A method and arrangement in a communication system for dynamically allocating communication system frequency spectrum after cross-talk interference between transmission medias used for upstream and downstream communication in the frequency spectrum. After detecting cross-talk interference on a disturbed transmission medium caused by transmission on a disturbing transmission of medium, the arrangement transmits a media control signal on the disturbed transmission medium and detects a reproduced media control signal that is reproduced due to cross-talk on the disturbing transmission medium. The disturbed or disturbing transmission media can then be dynamically allocated different frequency spectrum to eliminate the interference. In one embodiment, each transmission medium has a default transmission direction, and if one of the transmissions is in a direction opposite the default direction, that transmission is reassigned to another transmission medium.