Quark-gluon-string model and EAS simulation problems at ultra-high energies

The reliability of model calculations is of considerable significance because at energies above those attained by accelerators only model predictions enable us to extract primary cosmic ray characteristics. Different phenomenological models based on the quark-gluon picture of hadron interactions are of use now as a foundation for experimental data analysis at energies > 1015 eV. In this paper we consider the present status of quark-gluon string (QGS) model with allowance made for semihard processes and (in the framework of so advanced model) discuss experimental data obtained at energies 1015 ÷ 1019 eV. In doing so we assume mass composition following from the diffusion notions of cosmic ray propagation through the Galaxy. Some general problems of EAS simulation and among them the simulation of electron-photon cascades are also discussed.