in a dba method for an e - pon including an olt and onus , dbacns corresponding to processes in a tree algorithm are arranged in multi - level stages with the onus being in the lowermost stage . dbacns or onus in a specific stage are connected to a dbacn from its immediately higher stage in a tree structure . a process of allowing a dbacn to collect bandwidth request information from dbacns or onus in its immediately lower stage is performed sequentially from the lowermost dbacns to the uppermost dbacn . the uppermost dbacn allocates the smaller one of a total available allocation - bandwidth and a total requested bandwidth to dbacns in its immediately lower stage in their priority order . dbacns , other than the uppermost dbacn , each allocate a bandwidth allocated down from a dbacn in the immediately higher stage to dbacns or onus in the immediately lower stage in their priority order .