also determines whether a file that it previously considered to be active has been deleted. With this information, the backup server can reproduce the set of files that were on the backup system as of the most recent or previous backups. Deleted, or inactive , files are retained for a specified length of time before being permanently removed from the backup server. 2.4.6 Multilevel incremental A multilevel incremental backup is the same as an incremental backup, except it is backing up all files that have been modified since the last n-1 level backup. With multilevel incremental backup, a full backup is considered a level 0 backup. Multilevel incremental backups take advantage of both incremental and differential backups. 2.5 Backup and restore window and scheduling When planning for backup, consider the backup and restore window and when to schedule the backups.