in a communications system having a first modem transmitting via a communications channel , a method for adding a second modem , the method including learning crosstalk caused by transmissions from the first modem to the second modem while the second modem is in a transmitting state insufficient to cause crosstalk interference to the first modem in accordance with a predefined measure , deriving from the learned crosstalk an estimation of crosstalk that would be caused by the second modem to the first modem when the second modem is in a transmitting state , configuring the first modem to cancel crosstalk according to the crosstalk estimation , causing the second modem to enter a transmitting state sufficient to cause crosstalk interference to the first modem in accordance with a predefined measure , and causing the first modem to at least partially cancel crosstalk caused by the second modem in accordance with the crosstalk estimation .