We focus on the design of the logical layer on an optical infrastructure for three different architectures: fixed grid WDM with fixed-rate multilevel modulation, fixed-grid WDM with time-domain hybrid modulation formats and flexible grid. A fairly detailed physical layer modelling is considered in the network design algorithms. We perform a sensitivity analysis of network and physical layer parameters, such as the link length, the traffic load and the Q margin, to understand how each parameter influences the network design. Besides comparing the two paradigms of flexible and fixed grid networks, including both pure and hybrid modulation formats, in terms of their spectral efficiency and power consumption, we show the importance of integrating a detailed physical layer modelling in the network design phase.