Data Communications Networking Devices - 4th Ed.pdf

770 ___________________________________________________________ SPECIALIZED DEVICESCommon cable typesFive common types of optical cables are illustrated in Figure 7.28. Note that when aone-®ber cable has a transmitter connected to one cable end and a receiver to theopposite end, it functions as a simplex transmission medium. As such, transmissioncan only occur in one direction. The two-®ber cable can be considered a duplextransmission medium since each ®ber permits transmission to occur in onedirection. When transmission occurs on both ®bers at the same time a full-duplextransmission sequence results. Thus, the 18-®ber cable is capable of providing nineduplex transmission paths.Figure7.28 Common cable types. Reprinted with permission from Data CommunicationsManagement, # 1986 Auerbach Publishers,New York.) A Optical ®ber,B Jacketed Kevlarstrength member,C Engineering plastic tubes,D Plastic separator tape,E Braided Kevlarstrength member,F PVC jacket,G Rip cord)The light detectorTo convert the received light signal back into a corresponding electrical signal aphotodetector and associated electronics are required. Photodetection devicescurrently available include a PIN photodiode, an avalanche photodiode, a phototransistorand a photomultiplier. Due to their ef®ciency, cost, and light signalreception capabilities at red and near infrared IR) wavelengths, PIN and avalanchephotodiodes are most commonly employed as light detectors.In comparison to PIN detectors, avalanche photodiode detectors offer greaterreceiver sensitivity. This increased light sensitivity results from their high signalto-noiseratio, especially at high bit rates.