A rigorous numerical analysis is performed in order to discuss and reveal mechanisms responsible for the extraordinary transmission through 1-D and 2-D sub-wavelength arrays of infinite slits and holes, respectively. The investigation is mainly focused on the influence of structural parameters on the resonant enhancements of optical response in the sub-wavelength region. Zero-order transmission maps are calculated in the array periodicity and slit/hole filling factor plane where a behaviour of the existing modes can be clearly resolved. The effect of metallic film thickness is also considered. Identification of mode resonances playing a significant role in the EOT process is verified by studying the corresponding field profiles. It is found that for TE polarization in the case of 1-D periodic metallic array, the TE fundamental mode plays a significant role in the enhanced energy transfer, however, for TM polarization, TEM modes are responsible for energy transfer through one-dimensional infinite slits. Together with waveguide / cavity resonances investigation, surface plasmon-polariton excitations are studied, and their contribution is discussed in the connection to the enhanced transmission. For the case of 2-D array, map of modes, based on the propagation constants and the coefficients of attenuation, is constructed in order to identify the group of those modes strongly contributing to the EOT. Importantly, metal is not considered as a perfect conductor.