nt for the laser diode. The laser diode is a commercially available near-infrared laser diode operating at a wavelength of 850 nm. The driver circuit is configured to provide an output power of 10 mW from the laser diode and focused through a collimator lens for transmission. The receiver portion of the transceiver uses a convergent lens for the col- lection and focusing of the beam onto the optical-sensing element such as the avalanche photodetector diode (APD). The output from the optical- sensing element (typically APD) is transmitted through the interface cir- cuitry to the optical transceiver module, which is then transmitted out on the optical fiber to the network. The block diagram in Figure 9-8 depicts how the modules interwork.' 5 Having described the two designs that are typically used to construct an FSO optical transceiver, it is a good idea to discuss briefly the hardware and software required to interoperate with the optical transceiver. The hard- ware is usually a router and/or switch that is capable of accepting fiber handoffs from the output of an optical transceiver. For controlling the inter- operability between a transceiver and a router and/or switch, a network management interface is necessary. In most cases, this interface is based on the Simple Network Management Protocol (SNMP) and provides additional features such as network monitoring, a graphical user interface (GUI), and remote management capabilities. 9.1.3 Atmospheric Impairments and Other Problematic Issues Weather Attenuation The most significant difference between optical wireless (FSO) and fiber-optic laser transmission is the unpredictability Free-Space Optics (FSO) Housing Figure 9-8 Block diagram of typical system. Fiber Optic Driver Laser Optical Interface Circuitry Diode Collimator Module Fiber Optic Cable Lense Interface Circuitry of the attenuation of laser power in the atmosphere compared to fiber.6 Optical fiber cables attenuate at a consistent, predictable rate. Typical measurements of this attenuation range from 2 to 