To better utilize the capacity of the twisted-pair access networks, operators deploying very high-speed digital subscriber line (VDSL) systems need accurate parameters for power back-off (PBO). However, VDSL standards give almost no guidance on how these parameters should be established for a particular network. In this paper we present a new technique for optimizing PBO parameters for a cable bundle, which is based on the Nelder-Mead simplex search algorithm. In this way each operator can easily calculate PBO parameters that match its actual access network down to the individual cable bundle. Using the properties of the PBO, as defined in the VDSL standard, we show how a normalized FEXT coupling can replace the knowledge of the individual couplings during the optimization of the PBO parameters. By simulations based on measured cable data we show that our approach using cable bundle unique PBO (CUPBO) achieves a significant improvements compared to the performance achieved with the ordinary PBO. © 2007 EURASIP.