The nation’s four largest wireless companies are all building extra network capacity for the inauguration of President-Elect Barack Obama on Jan. 20 next year.

Verizon Wireless, soon to be the largest company when its acquisition of Alltel is complete, declined to discuss its plans. However, AT&T, Sprint Nextel and T-Mobile USA all made formal announcements.

The event is expected to draw more than 1 million people, possibly multiple millions, to the National Mall – more than any other event, from New Year’s Eve in Times Square to the Superbowl.

AT&T said it’s spending $4 million to boost network capacity for the event. The company is planning for an 80% boost in 3G capacity near the mall and parts of the Georgetown neighborhood, along with a 69% increase for its 2G network. There also will be two portable cellular base stations and an assortment of extra T-1 connections and nodes. AT&T is staffing the inauguration with 60 extra employees and 11 in-building systems.

Sprint said it will have 40% extra capacity for its CDMA network, 90% extra capacity for its iDEN network, a real-time monitoring team to handle unexpected issues and 25,000 Nextel push-to-talk handsets available for rent to public sector workers and the media. Sprint is installing nine in-building systems and extra coverage at major transportation hubs.

T-Mobile said it’s adding capacity equal to 100 extra cell sites and adding unspecified extra capacity for SMS and MMS.

Yet despite the best of carrier plans, “CTIA… and its member companies advise residents and visitors to be prepared for delays when attempting to use mobile devices during the days’ events due to network congestion,” the D.C.-based industry association said.

“The wireless industry is preparing for record traffic during the Inauguration, and companies are undertaking extraordinary efforts to expand their networks’ capacities,” CTIA CEO Steve Largent continued, in a statement yesterday. “But it’s important for the public to understand that there will likely be some delays. Just as restaurants, trains and highways have maximum capacity limits, wireless networks have also been built to meet the needs of a large, but limited amount of people.”

The CTIA advises people to text, not talk; to wait before sending pictures; and to make social plans ahead of time rather than relying on mobile communications.