The results of numerical simulation of JET L, H and VH mode are presented. It is shown that L-H transitions are accompanied by a fast and large modification of the transport coefficients not only near plasma edge but across a large fraction of the plasma cross section. Both experiments and numerical simulations show that the giant ELMs on JET also have a global character, and their penetration length increases with the amplitude of the D signal. Transport coefficients are of the order of the L-mode ones during these ELMs. Analysis shows also that within experimental accuracy transport coefficients are the same for VH and ELM-free H­mode discharges. The variation in the global energy confinement is mainly due to the differences in the impurity radiation, power deposition profiles and the difference in the plasma recycling and related convection.