Wireless Access Control Design 101

An exercise for end-users who are ready to implement the technology

Laying out the System
Let’s try these lessons out on a floor plan starting with Figure 4, illustrating a typical first floor layout with offices on the perimeter of the building and an open atrium in the middle. The building measures 275x100 feet and there are a total of 16 doors that need to be secured. One choice, the PIM-485 from Schlage, can support up to 16 doors with select access control systems.

Figure 4

In a perfect world you would only need one PIM-485; however, note the two squares with X’s in the center of the atrium. Those aren’t skylights — they are symbols for elevators. Ever noticed how elevators often have stainless steel doors and metal panels? These can cause an RF “shadow” or area where RF transmissions won’t propagate as shown in Figure 5.

Figure 5

As a general rule, it’s best to design around obstacles such as elevators. In this case, placing PIMs on either side of the elevator eliminates shadows entirely, as shown in Figure 6. This prevents potential intermittencies. For example, with a single PIM configuration, the system works when the elevator car is on a different floor, but when the elevator lands on the ground floor, the metal content in the cab blocks the signal, causing a momentary RF shadow, which could become an intermittent problem.

Figure 6

Dealing With Obstructions
The term “line-of-sight” is simply the unobstructed transmission path between your wireless access point modules and the PIM to which they are linked, as shown in Figures 7 and 8. Note how Building B obstructs the signals coming from Building A.

Figure 7

Figure 8

Earlier it was mentioned that the cell had a 200-foot coverage radius. That is true for indoor applications; however, outdoors with no obstructions like trees, fences and walls, you can expect up to a 1,000-foot line-of-sight. What happens when a stand of trees is in the way or when the access point is further than 1,000 feet?

That’s where a repeater comes into play. Figure 9 illustrates a building and two vehicle entrance gates, along with a stately tree in the northeast corner of the building. If you want to secure the campus and install vehicle gates at the two entrances, you have a few options to consider. The first is to eliminate the obstacle and cut the tree down. In this green society, that might not make you very popular.

Figure 9

The second option is to use a repeater. Note that in Figure 10 there is a clear line-of-sight between the main building and gate 1, and the distance is less than 1,000 feet. This a perfect scenario for the Schlage Wireless Gate Kit comprised of a PIM-TD2, an outdoor wireless reader interface (WRI-OTD), and an indoor/outdoor remote antenna. This allows the PIM to be mounted indoors, the remote antenna to be mounted outside, and the WRI to be mounted at the gate, as shown in Figure 11.

Figure 10

Figure 11

Gate 2 is a different story — the tree is in the way. However, we have clear line-of-sight between the building and Gate 1, and from Gate 1 to Gate 2. By placing a repeater at Gate 1, we can link it to the main PIM and also link the repeater to the WRI at Gate 2, as shown in Figure 12. This saves hundreds of feet of costly trenching and weeks of disruption.