Listening Through Walls
U.S. PUBLIC SAFETY agencies continue to wrestle with the problem of communications interoperability—the ability of different agencies and jurisdictions to talk to each other when they are using different radio systems. Meanwhile a simpler, equally dangerous problem threatens first responders when they work in many larger structures: the challenge of communicating in emergencies through the dense infrastructure of buildings’ stairwells, parking garages, and basements.
The revolution in consumer wireless technology that helps keep the civilian world connected anytime, anywhere, has provided a fix: distributed antenna systems (DAS). With that technology’s arrival, local building authorities are increasingly requiring that new structures provide first responders with robust communications throughout large buildings. This year, the National Fire Protection Association (NFPA) fire code, which many states and localities adopt, includes guidelines for minimum in-building capability.
DAS is what its name implies: a network of interconnected antennae. What the network provides is the ability to actively collect and transmit signals within an enclosed space, while transmitting them to and receiving them from outside networks either over the air or along cable. While WiFi systems operate within narrow bands of wireless spectrum and use computer-style Ethernet cable, DAS has made-to order frequency spectrum capacity and usually relies on fiber optic or television-style coaxial cable for added bandwidth.
In cities and towns where building codes now require building-wide publicsafety radio coverage, codes typically apply to buildings over a certain size. The regulations often set minimum coverage requirements, typically 95 percent, usually for specific areas of buildings, such as stairwells and underground areas. NFPA recommends 99 percent coverage in critical areas—including emergency command centers, stairwells, and exit passageways—and 90 percent coverage everywhere else.
In towns such as Folsom and Roseville, California; and Grapevine, Texas, authorities require up to 12 hours of backup power for the systems, according to the National Public Safety Telecommunications Council (NPSTC).
Most critically, the codes typically mandate specific radio frequencies that the buildings must support, such as the ultra high frequency (UHF) band used by public- safety personnel in Boston and the higher 800 MHz band more recently adopted in many other jurisdictions.
Currently, DAS with public-safety-frequency capacity is most common in large public venues, such as airports, that have recently added wireless data services for the public, says John Spindler, vice president of product development for telecommunications firm ADC. The company installed systems at several large U.S. facilities, including Dulles International Airport outside Washington, D.C., and Atlanta’s Hartsfield-Jackson International Airport.
While DAS is notorious for high cost— up to $1 per square foot—Spindler says that several variables, such as the number of antennae needed, affect prices, and rates can run as low as five or ten cents. Large halls in buildings like airports or convention centers might only require a single antenna, while dense structures like office buildings would require far more for the same area.
Frequency capability is also a factor: the higher the frequency, the smaller each antenna’s range.
Cost will obviously be higher where the decision-makers choose to implement redundant systems. Some building managers choose to install dual systems—one for day-to-day wireless, the other for public safety in emergencies—either due to code requirements or the client’s desire for redundancy.
Other relevant local building codes can also push prices higher. Las Vegas, for example, a national leader in safety codes, requires that all wire is run through conduits—essentially pipes—to mitigate fire risk, Spindler notes.
For multitenant buildings, the expense can be addressed through a cost-sharing agreement in which the system vendor charges each tenant separately, Spindler says.
In large public venues, wireless providers may fund or contribute to installation of DAS systems if it means improved coverage for their customers, says Wendy Chretien, senior network systems consultant with Elert & Associates in Stillwater, Minnesota.
Given the lack of standards, civilian customers may have interoperability problems between their devices and the network, but the systems’ public-safety components will not be affected because of the relative simplicity of public-safety transmission, Chretien says. Standards may be coming, however. The U.S. National Institute of Standards and Technology is working to develop standard metrics for DAS performance, says NIST spokeswoman Evelyn Brown.