Wireless: The Future of Access Control

Oct. 1, 2003
Flexible, convenient and reliable, wireless is positioned to make its mark on the access control market.
Wireless access control is the latest technology innovation in the access control industry. It sounds like the way of the future. As a result, there is a lot of interest in this new type of system architecture. But as we know, there's no value to innovation unless it solves a real problem or makes an existing solution better, faster or less costly. Proponents of this new technology claim it does just that. But how does it work, and is it as reliable and secure as the tried-and-true wired systems?

How Does It Work?
Wireless access control eliminates the wiring from the access portal being controlled to the main control panel. This is done by adding a transceiver at the entry portal and another in an RF panel interface module connected to the main panel. These panel interface modules usually wire to the panel just like a door using standard reader technology, so they receive RF signals and output Wiegand or magstripe signals via those wires. The transceiver at the portal can communicate through walls to the panel interface module that is connected to the door control panel. As a result, the main control panels, the door control panels and the panel interface modules in this new architecture are all installed in the same equipment closet. This saves space, time and money.

Some wireless access control solutions are also integrated. Instead of a series of separate components that have to be wired together around the door, they are all integrated into one unit. For example, Recognition Source incorporates the transceiver in its Modular Integrated Reader Lock. This product also has the electric door lock, card reader, door position switch, power supply, request-to-exit and optional request-to-enter functions combined. Preassembling these devices into one unit can reduce the installation time at the door by up to 90 percent.

Panel interface modules can control two to 16 doors, depending on the main-control-panel manufacturer, and are offered with Wiegand, magstripe or RS485 connections. With an RS485 connection, only four wires need to be connected to the panel interface modules to control up to 16 doors each. If the brand of control panel selected does not have an RS485 connection for wireless access, the panel interface module wires to the main panel or door-control sub-panel just as if it were a door using the Wiegand or magstripe protocol. Panel interface modules using the Wiegand or magstripe approach can control up to four doors.

Reliability and security are two big concerns for end users and integrators. The reliability concern questions if the communications are robust enough to be accurate year after year.

There are generally two kinds of security concerns. The first is whether a transceiver can be tricked into allowing unauthorized access. The second is whether any RF signal in the area will block a request or command message so a door won't open after a request to enter. There are several critical system features that can significantly reduce these reliability and security concerns.

First, to ensure reliability, wireless systems should use spread-spectrum techniques that help make the communications robust. The spread-spectrum direct-sequence technique takes information, encrypts it, and spreads it over a wide frequency band to resemble noise. The transmitted message now has a very low power spectral density compared to other modulation techniques. However, when correlated by the receiver's encryption keys, it is the equivalent of higher power density. This allows long communication distances.

Since the user data bits are coded and transmitted over a wide bandwidth, and since the receiver correlates using a set of keys, the communication system tolerates a relatively high degree of noise. This represents a basic benefit of the system's process gain, and it makes communications reliable even in RF-laden or electrically noisy areas.

Integrators who have never installed wireless equipment often express other types of reliability concerns. They question whether varying construction types can cause communication problems. While construction types can affect the signal reliability, RF problems are easier to overcome than typically believed. For example, the antenna for the panel interface module is internal, so the location of the panel interface module is important. Mounting it according to the manufacturer's instructions normally will result in satisfactory communications.

However, occasionally communication to one or two doors in a system is questionable or inadequate. Overcoming these communication difficulties is usually a simple matter of relocating the panel interface module (moving it a few inches or rotating it). More severe problems can be overcome by using one of the following steps. 1) Mount a remote antenna in a better location—for example, on the other side of a closet wall or closer to the door. 2) Use a gain antenna in the same way. 3) Use a repeater. This final step is rarely necessary.

The range for controlling interior doors on the same level within buildings is usually 200 feet. The type of construction (wood, concrete, plasterboard) may affect that estimate. However, because of the penetration and bounce characteristics of spread spectrum at 900MHz, the effects of different construction types are small if the unit is installed within the manufacturer's range specifications. Only steel or wire mesh walls will significantly reduce the range.

A receiver with high sensitivity (–90 decibels above 1 milliwatt) combined with a transmitter with high transmission power (greater than +20 dBm) will give a dynamic range of at least 110 dBm. This radiated energy and sensitivity gets used up as the two devices are separated from each other. For example, at 200 feet, about half of the 110 dBm are used, leaving 55 dBm of signal. Intervening walls absorb about 3 dBm each, which further reduces the dBm available. Even with a lot of signal, a margin of about 25 dBm should be left over to ensure reliable communications. So there can be 10 intervening walls over 200 feet and there should still be a sufficient signal of 25 dBm.

In addition to providing high signal strength and sensitivity in the transceiver, the system's request and command messages should be sent repeatedly or until a response is evoked to ensure desired performance levels. The system should pole every portal regularly, preferably every few minutes, and give a trouble signal if communications are ever intermittent or lost.

The information being sent should be transmitted on many and different frequencies. The transmissions should be encrypted by algorithms so the signals are unrecognizable, which will keep “sniffers” from deciphering the content. In addition, message bit streams should contain more information than the 26 to 32 bits usually carried by the identification card. A randomly generated, unique identity code should be generated internally for each portal in the system. The identity code should not be known to anyone, even the system administrator. With all these security measures in place, there would be billions of permutations that would need to be deciphered to reproduce the correct message.

Another security concern is whether a valid request to enter can be blocked. While it is possible, this can only happen if there is another signal on the same frequency and it is stronger than the signal the panel interface module is supposed to receive. The chances of this happening are remote. However, it can occur and can be solved. The solution to this problem is to automatically switch to a different channel. Called dynamic channel switching, this feature virtually eliminates the risk of signal blocking by searching for a clear channel before transmitting.

Security professionals who have installed wireless access control often recommend that pre-testing be done before a bid is submitted. Suppliers offer pre-installation test kits to quickly determine if the RF link will work adequately. Each portal can usually be tested in less than a minute, and the test is usually done at lower-than-normal power levels to ensure a signal surplus in normal use. Laying out a wireless system is not difficult, since many manufacturers offer training seminars on how to lay out and install a wireless system.

A Flexible Solution
The following example illustrates how, even with some architectural obstacles, wireless access control works extremely well. The site is a university dormitory that has 40 doors, all controlled wirelessly on four floors. The building has a steel-in-concrete frame with concrete over metal deck floors and brick exterior walls. All interior walls are either plasterboard over steel studs or stucco over wire mesh, normally an RF nightmare. Common windows in the lounges on each floor have wire mesh laminated within. The panel interface modules are located in cinder block and concrete closets with steel doors. Despite this problematic construction, the system's communications have worked well since first commissioned. No remote antennae, gain antennae or repeaters were needed to facilitate good operation.

There are wireless access control solutions for every type of portal in a system. Models will soon be available for panic-bar-actuated doors. For gates, there are gate controllers that eliminate the need for closing parking lots and trenching. For elevators, there are elevator controllers that eliminate the need for adding traveling cabling. There are even portable wireless card readers that allow identity checking at multiple locations during a day using the same equipment. There are also wireless sensors that monitor any change of state, like door openings.

Today there are thousands of wireless installations, from single doors, elevators or gates to hundreds of doors in a single building. In general, wireless access control equipment is able to work successfully wherever it has been installed.

Monica Keane is a marketing communications consultant with more than 20 years of experience in the high-tech and financial services industries.