Researchers identify why 911 calls are delayed, failed, or dropped

An estimated 240 million calls are made to 911 in the U.S. each year, with wireless devices accounting for 80% or more of these calls. Making sure emergency calls connect and stay connected to 911 dispatchers is essential and could have life-threatening consequences. MSU researchers found that in certain situations, 90% of emergency wireless calls failed to connect to 911 within 120 seconds but have developed an app to correct this.

Guan-Hua “Scott” Tu, an associate professor in the College of Engineering, and his team observed that despite sufficient wireless signal coverage, users may still experience prolonged emergency call setup times, call initiation failures, or call drops.

“The systems and networks that support cellular emergency services are a double-edged sword,” said Tu. “While they allow for widespread access, they are numerous and complex in their interactions, leaving cellular emergency services prone to errors.”

Regulatory authorities like the Federal Communications Commission or FCC in the U.S. and standard organizations such as Third Generation Partnership Project or 3GPP have specifications to enhance the availability and effectiveness of these cellular emergency services. The FCC requires carriers to transmit all 911 calls to a Public Safety Answering Point or PSAP such as a 911 call center, regardless of whether the caller subscribes to them or not. 3GPP allows wireless callers to access emergency services across cellular and Wi-Fi networks.

The researchers discovered that in locations with weak cellular signals but strong Wi-Fi signals, the problematic network selection can prevent 90% of emergency calls from reaching PSAPs within 2 minutes, compared to just 5.85 seconds for non-emergency calls in the same locations.

“These locations are more common in indoor environments,” said Yiwen Hu, a Ph.D. candidate working with Tu. “Additionally, emergency call failures and drops occur when the user is moving, even with sufficient wireless coverage, due to restrictions in emergency services.”

To identify potential design defects that lead to failures or delays in emergency services, Tu and Hu along with Li Xiao and University Foundation Professor Jiliang Tang collaborated with researchers from Purdue University and National Yang Ming Chiao Tung University in Taiwan to develop the M911-Verifier, an emergency-specific model-checking tool.

Tu’s team also developed Emerg-Call-Dialer which is a free app that phone users can download. It helps address issues with cellular and Wi-Fi networks and prevents unnecessary long emergency call setup times.

“In our study, most emergency-specific designs function properly,” Tu said. “Our goal is not to criticize emergency service designs but to push for improvements that make them more reliable.”

The research is published in the journal Proceedings of the 30th Annual International Conference on Mobile Computing and Networking in the Association for Computing Machinery’s digital library as part of MobiCom 2024.