The American communications infrastructure is changing quickly. The requirements of NFPA 721 for fire alarm system communications between a protected premises fire alarm system and the supervising station have kept up with technology in some ways and have been left behind in other ways. This article explains todays options for fire alarm system communications.

The Past
Prior to 1993, fire alarm communications requirements were found in NFPA 71, Standard for Central Station Service. With the combination of all NFPA signaling standards into one document, these requirements were relocated to Chapter 4 of NFPA 72.2 The organization of the National Fire Alarm Code has changed since then, but the requirements still can be found in the Supervising Stations Chapter (currently Chapter 26).


In the past, when a developer of a communications technology wanted to bring a product to market, the following procedure was required:

  1. The new technology would be brought to the technical committee, where it was evaluated during the current code development cycle.
  2. If the technical committee found the technology viable, the technology was added to the standard.
  3. Once the NFPA standard was published, the manufacturer could get the product listed and take it to market.

In the 1980s, when technology moved at a more leisurely pace, this process worked well. Entering the 1990s and the 21st century, the technical committee realized that this time-honored process was too slow.


Development of "Other Technologies"
In 1999, a new section was added to the National Fire Alarm Code titled Other Transmission Technologies.3 This section was developed by the supervising stations technical committee after much discussion and committee work and a decision by the committee not to add any more specific technologies. Other Transmission Technologies details a number of performance-based design features that any new transmission technology must meet to be listed. If a developer follows this section, then the product can betaken directly to the testing laboratory and get to the market in a timely manner.

The World of Transmission Technologies Prior to 2010
With the addition of Other Transmission Technologies in 1999, the lineup of possible technologies in the National Fire Alarm Code included the following:

  1. Active multiplex
  2. Digital alarm communications systems (DACS)
  3. McCulloh systems
  4. Two-way radio frequency multiplex systems
  5. One-way private radio alarm systems
  6. Directly connected non-coded systems
  7. Private microwave radio systems
  8. Other transmission technologies

This lineup provided specific technical requirements for each of the technologies listed, virtually unchanged since they were initially included in the standard.


NFPA 72-2010 Housecleaning Efforts
In 2010, the Technical Committee reexamined each of the transmission technologies in the standard, using the Other Transmission Technologies section as a litmus test to determine whether the legacy methods could be eliminated. After this examination, the following legacy methods were eliminated from the 2010 edition of the standard:

Active Multiplex
Active multiplex systems (also known as derived local channel systems) were developed by Wisconsin Bell in 1983 with the collaboration of several local fire alarm monitoring companies in Wisconsin. The system used a single telephone line, with a unit that split the bandwidth between voice and fire alarm data. Similar to todays DSL units, this was a leap forward for the era. With a single telephone line, one could have an always on connection to the fire alarm control panel and still use the telephone line for normal voice communications. Eventually, the specialized equipment required for this system became unavailable, and this method fell into disuse. Since all requirements of Other Transmission technologies were met with this method, elimination of this method would not disallow any derived local channel systems still in use.

McCulloh systems are an even older technology, dating back to the late 19th century. These were wind-up wheels with cams that would transmit a signal (originally via telegraph) to a fire alarm dispatch center. After transmission, the McCulloh wheel would need to be manually rewound, which was the beginning of a requirement for a runner service for central station service. (Todays requirement for a runner to be dispatched when equipment needs to be reset by the prime contractor1 continues this historical need.) Since direct copper connections were required, and the signals did not travel far, subsidiary stations were required, and as monitoring became regionalized, this form of transmission lost favor. Also, since McCulloh meets the minimum requirements of the Other Transmission Technologies section, McCulloh was eliminated in the 2010 edition of the National Fire Alarm and Signaling Code.


Directly Connected Non-Coded Systems
Directly connected non-coded systems were developed for use with remote station fire alarm systems. The remote station standard (NFPA72C) was first issued in 1960, when the fire service began monitoring fire alarm systems directly at the fire station or emergency dispatch center. Directly connected non-coded systems used sub-voice grade copper telephone lines, and incorporated a polarity-reversal technique to signal the fire dispatch center. Unfortunately, this system only transmitted a fire alarm signal, not supervisory or trouble signals originating at the fire alarm control panel. Also, since it was a general fire alarm signal, annunciator panels were required to be installed at each protected premises to indicate specific zone of initiation. Today, as local telephone companies are removing copper lines, and are not making the series 1000 sub-voice lines available, this technology is out-of-date and was removed from the 2010 edition of the code. In addition, any existing directly connected non-coded systems still meet the requirements of Other Transmission Technologies.

Private Microwave Radio Systems
Private microwave radio systems were provided by a handful of manufacturers in the early 1980s. Since no private microwave radio systems survive, and the testing labs reported that no private microwave radio systems are currently listed, this technology was removed from the 2010 edition of the National Fire Alarm and Signaling Code.


NFPA 72-2010 Organization

With the elimination of the four legacy methods, the organization of the transmission technologies section of NFPA 72 was changed to require compliance with Other Transmission Technologies (changed in 2010 to General), with exceptions for DACS and radio, which were found to be not in compliance with the performance requirements of Other Transmission Technologies.


If DACS is selected as a transmission technology, the DACS section should be followed. If any listed radio technology is used, the radio sections (one-way and two way) should be consulted for the requirements.


Digital alarm communications systems were first introduced to the technical committee in the mid-1980s, and were rejected by the committee twice because it had been determined that using regular telephone lines was not reliable enough. This was the first time that any proposed communications method was not controlled end-to-end by the technical committee. Instead, it was proposed to be under NFPA jurisdiction only to the point of demarcation at the protected premises, and once through the phone company, back under NFPA jurisdiction from the point of demarcation at the supervising station. This was relatively radical thinking, and it took a leap of faith in the telephone system for DACS to be approved, finally, on the third attempt. However, the technical committee members asked for, and received, some modifications, specifically regarding redundancy. DACS is the only communications method ever allowed for fire alarm that requires redundant lines throughout the process.

How DACS Work
A digital alarm communicator transmitter (DACT) is required to be connected to the public switched telephone network (PSTN) ahead of any customer-owned equipment. The connection needs to be on loop-start POTS telephone lines. (POTS is a telephone company acronym meaning plain old telephone service; i.e., standard telephone numbers.) DACTs are required to seize the telephone line and disconnect any other use of the line using a RJ-31X jack provided by the telephone company. DACTs cannot be connected to party lines or pay-phone lines. DACTs need to get a dial tone, dial the digital alarm communicator receiver (DACR) at the supervising station, get verification that the DACR is ready to receive, transmit the signal, and receive acknowledgement that the DACR has received and understood the signal.


Originally, the requirement for a DACS was that two telephone lines needed to be used. However, by 1996, the technical committee had changed that requirement so that only the primary means of communication needed to be a POTS loop-start telephone line. The secondary means of communication could be another phone line, a cellular phone, a one-way radio system, a derived local channel, a one-way private radio system, a private microwave radio system, or a two-way RF multiplex radio system.4


The traditional way of connecting a DACT to the fire alarm control panel uses two end-to-end copper POTS loop-start telephone lines, connected to the fire alarm system via an RJ-31X jack, as shown in Figure 1.


With telephone companies in the United States rapidly replacing copper telephone lines with fiber optic lines, the infrastructure that existed when DACS were originally approved has changed. Use of telephone company fiber optic lines should cause no technical problems with signal transmission, but there is one drawback secondary power. Where standard copper telephone lines are powered by the telephone company central office (at least 96 hours of standby backed up by storage batteries, generators and sufficient diesel fuel supply), telephone fiber optic lines are powered by eight-hour standby batteries located in the field (on the poles or in the street pedestals), which is a problem because the telephone lines are only verified by the fire alarm control panel at 24-hour intervals.


Figure 2, compared with Figure 1, shows that except for the transmission method off premises, there is no significant difference between telephone company copper and telephone company fiber optic.


The third option today is to use cable company telephone service. When the cable industry first began providing voice telephone service, there was a technical problem with the CODEC used for voice compression by the cable industry.


Since then, the major cable providers have modified their software to emulate the telephone company, so from a technical transmission viewpoint, there is no significant difference. The issues involved with cable company telephone service are essentially those of standby power. Like the telephone company fiber optic service, cable company standby power supplies are in the street pedestals, and are only sized for eight hours of service, so the line verification issues are the same. The added problem with cable company telephones, however, is that there is a cable box located at the protected premises that requires building power to operate. The loss of AC power at the protected premises may de-energize the cable box, leaving no telephone service. If there is a UPS connected to the box, the UPS is generally sized for less than eight hours of service, which exacerbates the line verification issue. In addition, there is a single point of failure between the cable box and the point of demarcation to the cable company, which, if compromised, takes out both telephone lines provided by the cable company.


Figure 3 shows the typical installation of a cable telephone connection to the fire alarm control panel. There are two issues not present with the traditional telephone company installations. First, there is a power requirement for the cable box. Additionally, there is a single point of failure between the cable box and the demarcation to the cable company.


There are currently several communication technologies listed to either the one-way or two-way radio legacy methods in NFPA 72.1 The most promising of these radio methods uses one of two technologies, which have been shown to be reliable and, since they provide heartbeats on a regular basis, will indicate loss of channel immediately. The two technologies are mesh radio and GSM cellular radio.


The Future of Fire Alarm Communications
There were no changes in technology incorporated into the 2010 edition of NFPA 72. The changes in this edition were housekeeping in nature, removing obsolete technologies and reorganization of the remaining technologies.


On Dec. 21, 2009, AT&T petitioned the Federal Communications Commission to eliminate the landline telephone system. In their petition, AT&T stated that with each passing day, more and more communications services migrate to broadband and IP-based services, leaving the public switched telephone network (PSTN) and plain old telephone service (POTS) as relics of a by-gone era. Obviously, this does not auger well for the future of land line-based telephone services, and when this happens, it will spell the official end of the digital alarm communicator as well.


In January 2011, the NFPA Technical Committee for Supervising Stations Fire Alarm Systems approved two proposals that, if they are accepted through the NFPA process, will also have an impact on the future of the DACT. First, there was a proposal to drop the requirement for a second telephone line, making the alternate communications method another technology. This will essentially kill the DACT, since all other communications methods in the standard are stand-alone methods. Second, another proposal will change the timer test for DACTs from 24 hours to six hours, to compensate for the change to an eight-hour standby power supply.


Art Black is with Carmel Fire Protection Associates.



  1. NFPA 72, National Fire Alarm and Signaling Code, National Fire Protection Association, Quincy, MA, 2010.
  2. NFPA 72, National Fire Alarm Code, National Fire Protection Association, Quincy, MA, 1993.
  3. NFPA 72, National Fire Alarm Code, National Fire Protection Association, Quincy, MA, 1999.
  4. NFPA 72, National Fire Alarm Code, National Fire Protection Association, Quincy, MA, 1996.