This paper is devoted to the analysis of optical signal to noise ratio (OSNR) for all optical networks (AON) with wavelength switching at the cascade switching of optical amplifiers and optical cross connectors, including modelling of transient interferences in optical cross connectors, due to the impact of which there is an additional communication lines’ power penalty appears. In order to create economically profitable all optical net or to increase the throughput capacity of existent one considering its high performance, it is necessary to review such architectural and wavelength routing factors as network’s physical layers restrictions. Available publications omitted any effective solutions of the wavelength routing task on the criterion of the structural cost and BER minimization in the respective optical wavelength path. Despite it, while designing (logically or physically) an optical network with wavelength switching in the most optimal way, we need to be sure that physical technology of the network will grant us with the acceptable level of negative impact from network elements’ physical parameters. That’s why the investigation of the AON physical layer impact by means of OSNR analysis in the optical channels with wavelength switching at the real transmission parameters is the scope of this work. It is shown, that optical signal probably should pass the set of optical cross connectors (OXC), distributed optical amplifiers and interconnected optical fibers that definitely will worsen the quality of the signal. This is due to the decrease in signal power within optical fiber attenuation, crosstalks in OXC and summing the useful signal with noise of Amplified Spontaneous Emission (ASE) in optical amplifiers. This article contains the result of OSNR calculation after the AON section analytical simulation and recommendations for network designers were presented. Resulted analytical expressions allow making justified choice of the AON with wavelength switching structure based on a given set of input parameters and optimize this structure on the criterion of maximum OSNR values on the corresponding wave transmission paths. It is also shown that FEC technology according to G.709/Y.1331 is applicable when we need to make our AON structure more scalable in the spatial dimension and more powerful when more sophisticated switching core needed to be realized. We could also conclude that worsening of OSNR value is under strong impact of the number of optical amplifiers used, and in the less scale under impact of the OXC number.