Dear Seema,
Usually, the notice period is kept the same for all employees, but if you want, you can keep a different notice period for a different class of employees. Please keep one thing in mind: the variation in the notice period must not create differences across various departments. For example, suppose you feel that Marketing is a critical skill set category and you keep a notice period of 3 months; on the other hand, in Operations, you keep a notice period of 1 month. This might give rise to grievances, as people will perceive that to the company, Marketing is more important and valuable than Operations, and you are also putting an official stamp by giving a different notice period.
I agree that in all companies, some departments are more important than others, but we should never officially admit the difference. Isn't it?
Regards,
SC