Thank you for your answer. However, there is a reason if want to keep ppp inside /etc/network/interfaces. Indeed, when finally the wlan is started, then the ppp will work, too, without human interference. Otherwise, if I remove the interface ppp, I will have to manually launch "pon dsl-provider"... ok, not a great effort, but I prefer to avoid it.
By the way, I am *sure* that my wlan will never take 1 minute to get up. So, in my opinion, there should be some parameter somewhere allowing the user to de/activate the "2 minutes sleep". Just my opinion.