A priority scheme is provided for ports assigned to respective channel units in a channel bank. The priority scheme is based on an amount of preallocated bandwidth unused by channel unit ports. A first water level in a first bucket is associated with an amount of allotted bandwidth unused by the channel unit and a second water level in a second bucket is associated with an amount of unused allotted bandwidth exceeding an overflow level of the first bucket. A high priority value is derived from the first water level when the first water level is above zero. A medium priority value is derived from the second water level when the first water level is equal to zero. A low priority value is derived when both water levels are zero. In another aspect of the invention, the total port bandwidth utilization history is preferably accounted for. It is differentially aged to favor recent history over older history, and optionally further compensated to favor long quiet times.