From Baltimore to San Francisco, computer experts are scrambling to uncover the cause of the phone outages that have hit millions of people in recent weeks.
Part of the problem in finding a quick answer to the outage problem lies in the complexity of the network: Known as "Signaling System 7" (SS7), it is an intricate web of computers, relay stations and transit sites.
First installed by AT&T in the mid-1970s, SS7 is used by all seven regional Bell phone companies.
SS7 is intended to replace the old network of independent, electronic switches. And it will be upgraded for years to come as new technology -- software and hardware -- becomes available.
In an SS7 configuration, supercomputers known as "signal transfer points" (STPs) send out electronic "feelers" across the network to check to see if lines are free before telling electronic switches to send calls through. Electronic switches, which actually handle calls, take their cue from the STPs.
As the "brains" of the network, STP sites are crucial to the performance of the entire, surrounding network.
In the case of the Baltimore outage, for example, the failure of two sets of STPs -- each STP has a mate -- shut down 600 switches across the region.
Prior to SS7, electronic switches worked independently and did not rely on a central STP for information on how to route and sort calls. As such, the failure of one switch did not adversely affect others.
Because of SS7, it takes less time to place a call -- milliseconds instead of seconds -- and phone companies don't tie up precious network lines with calls that can't be completed.
Other advantages of SS7: Companies can now offer enhanced services like Caller ID and Call Waiting.
SS7's superior call-routing functions also help cut down on toll fraud, which ultimately helps hold down local phone rates.