To clarify the important role of the transverse wave structure in real detonations, we conducted experiments in porous wall tubes, as to attenuate the detonation's transverse waves. Flow visualization and measurements of the attenuation and failure processes for three typical unstable mixtures (oxy-acetylene, oxy-methane, and oxy-propane mixtures), characterized by their highly irregular frontal structure, revealed the ongoing competition between transverse wave elimination at the porous wall and re-amplification of triple points within the reaction zone. At critical conditions, when these two effects balance, a unique failure limit is found, namely d(*) approximate to 4lambda. Below this limit, the detonations fail. These experiments thus illustrate that transverse wave interactions are essential in the ignition and propagation mechanism for such unstable detonations. In comparison, experiments with argon-diluted detonations displaying a regular cellular structure with weaker transverse waves indicate that their transverse waves do not play a significant role in their propagation mechanism. The failure of these stable detonations is due to the global curvature mechanism caused by the mass divergence into the porous wall, leading to the slow distribution of frontal curvature. The results obtained in diluted and undiluted mixtures are also compared with a model taking into account the mass divergence at the permeable tube walls. Very good agreement is found for the argon-diluted mixtures, while the agreement for the unstable mixtures is very poor. This further indicates that the weak transverse wave structure in argon-diluted detonations, unlike the undiluted ones, does not significantly contribute to the ignition mechanism. Hence these argon-diluted detonations can be well approximated by a ZND reaction zone structure.