Skip to main content
Accessibility Resource Center Skip to main content
Have a phone you love? Get up to $500 when you bring your phone. OR get iPhone 13, on us for a limited time. With Select 5G Unlimited plans. Buy now
end of navigation menu
DSL modem losing sync
Enthusiast - Level 3

What is the real cause of a modem losing syc; is it a drop in voltage or a change in frequency?


Re: DSL modem losing sync
Super User
Super User

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.