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What is the real cause of a modem losing syc; is it a drop in voltage or a change in frequency?
Modems generaly lose sync due to noise. Noise occurs if you have a poor signal coming in, being caused by the speed being set too high, poor quality copper and connections, or loud electrical interferers (like lightning) which can cause a marginal signal to drop below a signal-to-noise threshold. Otherwise, modems can lose sync if either the line card or the modem are faulty. If voltage gets too low for the modem or for the DSLAM (rare on this one), the equipment will lock up, power down, or reboot if there's simply a power "blip."
A change in frequency is sort-of correct in the sense that DSL uses multiple frequencies on a copper connection to establish your downstream and upstream speed. The modems select from a certain range of frequencies to use to obtain a speed, and will load a specific frequency with a certain amount of data. If a specific frequency becomes too noisy, the modem will be forced to allocate data to other frequencies and will keep doing this until that frequency can carry all of the data it's assigned reliably. If it's not able to do this, the modem disconnects and re-connects at a lower speed to compensate for the noise.