【正文】 electrical stage which drives an optical source to give modulation of the light wave carrier .The optical source which provides the electricaloptical conversion may be either a semiconductor laser or light emitting diode (LED).The transmission medium consists of an optical fiber cable and the receiver consists of an optical detector which drives a further electrical stage and hence provides demodulation of the optical carrier .Photodiodes (PN,PIN or avalanche ) and ,in some instances ,phototransistor and photoconductors are utilized for the detection of the optical signal and the opticalelectrical conversion .Thus there is a requirement for electrical interfacing at either end of the optical link and at present the signal processing is usually The optical carrier may be modulated by using either an analog or digital information signal.. Analog modulation involves the variation of the light emitted from the optical source in a continuous manner. With digital modulation, however, discrete changes in the length intensity are obtained (. onoff pulses). Although often simpler to implement, analog modulation with an optical fiber munication system is less efficient, requiring a far higher signal to noise ratio at the receiver than digital modulation. Also, the linearity needed for analog modulation is mot always provided by semiconductor optical source, especially at high modulation frequencies .For these reasons ,analog optical fiber munications link are generally limited to shorter distances and lower bandwidths than digital links . Initially, the input digital signal from the information source is suitably encoded for optical transmission .The laser drive circuit directly modulates the intensity of the semiconductor laste with the encoded digital signal. Hence a digital optical signal is launched into the optical fiber cable .The avalanche photodiode detector (APD) is followed by a frontend amplifier and equalizer or filter to provide gain as well as linear signal processing and noise bandwidth reduction. Finally ,the signal obtained is decoded to give the original digital information . Broadband Communication As can be inferred from the examples of videophone and HDTV, the evolution of future munications will be via broadband munication centered around video signals. The associated services such as video phone, video conferencing, video surveillance, cable television (CATV) distribution, and HDTV distribution to the highspeed data services such as highresolution image transmission, highspeed data transmission, and color facsimile. The means of standardizing these various broadband munication services so that they can be provided of standardizing these various broadband munication services so that they can be provided in an integrated manner is no other than the broadband integrated services digital work in an integrated services digital work (BISDN). Simple put, therefore, the future munications work can be said to be a broadband telemunication system based on the BISDN. For realization of the BISDN, the role of several broadband munication technologies is crucial .Fortunately ,the remarkable advances in the filed of electronics and fiber optics have led to the maturation of broadband munication technologies .As the BISDN bees possible on the optical munication technologies .As the BISDN bees possible on the optical munication foundation . the relevant manufacturing technologies for lightsource and passive devices and for optical fiber have advanced to considerable levels . Advances in highspeed device and integrated circuit technologies for broadband signal processing are also worthy of close attention. There has also been notable progress in software, signal processing, and video equipment technologies . hence, form the technological standpoint ,the BISDN has finally reached a realizable state . On the other, standardization activities associated with broadband munication have been progressing. The Synchronous Optical Network (SONET) standardization centered around the T1 mittee eventually bore fruit in the form of the Synchronous Digital Hierarchy (SDH) standards of the International Consultative Committee in Telegraphy and Telephony (CCITT), paving the way for synchronous digital transmission based on optical munication .The standardization activities of the integrated services digital work (ISDN), which menced in early 1980?s with the objective of integrating narrowband services ,expanded in scope with the inclusion of broadband services ,leading to the standardization of the BISDN in late 1980?s and establishing the concept of asynchronous transfer mode (ATM) munication in process . In addition, standardization of various video signals is being finalized through the cooperation among such anizations as CCITT, the International Radiomunications Consultative Committee (CCIR ) ,and the International Standards Organization (ISO),and reference protocols for highspeed packet munication are being standardized through ISO, CCITT, and the Institute of Electrical and Electronics Engineer (IEEE). Various factors such as these have made broadband munication realizable. Therefore, the 1990?is the decade in which matured broadband munication technologies will be used in cibhybctuib with broadband standards to realize broadband munication works. In the broadband munication work, the fiber optic work will represent the physical medium for implementing broadband munication, while synchronous transmission will make possible the transmission of broadband service signals over the optical medium. Also, the BISDN will be essential as the broadband telemunication work established on the basis of optical medium and synchronous transmission and ATM is the munication means that enables the realization of the BISN. The most impor