WiFoo..The.Secrets.of.Wireless.Hacking [Electronic resources] نسخه متنی

Figure 12-1. Unobtrusive Wall Mount

[View full size image]

Figure 12-2. Hidden Antenna

Appendix C, "Alternative Antennas," describes a ceiling-tile antenna. This antenna is attached to the top of a ceiling tile, and therefore is totally hidden from view. The downside to a ceiling antenna is the need for adaptor cables (and tying to match to existing ceiling tiles). As an alternative, Figure 12-3 shows a mounting system from Maxrad that provides a very unique mounting, for both an AP and for antennas. The entire fixture can be assembled and installed as a complete system. It contains a 2.4-GHz diversity omnidirectional antenna with an outward radiation pattern. In addition to that, the bracket is designed to hold the AP just high enough above the ceiling tile to permit an attached 5-GHz dipole or patch antennas to radiate as well as possible (for being above the ceiling). Of course, the survey engineer should first test with this type of antenna to ensure accurate deployment. This is just one reason why it is a great idea for survey engineers to understand installation issues.

Physical Security

The physical security of an AP is sometimes based on the old adage "out of sight, out of mind." In some situations, APs placed above ceiling tiles seem to meet this level of "security." In environments with heavy public traffic but inconstant monitoring, such as a school or other public building, however, this practice is not adequate. Although one would hope that students and others would not be prone to mischief, you must plan for just such mischief.

The mounting of the antenna must be such that it is secure and cannot be removed easily. However, the real security threat is the AP itself. Securely mounting the AP and the radios modules within the AP is critical.

Some devices, such as the Cisco Aironet 1200, provide a security slot that enables you to attach a Kensington-type locking cable (identical to the ones used for securing laptops). Some APs also offer unique mounting brackets that enable you to use a small padlock (see Figure 12-4).

At times you might need to provide a totally secure box for the AP (which is often the case when APs are mounted for public access and you do not have a secure location in which to mount the AP). Various boxes are available to enclose an AP. These can be as simple as a small plastic lock box available at any electrical-supply outlet or even some home-improvement stores. Plastic can be drilled easily (for cable entry) and, in some cases, the antenna might even work well through the plastic (but this should be tested as part of the site survey).

Environment

A few APs can handle outdoor or harsh environmental conditions (for example, the Intermec WA21 or Cisco Aironet 1300, shown in Figure 12-5), but the vast majority of APs available today are not up to such tasks. If you plan to place an AP in an outdoor area such as an open-air patio for public access or a beach house where salt spray and humidity are very common, you typically need some type of extra enclosure.

Several different enclosures are on the market today. They vary in size, features, and material. Some might offer heating and cooling systems, contain a power supply, or have AC line voltage fed into them. Others rely on Power over Ethernet (PoE), as do the enclosures available from TerraWave shown in Figure 12-6. When specifying enclosures, look for features such as these:

NEMA ratings. Some enclosures are intended for indoor use to provide a degree of protection against dust, falling dirt, and dripping noncorrosive liquids; others are intended for outdoor installation.

A opening large enough to facilitate component installation and maintenance.

A method for direct mounting or optional external mounting feet.

A seamless one-piece gasket to provide oil-tight and dust-tight seal.

A lockable door.

Dual antenna connections for external diversity antennas.

In many cases, these environmental enclosures are used for more than just environmental protection. For example, in many cases you can physically secure the AP by using the lockable door.

Building Codes

Building codes vary widely from location to location. This can be a problem for a company that is trying to standardize on an implementation for multiple sites located in different municipalities. There are different regulations regarding cable usage, fire ratings, accessibility to plenum areas, and so on. Although this section touches on a few of the issues, this is a topic that is beyond the scope of this book. Consult local authorities or use local contractors to guarantee that the installation meets the local regulations.

The National Electric Code (NEC) defines a plenum space as a compartment or chamber where one or more air ducts are connected and form part of the air distribution system used for ventilation into locations where people reside.

The space above acoustic ceiling tiles can be classified as plenum if the space extends beyond a single room. Typically, this space is common to the majority of the building (or at least to a particular floor). This type of construction is common. The reason for rating this as a plenum area is that in the case of fire, any smoke or fumes in the plenum area can travel through the air ventilation and into other inhabited areas. Therefore, equipment placed in the plenum must meet certain requirements for fire and smoke. If the walls extend above the ceiling tile and seal off the space, the area above the ceiling might be exempt from the plenum rating.

To meet the requirements of the NEC, you must use cables rated for plenum spaces (labeled as Communication Plenum Cable, or CPM). Similarly, any equipmentincluding APs, antennas, and antenna cablesinstalled above the ceiling must meet the plenum ratings.

Although the NEC defines the plenum ratings, and most cities use these same ratings, some localities have defined their own local codes. This is something that the installer must research before installing any equipment into a plenum area.

Another regulation in some localities concerns certification of the individuals accessing the plenum areas. Some regulations specify that any person accessing a plenum area must hold a valid heating, air conditioning, and ventilation certification (HVAC). This regulation might impact well beyond the installation portion of a WLAN project. If an HVAC contractor is used for the installation (installing the APs and running cables), what happens if an AP fails sometime down the road and needs to be replaced? If such a regulation applies in this locality, it is illegal for an IT person (unless HVAC certified) to access the AP above the ceiling.

One company had a custom AP enclosure made to meet this HVAC certification requirement, as well as any plenum requirements. Figure 12-7 shows the enclosure, which once installed is totally isolated from the plenum air space and opens from the user side of the ceiling. This permits anyone (with keys to the boxes) to access the AP for installation and maintenance. The box itself had to be installed by an HVAC contractor, but once installed and wired, the IT staff could handle all the necessary maintenance.