This article is from the storage FAQ part2, by Rodney D. Van Meter with numerous contributions by others.
It's now possible, in several fashions, to backup systems over a network or even a modem, for recovery from fires and even disk crashes. Channel extenders, such as the CHANNELink http://www.cnt.com/products/clnk/clnk2.htm from CNT and the Symmetrix Remote Data Facility http://www.emc.com/symmdoc.htm, are used by some mainframe systems to create remote copies of disks (remote mirroring) as a disaster recovery measure. Early systems used dedicated fibre or telephone lines and ran proprietary communications protocols. Newer systems from CNT are capable of communicating over general-purpose wide-area networks, thus saving the costs of the dedicated lines. It's also possible to backup PCs over your modem in an incremental fashion, through your ISP; one example is http://www.telebackup.com. Two other companies that do this over the Internet (out of, I believe, more than 30) are Connected Corp., Framingham, MA; Virtual Technology Corp., Minneapolis, MN.