Data transmission over long distance circuits

A method of handshaking between an originate modem and an answer modem interconnected by a communication line comprising the steps of:

• causing a ringing signal to be applied to the answer modem in response to a signal from the originate modem;
• connecting the answer modem to the communication line after receipt of the ringing signal;
• transmitting an answerback tone from the answer modem to the originate modem after a period of silence following connection of the answer modem to the communication line;
• connecting the originate modem to the communication line after receipt of the answerback tone;
• waiting sufficiently long to reactivate any echo suppressors in the communication line;
• thereafter transmitting first and second training sequences, one from the originate modem to the answer modem and the other from the answer modem to the originate modem, said first and second training sequence being sufficiently long to train up any echo canceller in the communication line;
• detecting if the lengths of the first and second training sequences as received are significantly shorter than their lengths as transmitted;
• and
• in the event the training sequences as received are significantly shorter than their lengths as transmitted, transmitting a sacrificial carrier at the beginning of each transmission of data in a new direction on said communication line and in the event that the training sequences are not significantly shorter as received than as transmitted beginning each transmission of data in a new direction on said communication line without a sacrificial carrier.