Thank you for the suggestion.

1. Listener is not communicating properly. If so stop and start your listener.
= Listener is runningup, altough I stop/start it again. I can query from other box.
2. Check to make sure that no other process/script that would take down the catalog database when the RMAN backup starts....
=There is no script that take the catalog down. I tried even at the time when there was no activity at all.

3. I would also check that I'm able to just open and close all those channels without doing any backup.
= I tried that and it was worked fine

4. check sbtio.log under target db's udump
=there is no log entery in sbtio.log

Appreciate if anyone has any suggestion....

abhijit