Wireless Protection

May 1, 2003
Systems operating without wires can help schools and universities reduce installation costs.

Technology is worthwhile only if it solves a problem or makes a solution better, faster or less costly. Wireless access-control equipment can solve problems for schools and universities.

Wireless access control is an integrated reader lock at the door and a panel interface module at the control panel, a combination that brings several benefits.

One integrated reader lock takes the place of everything needed at the door for “online” real-time access control. So only one device has to be installed. No one needs to spend hours installing an electric strike, hand-wiring up to six separate devices at the door, then wiring those back to a main control panel. Because the integrated reader lock has the electric door lock, card reader, door-position switch, power supply, request-to-exit and request-to-enter functions built in, the installer has only to remove the old lock, drill a few holes and install the new integrated reader lock. It's done in about 30 minutes. In addition to all of these functions being built into the integrated lock, there is a spread-spectrum transceiver built in as well. This eliminates the need for any wiring from the door to the main control panel. Any door-control sub-panel can now be mounted next to the main panel.

The benefits

Integrators can install everything needed for access control at the door in less than an hour per door. It can be done by one technician or can be subcontracted to a locksmith. Installers no longer need to place the door-control modules near the door; they can be installed right next to the main panel.

To communicate wirelessly to the integrated reader lock, a panel interface module is needed. This panel interface module is mounted next to the door-control module and then wired to it. The main control panel, the door-control modules and the panel interface modules can be in the same equipment closet.

Wireless access equipment can be used with every brand of main control panel. Each panel interface module can control from two to 16 doors. The panel interface modules come with Weigand, mag-stripe or RS485 connections. With an RS485 connection, only four wires need to be connected to the panel interface modules. These RS485 panel interface modules can 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 Weigand or mag-stripe protocol. Only two doors can be controlled per panel interface module using the Weigand or mag-stripe approach.

In university residence halls, a system can be installed between semesters or over holiday breaks. In a recent installation at a university, each of 40 suites in a four-story residence hall was outfitted using wireless integrated reader locks. The installation manager estimated that using wireless access equipment saved the university $80,000 to $100,00. Integrators who have run the numbers usually find that there are typically total savings of 20 to 25 percent.

How does it work?

An electronic card reader, door position switch, power supply, request-to-exit and request-to-enter sensor are packaged together with a transceiver and connected to an electric motor-driven lock to create the integrated reader lock. These integrated reader locks are available with most card-reader types including proximity, mag-stripe, Weigand and even biometric — smart-card readers should be available soon. Both cylindrical and mortise-type locks are available.

Power for integrated reader locks is provided by a battery pack, typically several alkaline AA cells. An excellent spec for the battery pack is that it will last for 60,000 swipes or up to five years, whichever comes first. A low-battery trouble signal is a “must” feature.

The electronic signals that need to be sent between the lock and panel interface module are sent wirelessly. These wireless signals use spread-spectrum techniques that help make the communications robust.

Is it reliable?

For a truly reliable system, the receiver should have high sensitivity (-90Dbm) while the transmitter should have high transmission power (greater than +20Dbm), giving a dynamic range of at least 110 Dbm. This radiated energy and sensitivity, measured in Dbm, is used up as the two devices are separated from each other. For example, at 200 feet about half of the 110 Dbm are used. Intervening walls absorb about 3Dbm each, which further reduces the Dbm available. Even with a lot of signal, a margin of about 25Dbm should be left over to ensure reliable communications.

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 monitor every door regularly, preferably every few minutes, and give a trouble signal if communications are ever lost.

The range for controlling doors within buildings is usually a couple of hundred feet on the same level. The type of construction will have an effect. But because of the penetration and bounce characteristics of spread spectrum at 900MHz, the effects are small if you stay within the manufacturer's specifications for range.

Manufacturers offer training seminars on how to lay out a wireless system. They also offer pre-installation test kits to quickly determine if the RF link will work adequately. Each door usually can be pretested in less than a minute. These tests usually are done at lower than normal power levels to ensure a signal surplus in normal use. The integrator usually does these tests before a bid is submitted.

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 will result in satisfactory communications most of the time. Occasionally communications to one or two doors in a system is questionable or inadequate. Overcoming these communication difficulties usually is a matter of relocating the panel interface module a few inches. More severe problems can be overcome by mounting a remote antenna in a better location, on the other side of a wall or closer to the door; using a gain antenna in the same way; or using a repeater.

How secure is it?

Security is a big concern. Someone could trick the integrated door lock and gain access. Also, an RF signal in the area could block the system's request and command messages so a door won't open.

To prevent those problems, wireless systems should use spread spectrum to split the information being transmitted and allocate it to many channels. Further, that information should be manipulated with pseudo-noise algorithms so the transmitted signals look like noise, which will help keep “sniffers” from deciphering the content. In addition, message bit streams should contain a lot more information bits than the 26 to 32 bits usually carried by the identification card. And to be more secure, a randomly generated, unique identity code should be generated internally for each door in the system. That identity code is not known by anyone, even the system administrator. If a facility takes all of these measures, billions of permutations would need to be deciphered to reproduce the correct message. It would be easier to knock a hole in a wall than to compromise the wireless system.

As far as “blocking” a request, it is only possible if there is another, stronger signal on the same frequency that the integrated reader lock and panel interface module is trying to use. The possibility of this happening is remote. But if it does occur, switching to a different channel automatically will solve the problem. This is called dynamic channel switching. Systems that have dynamic channel switching can avoid signal blocking.

Conforti has more than 30 years of experience in the electronics industry. He is one of the founders and a former president of First Alert, the residential smoke detector company that is now a part of Sunbeam.

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