ATIS Report Offers Recommendations to Counter 9-1-1 Call Overload (11/2/11)
Wednesday, November 02, 2011 | Comments

Changing call patterns including increased traffic on wireless networks have led to higher risks of overloading 9-1-1 systems during weather or other emergencies, resulting in abandoned or unanswered calls, according to a new Alliance for Telecommunications Industry Solutions (ATIS) report. The association’s network reliability steering committee (NRSC) made several recommendations to telecommunications service providers and public-safety answering points (PSAPs) for improvements to reduce 9-1-1 outages.

An industry workshop in October addressed the issue of overloaded 9-1-1 systems. The FCC asked ATIS in May to review why certain trunk lines serving PSAPs were being removed from service during heavy calling volumes. The NRSC completed its analysis with the publication of the 9-1-1 CAMA Trunk Throughput Optimization Analysis (ATIS-0100034) report.

"We appreciate the hard work of the NRSC on the 9-1-1 CAMA trunk overload issue,” said John Healy, chief statistician of the FCC’s Public Safety and Homeland Security Bureau (PSHSB). “Our collaboration should help prevent major 9-1-1 outages in the future and is a major step in improving the overall reliability of 9-1-1 networks."

PSAPs receive emergency calls and, at times of heavy volumes, “wink” failures can occur between service providers’ selective routers and PSAPs that temporarily disrupt trunk lines. A wink is a short signal carried over the network at the beginning and end of calls. During a high call volume overload to 9-1-1, the time needed for the PSAP’s customer premises equipment (CPE) to be ready for the next call can exceed the maximum call setup time, resulting in the PSAP’s equipment not providing a wink.

If all the other trunks remain busy, as is often the case during a heavy call volume event, the same call will again be offered to the same trunk. If this second offering results in a no-wink condition, the selective router trunk will be taken out of service in what’s known as a double-wink failure. This could lead to a cascading effect that takes several or all trunk lines connected to local PSAPs out of service, which happened during an East Coast blizzard in January.

“9-1-1 communications links are vital, particularly during emergencies,” said Susan Miller, ATIS’ president and CEO. “There are important steps that service providers and PSAPs can take to prevent emergency communications networks from becoming overloaded and failing. ATIS’ report outlines such steps, and I hope they can be put into place quickly to facilitate the flow of emergency communications.”

ATIS’ report developed multiple recommendations to prevent these types of outages, including:

• Service providers modifying their selective routers to prevent complete trunk groups from going out of service because of double-wink failures.

• PSAPs and service providers working together to develop overflow routing to backup PSAPs during high call volumes.

• Increased communications between service providers and PSAPs during high volume periods to minimize impact.

• Updated procedures that would allow PSAPs to handle more calls during high volume periods. This could include employing more call-takers during expected weather emergencies or shifting focus to handling as many calls as possible rather than returning calls during these volume periods.

The report is available at the ATIS document store at

ATIS is a technical planning and standards development organization and a founding partner for the Third Generation Partnership Project (3GPP).

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