This Recommendation presents several models to enable the reader to evaluate the effect of diffraction on the received field strength. The models are applicable to different obstacle types and to various path geometries. The ITU Radiocommunication Assembly@ considering that there is a need to provide engineering information for the calculation of field strengths over diffraction paths@ recommends that the methods described in Annex 1 be used for the calculation of field strengths over diffraction paths@ which may include a spherical earth surface@ or irregular terrain with different kinds of obstacles.