The most serious threat to wireless networks is call volume. Wireless architecture has enough redundancies built in that a failure along any point wouldn't significantly cripple an entire network.

The most serious threat to wireless networks is call volume. Wireless architecture has enough redundancies built in that a failure along any point wouldnt significantly cripple an entire network.

If one cellular base station or antenna - a node of the network that receives calls - were brought down, wireless phone and network devices would still be able to communicate with another one nearby. "Typically, your mobile phone is talking to from two [cells] to six cells," said Jim Freeburg, Lucent Technologies director of wireless architecture.

In addition, most wireless operators own spare cell sites, called cells on wheels. Carriers typically use them when they expect a surge in volume on a portion of the network, such as during major sporting events, but they can also be used to quickly replace a malfunctioning site.

Cell sites are connected to base station controllers and then to a switch, which connects the wireless network to the wireline public switched telephone network. Because as many as 25 to 50 cell sites might link to one controller, a failure at the controller could be damaging to a network. However, some wireless operators build their networks so that every other cell site in an area is connected to the same controller, so if one fails, the network can at least offer spotty coverage, said John Touvannas, senior product manager of Motorola.

Lucents gear combines the controller function and the switch, but most of its customers can reroute traffic to a different switching facility if one fails, Freeburg said.

In todays circuit-switched wireless networks, unusual call volume will always harm network performance as too many users compete for a limited number of circuits. But some mobile voice operators now using circuit-switched technology said they plan to move toward a packet-based network similar to that used by the Cingular Interactive data network.

"Its the difference between circuit-switched, where you need to have a direct connection and you only have so many circuits, versus packet, where data gets through in order of priority," said Jesse Perla, CTO of MobileQ, a developer of software for mobile e-mail and Internet access.

Motorola has developed an IP radio access network that it says will make networks more reliable. In such a configuration, all components, including the cell sites and controllers, are distributed and work as a peer-to-peer network. "If you lose one, the base station isnt specifically tied to one controller. Its routed based on load," Touvannas said.

But not everyone is convinced IP will make networks more robust; Lucents Freeburg is particularly worried that a software problem could spread across all the nodes. "The challenge I see for us is how to maintain the same level of reliability in the new IP networks. They arent automatically more reliable than todays networks," he said.