Netbackup 7.1 NDMP on Dell FS7500

We have close to 7TB of NAS volume , we do backup using NDMP . Having issues with level 1,2,xx when doing the backups , we do however can get a full backup without any issues. Our config is simple we don't have any variable set nor these parameters set -

-SIZE_DATA_BUFFERS
-NUMBER_DATA_BUFFERS
-SIZE_DATA_BUFFERS_DISK
-NUMBER_DATA_BUFFERS_DISK
-NET_BUFFER_SZ
-NET_BUFFER_SZ_REST

How can we optimize NDMP backups , do we need to have set Type =TAR and other variables set ? How can we exclude files and directories ? ( .EXE , .ISO etc). How can we get the backups completed faster ?