Hi Ravi,
With SAP ECC EHP 6 you can directly connect ECC and BO , without implementing BW, Still you can use BEX for creating BO reports.
While implementing BO on ECC you can directly report on ABAP Functions, Infosets and ecc Tables(using CR).we won't need to do modelling in BW, as you can consume ABAP logic directly in BO.
please see the following link: