In Oracle 10g we had basic compression in RMAN.
But Oracle 11gR2 has added some more compression features.

There are basically four compression levels.

HIGH = unmodified BZIP2
MEDIUM = ZLIB
LOW = LZO
BASIC = BZIP2

If you don't have advance compression but want to use
basic compression you can issue the following command:

Code:
RMAN> configure compression algorithm 'basic';


If you have advanced compression feature then you can issue
any of the following commands:

Code:
RMAN> configure compression algorithm 'high';


or

Code:
RMAN> configure compression algorithm 'medium';


or

Code:
RMAN> configure compression algorithm 'low';



If you want to reset to defaults
then you can issue the following command:

Code:
RMAN> configure compression algorithm clear;