Point-to-point airwave transmission systems that use focused infrared (IR) light beams between transmitters and receivers, much as microwave systems use focused radio beams. FSO requires optical line of sight (LOS) and will work through clear windows, depending on their chemical composition, but not through walls or other opaque objects. FSO transmitters use laser light sources in the form of either vertical cavity surface-emitting lasers (VCSELs) or laser diodes. FSO systems suffer from environmental interference, particularly fog, which absorbs, diffuses, and reflects the light beam, much as fog affects the beam from the headlights of an automobile. Under optimum conditions, transceiver separation is limited to about two to five kilometers, which limits the technology to short haul applications. Unlike radio systems, FSO systems are immune to electromagnetic interference (EMI) and radio frequency interference (RFI), require no licensing, and are inherently secure. FSO systems commonly operate at rates of 1.544 Mbps (T1), 2.048 Mbps (E-1), 34 Mbps (E-3) and 45 Mbps (T3), and are capable of rates up to 10 Gbps (OC-192). Systems running at up to 160 Gbps have been demonstrated in the labs. See also absorption, airwave transmissions, diffusion, EMI, infrared, laser diode, LOS, reflection, RFI, short haul circuit, and VCSEL.