A method for fairly allocating a differentiated bandwidth based on shared multicast traffic in transmission of downstream traffic in a Time Division Multiplexed-Passive Optical Network (TDM-PON) is provided. The method includes the steps of: deciding a total number of multicast groups to which services are currently provided, and a number of multicast groups to which a predetermined optical network unit (ONU) subscribes; calculating a multicast share degree in each of the multicast groups, and a total multicast share degree of the predetermined ONU in all of the multicast groups; dividing the total multicast share degree of the predetermined ONU by the total number of multicast groups to calculate a final normalized multicast share degree of the predetermined ONU; calculating an average multicast bandwidth of the predetermined ONU depending on a total bandwidth allocated to multicast traffic services on the basis of the final normalized multicast share degree; calculating the differentiated bandwidth of the predetermined ONU using ONU-specific service weights; and subtracting the average multicast bandwidth from the differentiated bandwidth of the predetermined ONU to calculate a maximum dynamic timeslot bandwidth of the predetermined ONU.
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