A remote site stores data received from a currently-used site and transfers the
stored data to a proximal site. The proximal site stores the data received from
the remote site and transfers the stored data to the currently-used site. The currently-used
site crosschecks the data returned from the proximal site against the data transmitted
to the remote site, and when the two data do not match each other, transmits the
data to the remote site again.