: Nowadays the communication systems and networks have been developed rapidly and without them life becomes approximately impossible. One of the most important kinds of these nowadays sytems is fiber optics network communication system. The speed at which information can be transmitted in these systems is increasing everyday. The accurate and perfect operation of the optical networks needs sensing and control methods systems for example distributed sensing systems. The distributed sensing systems (OTDR and its derivatives) can detect the faults of the optical cable by examining the reflected signal from the cable. Moreover, these sensors are used to detect and locate the vibrations in bridges and buildings, diagnose leak of oil in pipelines and its position, also locate and detect the faults in communication systems preventing the failure of them. In this case, because the cost of preparing and implementing these systems is less than the compensation cost in the case of having a damage in these systems (destruction of bridges and building, oil leak or when there is no access to the server for some users), so these systems are known as economical systems. One of the most important parameters of these systems is the sensitivity of them to events and faults that depends on SNR. In this thesis, these distributed sensors are studied and compared then simulated by Matlab and Optisystem. A combination system of POTDR and COTDR is suggested to improve the SNR, this combined system is simulated using Matlab. The intensity of scattered power has been increased by 11.5 dBm at 100 km, which means improvement in SNR. Keywords : Distributed optical fiber sensors, OTDR, Rayleigh scattering and Brillouin cattering.