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Changing Call Patterns Leading to Overloading of e911 Systems during Emergencies

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November 04, 2011

Changing Call Patterns Leading to Overloading of e911 Systems during Emergencies

By Anshu Shrivastava, TMCnet Contributor


Changing call patterns, including increased traffic on wireless networks, have led to higher risks of overloading 911 systems during weather or other emergencies, resulting in abandoned or unanswered calls, according to a new report published by ATIS.


This year in May, the Federal Communications Commission asked ATIS (News - Alert) to review why certain trunk lines serving PSAPs were being removed from service during heavy calling volumes.

To both telecommunications service providers and Public Safety Answering Points (PSAPs), the association’s Network Reliability Steering Committee (NRSC) made several recommendations for improvements to reduce 911 outages.

The NRSC completed its analysis with the publication of the “911 CAMA Trunk Throughput Optimization Analysis (ATIS-0100034) report.”

Noting that 911 communications links are vital, particularly during emergencies, Susan Miller (News - Alert), president and CEO at ATIS, said that there are important steps that service providers and PSAPs can take to prevent emergency communications networks from becoming overloaded and failing.

Miller said that ATIS’ report outlines such steps, and she hopes that 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. In addition to recommending that service providers modify their selective routers to prevent complete trunk groups from going out of service due to double wink failures, the report also recommends that PSAPs and service providers work together to develop overflow routing to backup PSAPs during high call volumes.

Additional recommendations include increased communications between service providers and PSAPs during high volume periods to minimize impact; and updated procedures that would allow PSAPs to handle more calls during high volume periods.

"We appreciate the hard work of the NRSC on the 911 CAMA trunk overload issue,” said John Healy, chief statistician of Public Safety and Homeland Security Bureau at the FCC (News - Alert), adding that the collaboration should help prevent major 911 outages in the future and is a major step in improving the overall reliability of 911 networks.

PSAPs receive emergency calls. According to the report, it’s seen that at times of heavy volumes, “wink” failures can occur between service providers’ selective routers and PSAPs that temporarily disrupt trunk lines.

A short signal, the wink is carried over the network at the beginning and end of calls. The time needed, during a high call volume overload to 911, for the PSAP’s customer premise equipment to be ready for the next 911 call can exceed the maximum call setup time resulting in the PSAP’s equipment not providing a wink.

The report notes that 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. In case the 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, which may lead to a cascading effect.

The cascading effect takes several or all trunk lines connected to local PSAPs out of service, which happened during an East Coast blizzard in January 2011, the report says.

ATIS is a technical planning and standards development organization. Last month, the organization responded to the FCC’s Notice of Proposed Rulemaking regarding emergency location information requirements for wireline and wireless communications services.


Anshu Shrivastava is a contributing editor for TMCnet. To read more of Anshu’s articles, please visit her columnist page.

Edited by Juliana Kenny







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