Full-duplex start-up for modems

A method of starting full-duplex communication between a first station with a calling modem and a second station with an answering modem, the answering modem, when called, sending an answering tone to the calling modem, the method comprising the following steps:

• a1. sending from each of the modems (a sending modem) to the other modem (a receiving modem), intermittently and repetitively:
• a set of channel probing tones at predetermined frequencies, and
• repetitive identification messages in a given frequency range, containing the sending modem's local time when the message is sent, and characterizing the sending modem by its capabilities in terms of parameters including supported modulation rates, center frequencies, and supported coding;
• a2. at the receiving modem receiving said probing tones and identification messages:
• establishing a running time reference from the sending modem's local time received in said identification message(s), and
• determining transmission channel characteristics, in particular determining a useful bandwidth from said received probing tones;
• b1. sending from the receiving modem:
• repetitive response messages, each containing the running time reference at said receiving modem and transmission parameters selected from the parameters supported by both modems, based on the determined channel characteristics, in particular the modulation rate and center frequency, for subsequent transmission of training and data signals from the sending modem receiving said particular response message;
• at the sending modem receiving said response message:
• determining the round trip delay by comparing the running time reference and the actual local time at the sending modem;
• extracting the information on the transmission parameters to be used for subsequent transmission of training and data signals to the receiving modem sending said response message;
• c1. sending from each modem, at the respective determined transmission parameters, full-band training sequences;
• c2. determining, at each modem that now sends and receives training sequences, the responses of the respective detected near-end and far-end echo paths and of the signal paths from the other modem to adjust receiver settings, in particular echo cancellors and equalizers as necessary for subsequent data transmission.