How is the placement of NIT, Delhi for M.Tech in CSE? Does every company allow M.Tech students to sit in placement?

Placements are good. See, it depends on the company, if they want a person for the research work then they will only hire M.Tech students and if they don't then they will not hire those students.

It is just below an average since now a company wants a student who not only make a product but also sell it on the market by good marketing planning. That's why now a days a trend of BTECH+MBA is more popular rather than BTECH + MTECH but in my opinion if one wants to continue his her specializatio