Wireless Access Control Design 101

Oct. 27, 2008
An exercise for end-users who are ready to implement the technology

The Challenge: You have had a series of thefts in your 84-year-old administration building, and you are ready to implement new technology to secure all perimeter and interior doors on the first floor. You want real-time alarm notification and recording of all access attempts. Your current building has the original mortise locks with skeleton keys on the interior, and you would like to migrate to proximity cards and standardize on an HID Corporate 1000 format.

Creating a wireless access control system for this application isn’t as difficult as you might think. Follow this wireless access control design exercise and see for yourself.
Wireless technology should be considered for just about any door access control application — if for no other reason than a wireless locking solution takes 45 minutes to install vs. an average of eight hours for a wired solution. That produces savings in materials, as there is no wire to install, and labor, as workers are in and out within the hour.

System Design Guidelines
Wireless systems typically operate up to 200 feet between the door and the wireless panel interface module (PIM) for indoor applications. The PIM is an important element of the system — it ties the door you are installing to a new or existing access control system, either wired or wireless. PIMs transmit in an omni-directional pattern and communicate with assigned wireless devices without line-of-sight. The signals can pass through plasterboard walls, cinderblock walls, brick walls and many other materials for simplified system designs.

For ease of design, think of the 200-foot wireless coverage radius as a cell, measuring 275x275 feet (75,625 square-feet). You will lay these square cells directly onto your floor plan to determine the minimum number of PIMs required to cover the space, as shown in Figure 2. You will also count the number of doors that will need access control to ensure you have sufficient PIM capacity. Then, determine the best locations for the PIMs such as IT or telco closets and adjust the squares accordingly. Finally, test your proposed systems layout.

Figure 2

Don’t simply put the PIMs anywhere. Here are some guidelines:

• Place PIMs in a secure, central location.
• Avoid close metal objects, like ductwork and circuit breaker panels.
• Mount PIMs at least 6 feet high to broadcast above obstructions.
• Consider proximity to your access control interface.
• Locate the PIM within 200 feet of the wireless lock.
• Do not design a single PIM to be used on multiple floors.

Next, identify any potential RF problems, such as shielded walls, metal lathe, elevator shafts, metal staircases, ventilation ducts, metal clad walls, metal lockers and any other 900 MHz equipment on site. Sometimes you will need to ask the facility manager, otherwise use a wireless test kit to verify if there are any obstructions.

As with any other system, you will need to detail your access points — will the lock type be mortise, cylindrical, exit device, electric strike or magnetic lock? Remember, for this system, you are using dark bronze mortise locks.

For gate applications, do you want 1- or 2-way control? For elevators, do you want floor control and/or call button? Do you want to monitor a door’s status to report open or closed? Do you want to simply validate someone’s access card? Do you have any automatic door openers for handicapped entrances?

Most security directors would also have to decide the type of credentials they want to use, including make of proximity card or type of magnetic stripe card. In this case, you will be using a Corporate 1000 format.
Other things to consider include the specific opening requirements:

• Door type — single, double or glass;
• Buzz-in — handy for unlocking doors for people without access credentials;
• Scheduled unlocks;
• ADA entry points for the handicapped;
• Power availability;
• Usage – card holder traffic;
• Door status — monitor when doors are opened and closed; and
• Checkpoints — validate card holder credentials

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.

Figure 12

System Design Exercise
So far, we’ve been discussing our project and what needs to be done. Let’s simulate the installation.

We have a 150x150-foot, L-shaped building to protect. Using the Pythagorean Theorem, the distance between the two corners in Figure 13 is 212 feet, which is pretty close to the 200-foot rating we established earlier. However, by putting PIMs in room 14, you should be able to cover the entire floor, as the RF can cut across the open courtyard between the two wings of the building (see Figure 14).

Figure 13

Figure 14

The only problem with this scenario is that you do not have control over the courtyard. When looking at a blueprint, all you see are the building dimensions — the challenge is to plan for the unexpected and visualize how the space will be used in a year or more down the road. For example, after building completion, landscapers install trees, shrubs, flowers, picnic tables and a walking path as shown in Figure 15.

Figure 15

How will this effect RF performance? Chances are the trees will eventually impede the signal from reaching doors 1, 23, and 24 as shown in Figure 16. That’s why is best to be more conservative in your planning as in Figure 17, where PIMs are split between Rooms 6 and 14 to ensure 100-percent coverage.

Figure 16

Figure 17

To quickly recap, the 200-foot coverage radius is considered a cell of 275x275 feet. We counted the number of doors needing access control to ensure we had sufficient PIM capacity. We laid these square cells on top of our floor plan to determine the number of PIMs required and, then, found the best locations for them, adjusting our squares accordingly. Using our worksheet, we wrote down all the equipment we needed, leaving nothing to chance.

Once you have created your first wireless access control system, the next will become even easier. This exercise should start you on the right path.

Andy Geremia is Product Manager for Schlage Electronics, a division of Ingersoll Rand Security Technologies.