A coax line terminal (CLT) transmits allocations of upstream bandwidth to a plurality of coax network units (CNUs). In response to the allocations, the CLT receives frames with data in a plurality of physical resource blocks that each correspond to a distinct set of subcarriers. The plurality of physical resource blocks includes a first group of physical resource blocks that all have a first constant allowed capacity. Sizes and modulation orders of respective physical resource blocks in the first group vary as defined by a first modulation profile. The data in the first group are received from one or more CNUs that are assigned the first modulation profile.