ill randomly get off course of the site path. This effect is described as beam wander (see Figure 9-10).4 The mixing of both scintillation and beam wander leads to the fluctua- tions in overall signal stability. Therefore, the optical beam experiences an in-homogeneous spatial and temporal propagation path, causing the beam to propagate in a random fashion. When combined together, both effects produce an overall noise component to the received optical signal. Atmos- pheric attenuation is the dominant cause of error bursts in free-space com- munications systems and it becomes very important to find ways to characterize and quantify it. Moving Buildings The second physical issue that affects the perfor- mance and reliability of an FSO system is the natural movement of build- ings. Although it is rarely noticeable by humans, high-rise buildings occasionally experience minimal oscillation, or jitter, due to the movement or vibration of objects such as a heavy-duty electric generator. This creates a potential problem when using optical transceivers because the transmit- ters and receivers must be aligned accurately to each other in order to min- imize transmission error. Flying Objects Flying object obstruction is the third potential issue that can deteriorate the performance of an FSO system. The most significant instance is the flying pigeon or bird scenario. For network topologies such as the point-to-point or hub-and-spoke configuration, this issue can intro- duce a potential problem because these topologies only provide a single Figure 9-9 Scintillation. Input Output Power Power Figure 9-10 Beam wander. Input Output Power Power Free-Space Optics (FSO) path for transmission. However, for a meshed or a ring topology, this issue would not affect the availability of the FSO system because there is an alternate path to route the signal in these two topologies. Eye Safety Almost all wireless optical laser transceivers from various FSO vendors currently operate at a wavelength between 785 and 1,550 nm. This particular m