Dear Seema,
Usually the Notice Period is kept same for all employees, but if you want you can keep different Notice Period for Different Class of employees. Please keep one thing in mind, the variation in Notice period must not create differences across various department.
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 Operation and you are also putting an official stamp by giving a different Notice Period.
I agree that in all Companies some department are more important than the other but we should never put an officially admit the difference. Isn't it?
Regards,
SC