Nanotechnology is enabling the development of devices in a scale ranging from one to a few hundred nanometers, known as nanomachines. How these nanomachines will communicate is still an open debate. Molecular communication is a promising paradigm that has been proposed to implement nanonetworks, i.e., the interconnection of nanomachines. Recent studies have attempted to model the physical channel of molecular communication, mainly from a communication or information-theoretical point of view. In this work, we focus on the diffusion-based molecular communication, whose physical channel is governed by Fick's laws of diffusion. We characterize the molecular channel following two complementary approaches: first, we obtain the channel impulse response, transfer function and group delay; second, we propose a pulse-based modulation scheme and we obtain analytical expressions for the most relevant performance evaluation metrics, which we also validate by simulation. Finally, we compare the scalability of these metrics with their equivalents in a wireless electromagnetic channel. We consider that these results provide interesting insights which may serve designers as a guide to implement future molecular nanonetworks.