Assessing Officer is under obligation to issue a notice under section 143(2) for making assessment or reassessment as case may be; in case Assessing Officer has not done so, order framed under section 143(3) read with section 147 becomes invalid. Assessing Officer has no power to issue notice under section 143(2) after expiry of 6 months from end of financial year in which return has been field. Any assessment made based on notice which itself is not valid will also become void ab initio. Therefore, where notice under section 143(2) had been issued beyond prescribed time, assessment order framed under section 143(3) read with section 147 would become invalid.  Where mandatory notice under section 143(2) was issued beyond statutory time limit prescribed, provision of section 292BB would not extend any benefit to revenue.(AY. 2011-12,2012-13)