Limitations Affecting Equipment Installations
As with any installation, when installing a WLAN restrictions are always put into place by one or more of the following:
Customer requirements
Regulatory limitations
Environmental concerns
Take care to note any and all restrictions that might be in place. Also document any restrictions in the final site survey report so that the customer understands why a particular method was used (based on a given restriction).
Customer Restrictions
As you have learned in this chapter, understanding the customer's environment is important. Installation limitations can vary widely from customer to customer. Based on the sur-roundings, imposed limitations might require a survey engineer to make minor changes to a design. For instance, where a 5.2-dBi 2.4-GHz omni antenna is needed to provide coverage, it might not be possible to use one because of the antenna's physical size. A typical 5.2-dBi omni, which is approximately 10 inches or longer, might be too large to hang from a ceiling in a facility such as a school or hospital, for example. A Yagi antenna might work well in a long hallway, but the owner might require that the antennas remain unobtrusive. At 18 inches long and 3 inches in diameter, the 13.5-dBi Yagi does not meet the customer's unobtrusive requirement. In both cases, you need to use different antennas.
Perhaps the customer has required that the equipment be totally out of sight, as would possibly be the case in a museum or theme park. You could place the APs above the ceiling or in isolated locations, but then the antennas become an issue. You will need to verify that the AP supports external antennas and that the antennas available have plenum-rated cables.
Another such issue generally comes up in all locations where there is public access. Anten-nas and APs must be totally out of sight, hidden, or disguised. In the case of one theme park, all the APs are located in little cupboards, which have hidden and locked doors. The antennas selected were very slim patch antennas, which were attached to the wall and painted to blend in.
In one college, a WLAN system was installed for student use, and the APs selected used standard PCMCIA-type radio cards that plug into the AP. (This is a common design method for APs, and is popular among customers because of the easy radio upgradability for the AP.) The APs were just mounted in hallways near the ceiling. The IT staff of the school started to notice that some APs were no longer handling traffic. Upon inspection, it was found that the radio cards were being removed from their APs. The students had learned that the same radio card used in the AP could be used in a laptop!
An environment such as this university might require some type of enclosure for the AP that can be locked for the sole purpose of maintaining visibility but ensuring security against tampering hands.
Regulatory Limitations
Although discussed at length in Chapter 3, "Regulating the Use of 802.11 WLANs," limitations based on regulatory domain requirements are worth reiterating here. Be sure to address each of the following questions for the specific site in which you are working:
Is the RF system that is specified in the design legal in this location?
Are there limitations on the antenna gain (and style) that can be used at this facility?
How many channels are available for use in this location?
These questions require some investigation so that the system, once installed, meets the local regulatory specifications.
As a reminder, some countries specify a maximum of 20-dBm Effective Isotropic Radiated Power (EIRP), which limits antenna choices. Other countries do not permit 5 GHz, or limit the 5-GHz channels that can be used. Still other locations restrict certain channels for use with indoor applications only.
See Chapter 3 for the details of the various regulatory restrictions.
Environmental Concerns
Many WLAN installation areas require special designs. The most common is the plenum area above a ceiling in a standard office environment. As defined in Chapter 3, a plenum area requires that devices meet certain specifications regarding flame and smoke. Many APs use a plastic housing and, depending on the type of plastic, might meet these requirements. However, many plastic devices have not been tested or do not meet the necessary ratings and are not intended for use in these areas. For this reason, several vendors offer a metal housing for the AP.
Similarly, antennas also have environmental restrictions. Most external WLAN antennas on the market today that have been designed for WLAN usage are not plenum rated. This means they cannot be located above most ceilings. The cables on the antennas may use plenum cable, however, which means you can legally attach them to a plenum-rated AP and run the cable through the ceiling to the open air space (below the ceiling) where the antenna itself is located.
Local regulations also address plenum use. In some municipalities, any work done in plenum areas (such as above the drop ceilings) requires a licensed heating, ventilation, and air conditioning (HVAC) technician. If the AP is mounted above the ceiling, a licensed HVAC technician must be on site to supervise the installation. But what happens if an AP fails or needs to be replaced or accessed? Most IT professionals are not HVAC licensed. One possible solution (discussed later in Chapter 12, "Installing WLAN Products") is to place the AP inside some type of enclosure that is located in the ceiling, where the cables enter from above the ceiling. The metal box itself, installed by the HVAC professional, isolates the plenum area from the open air space of the room. The AP is now located outside the plenum area, and therefore plenum-rating requirements do not apply to the AP. This setup also obviates the need for an HVAC technician to be on site during AP servicing. In addition, the locked door provides physical security for the AP.
Most WLAN devices are designed to be located in a "friendly climate," meaning areas where the temperature ranges from 0 and 60 degrees Celsius. In addition, a friendly envi-ronment is one in which the AP stays dry and clean and vibrations are minimal. If you are putting an AP into a retail store or office building, a friendly climate might very well exist. In many scenarios, however, such as a food distribution center with a large freezer that might reach temperatures of –20 Celsius, or for outdoor installations in climates with extremely cold or hot weather, you need to design the WLAN based on the worst-case conditions.
WLANs are deployed in a wide range of locations, from climate-controlled offices to truck loading docks, outdoor rental car lots, airport tarmacs, shipyards and railyards, and even on board luxury cruise liners, locomotives, and passenger trains. Each location entails slightly different circumstances and therefore requires different survey tactics and installation techniques.
For applications that require the AP to be mounted outdoors, use an AP that has an appro-priate National Electrical Manufacturer Association (NEMA) rating, or place the AP inside a NEMA type of enclosure. This provides proper weatherproofing to prevent moisture damage and corrosion. Salt water is extremely damaging to most APs, so consider this fact if your WLAN installation area is anywhere near an ocean or sea.
Some WLANs are even used in hazardous areas, including industrial facilities where chem-icals are manufactured or stored, locations where painting is occurring (for example, auto-motive factories), or sites where explosives might be used (mines, for example). At such a hazardous site, use products that meet the site's intrinsic requirements or find suitable enclosures. Check with the customer to determine the level of safety necessary and the degree of isolation required.