WASHINGTON (AP) -- Concerns about the effectiveness of emergency radios on Sept. 11 have focused on technological flaws, but probers on Tuesday identified another problem: too many people trying to talk at once.
The overwhelming rush of radio transmissions by emergency personnel responding to the World Trade Center may have cut off one-third to one-half of radio calls, according to preliminary findings by the National Institute of Standards and Technology.
NIST is examing the construction and evacuation procedures of the towers.
Communications breakdowns among rescue personnel have been blamed for slowing the evacuation of office workers and rescuers, but most of the complaints have centered around poorly-designed equipment or spotty reception in the 110-story buildings.
NIST investigators, offering an update on their long-running inquiry, said the radio systems ``experienced surge load conditions after the attack.''
The condition existed on systems of the Port Authority, which operated the complex and has its own police force, and the New York City police and fire department personnel which rushed to the scene.
``Traffic volume made it difficult to handle the flow and delivery of information,'' the NIST report said. ``Multiple, concurrent radio transmissions on the same frequency, or doubling, made it more difficult.
``It is estimated that roughly a third to a half of the communications were not complete due to surge load conditions.''
Shyam Sunder, the lead investigator on the project, said it was still too early to say how much of the overall communications problems experienced that day can be blamed on ``doubling,'' which is caused by too many people trying to talk at once on the same frequency.
Shyam said the radio systems were probably not designed to carry the heavy load of traffic caused by such an extreme emergency.
NIST probers have reviewed much of the radio traffic among the Port Authority and some of the city's rescue services, Sunder said.
The group has not yet finished its analysis of all the transponders, devices used to relay emergency radio broadcasts where reception is poor, as in skyscrapers.
In the wake of the attacks, the federal government has pushed local authorities to find ways to allow different responders, like fire, police, and emergency medical personnel, to talk effectively to each other.
NIST's initial results suggest another danger may lie in allowing too many people to talk at once, clogging frequencies.
Sunder said his group is also seeking additional information on work done in 1964 by the Port Authority showing the towers could survive the impact of a 707 jetliner.
The modern planes that struck the building in 2001 are about 20 percent bigger than a Boeing 707, but the investigator said the Port Authority's analysis is still important because the 1964 scenario was ``strikingly similar'' to the events of Sept. 11, and might lead investigators to determine where the old analysis was faulty.
Sunder said the group has obtained two ``white papers'' referring to the early work, but cannot find the corresponding calculations and analysis, records of which were presumably destroyed in the building collapse.
- On the Net: National Institute for Standards and Technology