Clarification: NBU 7.5 Dedupe retentions

Hi all,

Let’s say I have an SLP with the following config:

Backup                 Dedupe STU       FIXED 9 weeks

                Duplication         OFFSITE_DATA_CENTER_DEDUPE_STU     FIXED 9 Weeks

                                Duplication         OFFSITE_DATA_CENTER_TAPE     FIXED 9 Weeks

Let’s say the initial backup is 30GB.  All steps complete successfully. 

Now this runs every weekend and data change is minimal. When the initial backup and duplication expire, will the next backup/duplication job have to dedup the full data amount of the 30GB across the WAN?

Now take it a step further with a large data set of terabytes and small WAN pipe of about15mb.  What should I do so I do not have to resend terabytes of data every 9 weeks? Just set a long retention?

Thanks for any information and clarification!