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 locationÃ¢â‚¬â€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 (Ã¢â‚¬â€œ90 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 Ã¢â‚¬Å›sniffersÃ¢â‚¬Å¥ 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.