chronjob backup

[Peter Mels]

I would like to backup my data to a other Ubuntu box in the same
Network. My one is 192.168.0.2 the backup box is 192.168.0.4
How does that look like in a chronjob and what's happen if the backup
box is not on-line?
There must be also a work around for the password question.

rsync -v -u -a --delete --rsh=ssh --stats data .mozilla-thunderbird 
alpha at 192.168.0.4:/home/alpha/transfer

thanks
Peter