Printer Friendly

Securing your signals.

A BURGLAR ALARM SYSTEM IS often an integral part of an orginazation's security program. Such systems are crucial to the protection of a company's assets and may also be necessary for insurance. Security managers need to understand how different burglar alarm systems operate and how the transmission of alarm signals is secured.

A burglar alarm system consists of sensors, a control unit, and a reporting device. The sensors are installed at the protected premises and are intended to detect intruders. These sensors are usually connected by an electrical circuit to the control unit, which is the brains of the system. The control unit is the means by which the system is turned on and off. This unit provides power for the sensors and receives signals from them.

The reporting device can be either a local bell at the protected premises or an alarm receiving unit at a remote location. When the control unit receives an alarm signal from a sensor, it then communicates with an alarm receiving unit to initiate an alarm. It is the reporting device that categorizes a burglar alarm system as a local, police-connected, or a central station system.

A BURGLAR ALARM SYSTEM IS MOST reliable when the alarm signal is transmitted to a remote monitoring station or a central station. At this remote location, trained operators provide supervision, record keeping, and maintenance services for monitored alarm systems 24 hours a day. When operators receive an alarm signal, they can notify the police and business owner and dispatch security officers to the protected premises.

Alarm signals can be transmitted in a variety of ways. Historically, alarm signals have been transmitted through telephone lines because they are convenient and cost-effective. In recent years, however, radio transmission of alarm signals has grown in popularity.

The following methods or systems are recognized by Underwriters Laboratories Inc. (UL) as being suitable for transmitting central station burglar alarm signals:

* Direct wire systems connect a single protected premises to a single alarm receiving unit at the central station.

* Transmitter systems connect more than one protected premises to a single alarm receiving unit. These systems are also referred to as McCulloh circuits.

* Multiplex systems use a method of signaling characterized by the simultaneous or sequential transmission and reception of many signals over a single communication channel. These systems also include a means of positively identifying each signal.

* Digital alarm communicator systems consist of a digital alarm communicator transmitter (DACT) located at the protected premises. The DACT contacts a digital alarm communicator receiver (DACR) located at a central station through the telephone company's switched network and transmits a message identifying the change in condition at the protected premises. The DACR verifies to the DACT that the message has been received.

* Derived channel systems use the switched telephone network to communicate between a protected premises and the central station. But unlike the digital alarm communicator system, this system allows for continuous supervision of the telephone line without interfering with normal telephone usage.

* One-way radio systems have a radio transmitter at the protected premises that transmits status signals to a receiving antenna located, typically, at ] a central station. In these systems, the central station does not acknowledge to the protected premises the receipt of the signals.

* Two-way radio systems are similar in configuration to one-way radio systems except that normal communication between the protected premises and the central station is based on an interrogate and response cycle. Transmissions emanate from the central station to individually addressed subscribers (protected premises) that, in turn, report their status condition.

* Combination systems provide an alarm sounding device at the subscriber's premises and also employ a direct wire, transmitter, multiplex, digital alarm communicator, derived channel, or radio system to transmit signals to the central station.

WITH THE EXCEPTION OF RADIO, ALL systems cited use telephone lines as the alarm transmission medium. Essentially, three types of telephone lines are used to transmit burglar alarm signals: metallic circuits, voice-grade lines, and a switched network. Line supervision is provided to verify that telephone lines in a central station burglar alarm system are functional (meaning they will allow alarm, opening and closing, and test signals to be transmitted from the protected property to the central station for interpretation by the operator).

Metallic circuits are pairs of copper wires that connect the protected premises directly to the central station. A balanced current system in which a direct current (DC) flows through the continuous, closed loop formed by the copper wires provides line supervision. An increase or decrease in the preset value of this current results in an alarm signal. Metallic circuits are used in direct wire and transmitter systems.

Voice-grade lines are similar to metallic circuits in that they are leased from the telephone company and dedicated to alarm signal transmission. However, voice-grade lines do not provide a continuous path from the protected premises to the central station. The lines are interrupted by transformers, amplifiers, and other telephone equipment. Since the path is not continuous, supervision is generally provided by an alternating current (AC). When this AC current deviates from the normal current by more than a preset value, an alarm is initiated. Voice-grade lines are used in multiplex systems.

A switched network is the same network over which ordinary telephones operate. Signals are relayed through an extremely complex system of cables, switching centers, microwave dishes, and satellites. Since the circuit is not dedicated to alarm transmission but is used also for telephone communication, supervision cannot be readily provided. Digital alarm communicator systems use a switched network. Derived local channel systems use a portion of a switched network for transmitting alarm signals.

Since a switched network is also used for ordinary telephone service, daily use of the telephone verifies its integrity. Additionally, UL requires that at least one communication must occur every 24 hours between the transmitter and receiver of a digital alarm communicator system.

THE MAJOR LIMITATION OF A CONVENTIONtional burglar alarm system is its susceptibility to defeat or compromise. As used here, conventional means that the system operates over telephone lines and has lione supervision. With such a system, burglars with the proper knowledge and tools can gain access to the telephone lines and defeat the alarm system. UL defines a compromise as the disconnection of the protected premises from the connecting line or communication channel in such a manner that does not cause a signal at the central station and, therefore, allows entry into the protected premises without initiating a signal at the central station.

The first reported defeat of a direct wire system was in 1959. Until that time, a balanced current system--in which the supervisory current in the circuit is continually measured--was considered state of the art in line security. However, the system could be easily defeated by voltage and resistance substitution techniques.

In an attempt to reduce the vulnerability of direct wire systems, line security systems were developed. UL 1610, Standard for Centra-Station Burglar-Alarm Units, gives the requirements for line security. When added to a conventional burglar alarm system, line security provides additional protection against compromise attempts.

Line security for direct wire and transmitter systems. The first line security system, referred to as tone line security, was introduced in 1965. This system required that the alarm system be resistant to defeat from any single compromise attempt. This method of line security was accomplished by placing an AC tone on the telephone line along with the supervisory DC signal. Both the AC and DC signals would have to be substituted simultaneously to compromise the system. In 1975, the first successful compromise of this system was reported.

The next generation of line security for direct wire and transmitter systems was introduced in 1977. Called pseudorandom digital line security, it is the present UL standard. Under this standard, UL requires that the line security system have even higher defeat resistance and that compromise attempts be detected within six minutes if

* circuit resistance is substituted.

* circuit voltages are substituted,

* equipment of the same design and manufacture is substituted,

* signals are recorded and played back, or

* signals are synthesized and played back on the protected line.

In a pseudorandom digital line security system, the AC tone is replaced by a digital signal. This signal is generated by a digital pattern generator at the protected premises and transmitted to a receiving unit at the central station. Pseudorandom implies that the digital signal, at some point, will repeat itself. Computer analysis of the communication can determine when the repeat cycle occurs, and this information can be used to compromise the system.

In 1982, there was a reported defeat of a pseudorandom digital system and, shortly thereafter, othe defeats occurred. These defeats of the system caused concern in the alarm industry and among users of line security equipment.

By definition, UL standards are minimum standards. Thus, UL lists line security systems that just meet the minimum requirements of the standard, as well as those that far exceed the requirements. However, UL did not upgrade its line security standard because the 1982 compromises resulted from design limitations of particular line security equipment. Apparently, the systems that have been compromised to date had either a short repeat cycle or a digital signal that returned to its starting point each time the alarm system was reset.

UL presently has listings for pseudorandom digital line security systems that are able to resume transmitting from the point at which the signal was stopped and that have repeat cycles of 99 or more years. Other UL-listed line security systems, referred to as encrypted line security systems, use the Data Encryption Standard (DES) of the National Institute of Standards and Technology to produce a nonrepetitive, highsecurity signal. These systems may also employ a message authentication code to ensure that the alarm message received at the central station is the same as the message sent.

Line security for multiplex systems. By the nature of their operation, multiplex systems provide line security. A multiplex system uses a party line to interconnect a large number of protected premises to a common channel. These systems operate through transponders at the protected premises that permit two-way communication -- interrogate and response -- with a receiver at the central station.

A transmitter at the central station sends to all the transponders a series of coded, pulsed signals along the common channel. On receipt of their assigned codes, the transponders transmit confirming signals. Any absence, interruption, or delay in the confirming signals causes an alarm. This scanning or polling of each protected premises can be done sequentially or randomly. A computer at the central station automates the polling and handling of signals.

In theory, a multiplex system can be compromised by substituting equipment. However, because each transponder is interrogated in rapid sequence (at least once every 200 seconds according to UL standards), the time frame for this substitution is so short that it is virtually impossible to defeat the system. To date, no multiplex systems have been defeated.

Line security for a switched network. Like multiplex systems, derived local channel systems inherently provide line security because of their method of operation. A derived local channel system uses a portion of a switched network to transmit multiplex alarm signals between the protected premises and the central station.

A subscriber terminal unit (STU), or remote module, monitors the alarm system sensors at the protected premises. Scanners installed at a telephone exchange receive signals from the STU or remote module over a switched network. Through a host computer, the signals are relayed over dedicated telephone lines to the central station that services the premises.

At the central station, equipment receives signals from the telephone exchange and outputs them in usable form to the central station operator. Line security is accomplished by either interrogation and response of the individual STUs or continuous transmissions from each remote module to the scanner at the telephone exchange.

HISTORICALLY, THE CENTRAL STATION industry has relied on telephone lines to transmit burglar alarm signals in a cost-effective and convenient manner. Recently, however, this dependence on telephone lines has been a mixed blessing. The unreliability (and, in some cases, unavailability) of certain dedicated telephone lines, such as the metallic circuits used for McCulloh signaling, and the high rental costs for others, such as the voice-grade lines used by multiplex systems, have created a need for alternative transmission means. This need has resulted in the use of radio to transmit alarm signals.

Radio equipment for this purpose has been around for about 20 years. Even so, many alarm companies have accepted little of this radio technology. Several reasons can be given for this lack of acceptance:

* Radio use has not been cost-effective compared to the more traditional forms of central station service.

* Radio frequencies allocated by the Federal Communications Commission (FCC) have been in the lower end (27 MHz) and mid-range (450 MHz) of the radio spectrum, where interference and congestion create problems.

* Allocated frequencies are not exclusive to the central station industry and have to be shared with other businesses.

* Radio equipment has not been UL listed for central station burglar alarm service.

In 1982, the FCC opened the 900 MHz band, which is at the upper end of the radio spectrum. Fourteen pairs of channels were made available. As long as they have a license, alarm companies and other businesses that need multiple address frequencies can have exclusive use of a channel in a 75-mile radius within this band.

The exclusivity of the channels and the capability of two-way polling, which is more practical at the less-noisy 900 MHz band, plus UL recognition of radio for central station service, have revived interest in radio transmission. This interest has led to more widespread use of radio for central station alarm signaling. As stated previously, radio systems, also referred to as RF systems, can be either one-way or two-way.

In a one-way radio system, signals are transmitted in only one direction -- from the protected premises to the central station. A radio transmitter is installed in the control panel for the burglar alarm system at each protected premises. The transmitter communicates with a single receiving antenna at the central station. However, since radio systems operate via line of sight, the receiving antenna is usually located at the highest point in the area. A variation on this configuration occurs when the transmitter has sufficient range to be in contact with two receiving sites at any give time. The two receiving sites provide redundancy in case one of the two is out of commission.

The transmitter is capable of constant communication with the central station, but inherent limitations of radio systems restrict the communication to a less freqeunt basis than in telephone systems. Thus, opening or closing signals or other identifiable signals provide supervision in a typical one-way radio system. This type of supervision meets the UL requirement that the transmitter contact the receiver at least once every 24 hours and is comparable to the degree of supervision afforded by a digital alarm communicator system.

In a two-way system, communication between the protected premises and the central station is on an interrogate and response basis. Long-range radio systems can also incorporate a local bell or digital alarm communicator to form combination systems. A two-way radio system is similar to a one-way radio system -- each protected premises is equipped with a radio transmitter that communicates with a single receiving antenna located at the central station or at the highest point in the service area. However, with a two-way system, normal communication between the protected premises and the central station is based on an interrogate and response cycle of the system.

Signals originate from the central station and are sent to individual protected premises. In turn, these protected premises report their status condition to the central station. The interrogate or scanning rate for a two-way radio system is the same as the rate required by UL for a multiplex system -- each protected premises is interrogated at least once every 200 seconds. In the event of an alarm condition, however, alarm signal transmission to the central station is immediate.

UNDER UL'S STANDARDS, GRADES OF service for central station burglar alarm systems primarily indicate the response time required for central station alarm investigators to reach the subscriber premises following an alarm. However, the grade of service also indicates the method used to transmit the alarm signal and whether line security is provided. The accompanying exhibit summarizes the grades of service available for central station systems based on the method of signal transmission and the maximum response time. UL identifies burglar alarm systems with line security by AA, BB, and CC grade designations.

When burglar alarm protection is required for insurance, the risk should be analyzed to determine what grade of burglar alarm protection is necessary. The risk analysis should include the following questions: What goods are being protected? What is the demand for the goods? Are the goods easily transported?

For instance, building materials, refrigerators, dishwashers, pet foods and supplies, and lumber, although desirable from a consumer standpoint, are not easily transported. Thus, the goods are not likely to be burglary targets. In these instances, a conventional burglar alarm system of grade A, B, or C may be appropriate.

Items such as stereo equipment, clothing and accessories, televisions, and tools are likely targets of burglary attack because they can be resold easily. When these items are being protected, a grade AA, BB, or CC burglar alarm system may be required.

Jewelry, precious metals, rare coins, stamps, furs, and currency are examples of items highly susceptible to burglary. They require, as a minimum, grade AA, BB, or CC burglar alarm protection. For extremely valuable goods, an encrypted line security system should be used to protect items from the sophisticated burglar -- a burglar who may have the skills to compromise line security systems that just meet UL requirements.

For the majority of businesses with burglar alarm protection, the threat of telephone or radio system compromise is minimal. But as the value and desirability of the goods being protected increases, so does the risk of sophisticated burglary attack.

About the Author . . . Allan M. Apo, CPP, is manager of crime prevention for the Engineering and Safety Service of the American Insurance Services Group Inc. in New York City. He is a member of ASIS.


Baum, Ronald J. "On the Line with DES." Security Management, April 1989, pp. 84-87.

Papier, Isaac. "New Signaling Technology: State-of-the-Art Techniques in Burglary and Fire Protection Systems." Lab Data, Vol. 16, No. 2, 1985, pp. 4-14.

Wilson, Dean K. "Alarm Transmission via Digital Alarm Communicators and Derived Local Channel." Federal Fire Forum: Fire Alarm Systems, New Information, June 6, 1988, pp. 1-10.
COPYRIGHT 1991 American Society for Industrial Security
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:effective transmittal of alarm signals in a burglar alarm system
Author:Apo, Allan M.
Publication:Security Management
Date:May 1, 1991
Previous Article:An aquatic adventure at the National Aquarium in Baltimore.
Next Article:Treating hazardous waste.

Related Articles
On the line with DES.
A look at cellular's alarming technology.
On the line with data transmission.
Better backup.
Grade A security.
For whom the bell tolls.
Alarm design that rings true: in designing alarm systems, consultants and end users often forget to consider the basics--such as how the system can...

Terms of use | Copyright © 2017 Farlex, Inc. | Feedback | For webmasters