels often change in a warehouse or retail facility. At certain times of the year (such as early November, when stock levels rise for the holiday shopping season), stock levels in some facilities may reach beyond 100 percent, with material placed in any possible free space, such as directly in front of the AP that provides coverage to the area. This poses a real problem for the survey engineer who is trying to survey when the stock level might be at a low level corresponding to the season. (Many installations occur during the off-season, when facilities are not running at peak capacity.)
Defined User Areas and Densities
The topic of user density has been brought up many times in this book. As stressed prev-iously, defining user areas and densities is a crucial part of the design and must be on the minds of design engineers and survey engineers at all times. The overall performance of the WLAN system depends on proper user density.
There have been surveys based on nothing but user density. At one very large software company, the buildings were all built in a very similar manner, and with identical internal design and contents. All cubicles were identical, all office construction was identical, and the number of users in a given area was very similar.
For this customer, it was decided that the applications used by nonengineering employees would permit between 20 and 25 users per AP. This provided adequate performance for normal operational network load. The engineers, however, required a bit more perform-ance, and the user density was lowered to between 10 and 15 users per AP.
Based on information such as this, some of the design can be done up front. You can use the site map to determine how large the cell coverage needs to be. For example, a survey determined that a single AP set to default power levels, with dipole antennas, could provide coverage for many more users (based on their seating locations) than the design calls for. In this particular case, the desired coverage turned out to be a small circle on the site map, and was about the size of a coffee cup. From this point, it was a matter of defining how many "coffee cups" were needed (see Figure 8-7). The engineers then selected the power setting to provide the proper coverage for the user density in the appropriate areas. Finally, testing was completed to prove the guesstimations of the coffee cup survey.
Figure 8-7 Coffee Cup Layout
If voice is to be used over the WLAN, it is vital to the design to understand the capacity of the AP versus the number of calls that can be carried by one AP at any given time. Typically, a standard 802.11b AP supports only between six and eight calls with the standard com-pression used in 802.11 voice over IP (VoIP) phones. As compression techniques improve, or as the wireless 802.11 VoIP phones move to support 802.11g or 802.11a, the number of calls per AP will increase.