Critical Infrastructure Warning Information Network
CWIN is used to transmit voice and data on infrastructure protection, communication and coordination, alert, and notification. In the event that a significant attack disrupts telecommunications networks or the Internet, the Critical Infrastructure Warning Information Network is to provide secure capability for communications across key government network operations centers, the private and public sectors, and trusted foreign partners. According to DHS, it is the critical, survivable network connecting DHS with the vital sectors that are essential in restoring the nation’s infrastructure during incidents of national significance.
(Description comes from from General Accounting Office document GAO-07-455)
The GovITwiki Intelligence Agencies page lists the U.S. government's major secret and sensative but Unclassified networks.