Two deadly explosive devices detonated at the Boston Marathon yesterday, slicing the crowd at the finish line with nails and other shrapnel. In the aftermath, frantic onlookers tried to reach their loved ones but encountered difficulties placing cell phone calls. Many looked at their sophisticated phones and wondered how the system could get overwhelmed so easily.

Experts in emergency telecommunications and management say that cell phone logjams like this will likely happen again after a major unexpected event. They also point to alternatives available now and other systems in the works.

“In public wireline and wireless networks, it’s simply not economical to provide enough resources for all the people who are subscribers to talk to all the other people at the same time,” said Stu Lipoff, an IEEE fellow and an engineering consultant for telecommunications companies.

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Instead, service providers usually manage traffic based on average use with limited surge capacity. This is called blocking, Lipoff explained. The probability that your call will get through on a typical day is around 90 percent.

Available circuits filled up quickly in Boston. When cell phones can’t get through they encounter fast busy signals or automated messages, Lipoff said. A rumor circulated that cell service was cut to prevent a bomb from being detonated by cell phone. Wireless carriers later issued statements disputing that report but did say the heavy call volume overwhelmed their towers.

Instead of calling, the carriers recommended their users try communicating via text or email from their phones. While this may seem counter-intuitive, voice communications do require a constant connection. Email messages, however, travel by packet switching. In the short time between one phone call and another, you could send a thousand email messages, Lipoff said.

When wireless companies have time to plan for a surge, they can increase capacity. Lipoff pointed to the annual Consumer Electronics Show in Las Vegas, where cellular base stations were added to make dozens of little cell sites at the convention center. After natural disasters like Hurricane Sandy, quick deployment sites were also brought in on trucks.

Emergency responders such as firefighters, police officers, utilities workers and government employees use non-cellular radio communications networks that are maintained by licensed government agencies, Lipoff said. Improving their effectiveness remains a work in progress but those networks don’t even touch the cellular ones that the public uses.

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“The difficulty is that cell phones are individual, user-to-user,” said Louise Comfort, a professor of public and international affairs at the University of Pittsburgh and director for the university’s Center for Disaster Management. The key is to establish a public network where information can be posted and people can also respond saying they’re safe, she added.

Reverse 911 has a lot of potential to disseminate crucial information about an emergency, but that only works when a community sets it up first. “We all used it last year because the University of Pittsburgh had a series of serious bomb threats that went on for 6 weeks,” Comfort said. “This is something that would have to be done at the local level.”

One relatively new system is the Commercial Mobile Alert System or CMAS that’s been adopted by the Federal Emergency Management Agency (FEMA) and is in trial stages now, Comfort said. The system is designed to automatically send short text-based government alerts to subscribers in scenarios like an Amber Alert or an imminent threat like a tornado.

That system can also give priority to communication with cell phone users who have disabilities or who need extra help during an emergency, Comfort said. Although cellular companies have agreed to carry CMAS and procedures for using it have been developed, the system isn’t widely known yet.

Adding to the complexity around communications, serious marathoners tend to ditch smart phones to run lighter. Perhaps the tiny devices they use to monitor heart rate, speed and distance could have two-way communication added without sacrificing too much weight or adding too much cost, Comfort suggested. The challenge would then be figuring out which network to use.

Yesterday when the public turned to their computers and smart devices for local news online, some sites were slow or down completely. However, compared to wireless phone service, the Internet is far sturdier.

“There may have been an overload in Boston -- or the speed may have slowed -- but usually, given the redundancy in the Internet, it would continue to function,” Comfort said. “The genius of the Internet is that if one station goes down, another node picks it up.”