Dear Sis, what NW does is nothing else but to remotely execute the appropriate backup command on the NW client, in this case the appropriate RMAN command/script. NW has been designed to start backups on a scheduled base, not on a specific criteria. These backups should run so often (in smaller than daily intervals) that the disk space will never be filled. You can configure a 'probe-based backup' to execute a backup but in general this is nothing else what I already suggested. There is no way to configure this from the NW Admin GUI.    
I have writer a script to monitor the archive log usage space on oracle, when i detect the archive log space is low, i want trigger the archive log backup job of networker. how can i do?
how to trigger the archive log backup of networker from client???