# How to calculate the transmission ( channel ) bandwidth in a digital communications system?

Is it the bandwidth ( BW ) of the individual pulse or should I generate a random process and calculate the BW from its PSD.
In my class we considered it the bandwidth of the raised cosine when we were studying the ISI. but then when we came to the modulation techniques we calculated the PSD of the random process ( and we considered that each bit is a rect not a raised cosine ).
What is the difference between the two approaches and which one is the transmission bandwidth?