nife-edge diffraction takes place when the signal grazes over pointed objects such as trees, etc., making a loss of approximately 6 dB. The diffraction loss depends upon the path and type of vegetation and will vary between the maximum of the smooth sphere diffraction and minimum of the knife-edge diffraction. There are various methods used to calculate diffraction. Terrain Averaging Model If the obstacle is neither sharp nor rounded, this method is used as it gives quite a good estimation of the diffraction loss. The loss can be calculated as (3.43) where h = difference between the path trajectory and the most significant obstacle F1 = radius of the first fresnel zone Knife-Edge Model This is used when the object is sharp and is obstructing the first fresnel zone. These models are used when there is more than one obstacle inside the first fresnel zone. The diffraction loss is given as L = 201og(1) (3.44) where 1 for V < -0.8 =0.452 -V(v-0.1)21-(v - 0.1) for (3.45) and the fresnel-Kirchoff diffraction parameter can be calculated as (3.46) where a is the wavelength, d1 and d2 are the distance to the sites from either side of the obstacle and h is the height of the obstacle. There are some models that are used to calculate the knife-edge diffraction loss. For obstacles that are separated widely the Epstein-Peterson model is used; for obstacles too closely placed the Japanese Atlas model is used; for simple calculations the Bullington model is used; and for many obstacles the Deygout model is used to calculate the knife-edge diffraction loss. Transmission Network Planning and Optimisation Another factor that may cause slow fading, especially in a longer path above frequencies of 10 GHz, is rain. Attenuation due to rain is lower at lower frequencies (8-10 GHz) and increases when the frequency of operation increases. However, other factors such as the rain rate, link length and polarisation play an equally important role in the attenuation due to rain. A rainfall-like atmosphere is quite unpredictable, varying with ti