This paper first introduces the basics of GPON and analyzes the upstream DBA （dynamic bandwidth assignment） mechanisms of the MAC layer. And then a GPON upstream DBA algorithm based upon cyclic polling is proposed according to the ITU-T specifications. The proposed algorithm can support five types of T-CONT and four types bandwidth assignment

thus can support various Qos services. At the end of the paper

the author give a brief analysis of the simulation results of the algorithm.