Abstract: Allocations of bits per transmission symbol to subchannels in a transmission system using multicarrier modulation are updated in response to requests from a receiver to a transmitter of the system, each request identifying a carrier whose bit allocation can be increased and a carrier whose bit allocation can be decreased, so that the total number of bits per symbol can be increased, decreased, or be unchanged. In order to synchronize bit allocation changes, a transmitted symbol counter at the transmitter and a received symbol counter at the receiver maintain synchronized symbol counts. In response to a request for a change in bit allocations, the transmitter sends a future symbol count value to the receiver, and the transmitter and the receiver implement the change when their respective counters match this count value.