Abstract: A dispersion compensation type optical signal receiving apparatus includes: an APD element for converting input signal light inputted from a transmission line into an electric signal; an amplifying device constituted with a preamplifier circuit and a limit amplifier circuit, which amplifies the electric signal converted by the APD element; an EDC IC for compensating the dispersion in the transmission line electrically; and a clock/data reproducing circuit for reproducing the clock and data signal contained in the input signal light, wherein there is provided a VOA for limiting the amplitude of the input signal light inputted to the APD element and an attenuation amount controlling circuit for controlling the attenuation amount of the VOA in accordance with the bias current of the APD element.

Abstract: To obtain a dispersion compensation type optical signal receiving apparatus constituted in such a manner that an electric signal outputted from a light receiving module can be properly inputted to an electronic dispersion compensation IC without being distorted, even when the input level of the optical signal is high.

Abstract: A wavelength division multiplexing optical transmission apparatus and a communication system employing the same are economical and highly reliable. The apparatus includes a plurality of transponders inputting and outputting branched optical signals having respective wavelengths and at least one variable wavelength transponder arranged in parallel to the plurality of transponders. Upon occurrence of failure in any one of the plurality of transponders, the optical signal having wavelength corresponding to the wavelength of the optical signal to be transmitted from the faulty transponder is transmitted from the variable wavelength transponder.

Abstract: First identifier is assigned to transaction information sent from a buyer's terminal and is sent back to the buyer's terminal. A second identifier associated with the first identifier and a part of the transaction information are sent to a vendor's terminal. An instruction indicating that goods to be sold in the transaction, to which the second identifier is attached, have been delivered to a retail store is sent to the buyer's terminal. Upon receiving the first identifier and the second identifier, a check is made to see if there is a correspondence between them. If there is a correspondence, an instruction is output to allow the delivered goods be passed to the buyer.