Most servers have an "idle time" value that specifies how long a user's session can last when no activity is detected. When the user exceeds the time limit, the server connection is closed. To prevent the server from closing the connection due to inactivity, you can configure the FTP Client to send "keep alive" commands.
To preserve a connection to the server
Open the Site Properties dialog box and then click the Connection tab.
Select Send keep alive every <n> seconds and specify how many seconds to wait in between sending consecutive keep alive (NOOP) commands.
NOTE:This setting has no effect on SFTP connections. For SFTP connections use Server Keep Alive on the General tab of the Secure Shell Settings dialog box.