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What is retraining? What is fall-back/forward?
Retraining and fallback are ways for modems to adjust to changing line conditions as the connection progresses. This allows modems to optimize their transmission speed and re-negotiate midstream, so connections do not hang or drop. Retraining is like the initial handshake -- several seconds where the modems probe the line in order to configure themselves. Either modem can request a retrain, but the retrain will only occur if the other modem grants it. You can hear the distinctive training sounds if the modem speaker is on. During the several seconds of retraining no data will flow.
Fallback, on the other hand, is simple speed shifting that is relatively quick, certainly quicker than retraining. Modems can fall back to a pre-determined transfer speed to keep the error rate low, and fall forward when possible to improve throughput. With the modem speaker on, you will hear only a small beep or blip in the carrier hiss. If a noise burst causes a high number of transfer errors, the receiving modem sends a quick fallback blip, which the sending modem must permit. After the sending modem falls back to a slower speed, and the recieving modem sees better line conditions (fewer errors), a fall-forward blip is sent and the connection speeds back up.
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