A digital subscriber line arrangement includes a downstream modem, an upstream modem and a copper pair connection between the modems. Each modem includes electrical length estimation function code means which causes the respective modem to measure the attenuation of signals transmitted by the other modem at a plurality of different frequencies, and to combine these measurements, or functions thereof, together to form a combined attenuation measure and to use this combined measure to form an electrical length estimation, KL0bis. The downstream modem is operable to transmit its electrical length estimation to the upstream modem, and the upstream modem is operable to generate a definitive electrical length estimation in dependence upon both the received estimation from the downstream modem and the estimation made by the upstream modem. Having generated a definitive electrical length estimation, the upstream modem is operable to transmit this to the downstream modem. Finally, the downstream modem includes Upstream Power Back Off mask generation function code means which causes the downstream modem to use the received definitive electrical length estimation to generate an upstream power back off mask for use in controlling the amount of power used when transmitting upstream signals over the copper pair connection to the upstream modem in one or more upstream transmission bands.