Akshay,
You will find the answer to this question in the PMBOK Guide itself - particularly in the introduction to the knowledge areas. For instance, this is from Time:
On some projects, especially those of smaller scope, defining activities, sequencing activities, estimating activity resources, estimating activity durations, and developing the schedule are so tightly linked that they are viewed as a single process that can be performed by a person over a relatively short period of time. These processes are presented here as distinct processes because the tools and techniques for each are different.
So the question isn't WHO does it but WHAT are they doing, HOW are they doing it and WHAT skills, T&T etc. are being used.