This paper uses small-signal analysis and time-domain dynamic simulation techniques to test and study the system frequency response of power system network under generator trip disturbances. As the complexity of large scale power systems obscures the basic nature of the inter-Area modes, the simple but the practical and modified Klein-Rogers-Kundur (KRK) two areas, four generators test system is selected to determine the fundamental nature of inter-Area modes of oscillation. All the nonlinearities of the system and devices including governor, generator, power system stabiliser (PSS) and automatic voltage regulator (AVR) have been considered in the simulation studies and neglected from the modal analysis in the model. In this study, the free-input motion of the system states as well as electromechanical oscillatory mode formulations is provided and their link to the frequency response of the system is analysed. Then, the effects of disparate devices like PSS, A VR and governor are investigated by small-signal analysis and time-domain simulation studies in DIgSILENT PowerFactory software, respectively. The summary would be useful for system operators to know the local and entire frequency response of the system using the small signal analysis.