A wireless telephone network maintained by Cingular Wireless LLC was knocked offline for much of the weekend, leaving about 900,000 mobile phone customers nationwide without service.
The outage stretched from Friday afternoon until Sunday morning, preventing incoming and outgoing mobile phone service and access to voice mail for about 4 percent of Cingular's 22 million customers, according to Cingular spokesman Clay Owen. The service outage hit customers using Cingular's GSM (Global System for Mobile Communication) network in the South, Northeast and Midwest regions of the U.S., Owen said.
The massive network failure was the first widespread outage since Cingular began building a national GSM voice and data network on top of its older TDMA (time division multiple access) network in 2001, Owen said.
Only the newer GSM "overlay" networks were affected. Cingular-owned GSM networks in California, Nevada, Washington state and the Carolinas that predate the overlay network use different network infrastructure and were unaffected by the weekend outage, Owen said.
While Cingular could not comment on possible causes for the blackout, the company is investigating the failure of an HLR (Home Location Register) in Atlanta, Owen said. HLRs are databases of mobile phone subscribers that contain account and billing information as well as phone preferences. The HLR databases are maintained by the cell-phone carrier and play a key role in call switching on GSM as well as TDMA phone networks.
Following the failure of an HLR in Atlanta, Cingular responded by routing call traffic through a Chicago HLR, Owen said.
Rerouting HLR traffic is a standard procedure in the event of the failure of an HLR site, Owen said. He could not comment on whether the traffic routing through Chicago contributed to the widespread service outage there, but said that traffic volume is lower over the weekend, so rerouting traffic from Atlanta should not have caused problems for Chicago customers.
Despite rumors on the Internet about a computer virus bringing down the HLR, Cingular had no evidence that computer viruses, worms or hacking played a role in the outage, Owen said. Customers who were unable to make calls over the weekend are entitled to receive credits for the downtime, he said.
Cingular encourages customers who were affected by the blackout to contact Cingular customer support or walk into one of the company's retail outlets to find out more about having their account credited. The company's marketing department is also looking into whether blanket credits should be assigned to customers in affected areas, but has not reached a decision about that, Owen said.
The standard for mobile communications in Europe, GSM is less common in the U.S., where Cingular is in the process of building a separate national GSM network on top of its TDMA network.
The network is 75 percent complete and will be 90 percent complete by the end of the year. In the meantime, Cingular is slowly migrating customers from its TDMA to its GSM network, encouraging them to take advantage of "cool" features like camera phones and other data services that GSM makes popular, Owen said.