A coax network unit (CNU) is coupled to a coax line terminal (CLT). In first and second modes of operation, the CNU transmits data during an upstream window and receives data during a downstream window. In the first mode of operation, a duration of data transmission for the upstream window or a duration of data reception for the downstream window is reduced by a specified amount with respect to the second mode. A sounding signal is transmitted in the first mode in a probing slot that has a duration corresponding to the specified amount.