Distributed antenna system (DAS) networks for public safety communications and for commercial cellular systems have much in common. But Minfei Leng, manager of systems engineering and applications engineering for Bird Technologies, says that, as with all modern technologies, there are many differences worth taking the time to understand. And he proved his point with his presentation, “Understanding NFPA Code for DAS,” at the 2017 Network Infrastructure Show, a part of the International Wireless Communications Expo.
Leng is a principal on the technical committees of the National Fire Protection Association’s NFPA 72: National Fire Alarm and Signal Code and NFPA 1221: Standard for the Installation, Maintenance and Use of Emergency Services Communications Systems.
Leng said that, as with most regulatory requirements, the need to meet the NFPA 72 and 1221 codes increases business activity. In a simplified form, the figure on this page shows how a converged DAS for both public safety and cellular communications would be configured. For public safety, a bidirectional amplifier, and for cellular, a repeater, would feed a headend. Some larger cellular systems might use a direct connection with the base stations, Leng said. Because of the number of radio-frequency (RF) bands involved and the differences in their positions in the RF spectrum, Leng said the designer of such a converged DAS would have virtually no choice but to use fiber-optic cable, which in turn would require the system to convert all of the RF signals into light waves to carry them to the remote units.
Although the NFPA code applies in most of the United States, Leng said that the International Code Council’s International Fire Code (IFC) applies in many jurisdictions, mostly concentrated in California, Oregon and Washington. He said the two codes are highly similar, with differences involving the percentages of area signal coverage required and where, and the duration of backup power required. “The bottom line is that NFPA and IFC requirements only apply to public safety communications,” Leng said. “Right now, they’re not concerned about the commercial side because it serves your convenience, whereas public safety communications is life-critical for the community — people’s lives are at stake.”
For cost and technology reasons, much public safety DAS uses an active headend that picks up the signal from the air, amplifies it, and feeds it through couplers and either leaky coaxial cable or ordinary coaxial cable connected with antennas. With such a system, Leng said, all the components that carry the signal after it leaves the booster are passive. In contrast, a commercial DAS uses fiber-optic cable with active components at the headend and in every remote unit, which Leng likened to signal boosters. Because the NFPA code applies to all RF-emitting active devices, everything on a converged DAS falls under the code jurisdiction.
“Public safety DAS has been around since the 1970s. But in the past 10 to 15 years, since the watershed moment of the 9/11 terrorist attacks in which many lives could have been saved if the World Trade Center had had a functional DAS, we have seen a bigger push for public safety DAS,” Leng said.
NFPA publishes updated versions of its NFPA 72 code every three years. Leng said the latest version from 2016 omits detailed requirements on DAS, which it previously included. Instead, the code makes reference to NFPA 1221 for the details.
Leng reported that a new requirement as of 2016 defines feeder and riser cable and their fire rating. In a building, the riser cable makes a vertical run between floors. Feeder cable extends horizontally. “Think about a tree — riser cable is like the trunk, and feeder is like the branches,” Leng said. The NFPA code requires that cables rated as plenum cables must be used for risers and feeders. It further requires that risers must be routed through a two-hour-rated enclosure. Passage of the feeder cable in and out of the two-hour-rated enclosure must be fire-stopped to two-hour ratings.
“The idea is you do not want a fire on one floor spreading to the next floor,” Leng said.
The NFPA code covers interference and signal degradation, requiring that no system capable of operating on frequencies or causing interference shall be installed without prior coordination and approval of the authority having jurisdiction. It says that the building owner or manager shall suspend and correct installations that degrade the performance of the public safety radio system. Moreover, the code says that systems that share infrastructure with non-public safety services shall ensure that the coverage and performance of the public safety channels are not degraded, regardless of the amount of traffic carried by the non-public safety services.
Leng said that in this way, the NFPA acknowledges that some systems exist that put public safety and non-public safety (cellular applications) together. “It essentially means public safety has their own protected bandwidths, their own protected capacity,” he said.
The NFPA code limits group delay, a technical term that refers to how long a signal is delayed as it goes through the DAS. “We’re talking about microseconds,” Leng said. “To the human mind, microseconds of delay doesn’t mean much; but to a radio, it means a lot. If a portable public safety radio receives a delayed signal through the DAS and it also picks up a signal directly from the macro network, and if the delay between the two is more than about 25 microseconds, it will cause a reception problem. The code doesn’t specify the maximum amount of delay. It lets the authority having jurisdiction decide what’s acceptable.”
— Minfei Leng, manager of systems engineering and applications engineering for Bird Technologies
Since the 2016 version of the code, the NFPA has required that the DAS deliver 99 percent floor area coverage for critical areas, which include fire command centers, fire pump rooms, exit stairs, exit passageways, elevator lobbies, standpipe cabinets, sprinkler sectional valve locations and other areas deemed critical by the authority having jurisdiction, with 90 percent coverage for general building areas.
Another departure from past code versions that Leng identified has to do with signal strength. Versions of the code previous to 2016 defined the minimum measurable RF signal as −95 dBm. In 2016, the code said that the signal has to be strong enough to provide a minimum audio quality of 3.0, which is defined as speech understandable with a slight effort, and which requires occasional repetition because of noise and distortion. Elsewhere in the code, the text recommends an audio quality of 3.4.
In Leng’s view, this means the code shifted from using a hard number of −92 dBm that can be measured on test equipment and instead uses a subjective audio quality specification. He said people have debated which is better.
“With a specification of −95dBm, you can’t get away having less signal,” Leng said. “It’s a clear measurement in black and white. But a −95 dBm signal in some cases is more than enough. In a noisy RF environment, it may not be enough. On the other hand, definition of audio quality is subjective. It depends on who makes that judgment — an 80-year-old who wears a hearing aid versus a 20-year-old, for example.”
The NFPA code addresses another technical aspect of a DAS: antenna isolation. “The idea is that you have your antenna on the roof,” Leng said. “It talks back to the base station with antennas within the building, talking to a portable. You want to make sure the signal coming out the inside-of-the-building antennas does not go back into the roof antenna. Otherwise, the DAS goes into a ring-around situation.” The code sets the isolation at minimum of 20db above the signal booster gain under all operating conditions.
Leng identified a key change in the 2016 code on component enclosures. The fire code says that every active component — including repeaters, transmitters, receivers, signal boosters, filters and battery systems — must be contained in enclosures rated NEMA 4 or NEMA 4X by the National Electrical Manufacturers Association.
An enclosure meeting the NEMA 4 rating has no fans or vents, rendering it dust proof. It doesn’t have to be waterproof (able to withstand submersion), but it has to be able to prevent water ingress when subjected to the spray from a fire hose. The additional X rating for NEMA 4X means the enclosure also resists corrosion. “Typically, a NEMA 4 box will be made of plain steel or aluminum, and a 4X box will be made of stainless steel,” Leng said.
Prior editions of the code did not allow the use of external filters, specifying that everything had to be integrated into one enclosure. The 2016 version implicitly allows external filters because it says external filters have to be in a NEMA 4 box.
“You could argue that the filter is probably more important than the amplifiers,” Leng said. There are people who want to save money or cut costs. They will buy a cheap $200 booster. Then, they go to a more reputable manufacturer and try to buy the filters, basically making their own signal boosters. The old code doesn’t allow that, and for a good reason. But the new code starts to allow it. It is a concession, so to speak, to the convergence of public safety and commercial because if you ever see a fiber DAS for cellular, you will see that their remote units are probably no bigger than a slide projector.”
The reason the filters are so compact is that the cellular frequencies are all broadband, Leng explained. The nature of the cellular systems seldom gives their operators any reason to worry about interference or transmit-receive desensitization. But Leng said that in a fiber DAS with either VHF or UHF public safety signals, there’s no way that a compact, broadband filter would be adequate.
“We make some of the VHF filter cavities,” Leng said. “For a system that has to filter a very challenging guard band, some of the cavities we make are 6 feet tall and 10 inches in diameter.” Leng said it is obvious that it would be ridiculous to attempt to design a remote unit that could include such cavities in a single enclosure. Thus, the change in the code about using external filters is a concession to the need for such large filters in some systems.
The NFPA code says a public safety DAS has to have a primary power source from a dedicated circuit, which means the connection has to be hard-wired, not plugged into a wall socket. The code requires a secondary power source, such as a storage battery or a fuel cell.
Leng said the code further requires the use of a supervisory signal for all of those components to indicate antenna malfunction, the failure of active RF emitting devices that would include the headend and any remotes and battery capacity that falls to 30 percent. If the AC power fails and the DAS switches to DC power, it has to send an alarm signal. If the battery charger begins to fail, the supervisory equipment has to send an alarm.
Referring to the building in which he was speaking, Leng said the Las Vegas Convention Center has more than 2,200 Wi-Fi access points in its DAS, and if it’s a converged public safety and commercial wireless carrier DAS, it would have to have an alarm for each one. Leng said that providing a backup battery to each access point would send the system operator’s cost through the roof.
Leng said that the authority having jurisdiction still can cite NFPA 72, noting that the code has been modified and will continue to be modified to keep up with market trends and advancing technology. He said the code requirements for building owners to provide a signal distribution system to support public safety communications is, in effect, a tax on business. “It’s for a good reason, you can argue, but it’s still a tax,” he said. “It adds to the cost of business for the building owners.”
Referring to the role played by NFPA, Leng said, “We try to be sensitive about where to enforce code requirements. But ultimately, it’s for the benefit of everyone.”
Don Bishop is the executive editor of AGL Magazine. He joined AGL Media Group in 2004. He was the founding editor of AGL Magazine, the AGL Bulletin email newsletter (now AGL eDigest) and AGL Small Cell Magazine. He is a frequent moderator and host for AGL Conferences. Don writes and otherwise obtains editorial content published in AGL Magazine, AGL eDigest and the AGL Media Group website.