Fixing One Problem Sometimes Creates Another

Fixing One Problem Sometimes Creates Another

From time to time people ask me what is the meaning of the flashing lights on their modems. The meaning of a modem’s LEDs varies from one model to another, but for one of my clients the problem was that the flashing lights on the modem in her bedroom were keeping her awake at night. She solved that problem by unplugging the modem when she went to bed, but as is sometimes the case, fixing one problem only creates another.


After several frustrating days trying to troubleshoot why this lady’s internet connection was slow and going on and off intermittently, I eventually figured out she was turning off the modem every night.


Internet Service Providers (ISP) all recommend that your modem be turned on 24/7/365; that means never turning it off. Why—my client wanted to know. The simple answer is that the internet provider says so and that being the case it is best to follow their suggestion. The technical reason is, well, technical.


The modem in your house is a part of your Internet Service Provider’s citywide network. It needs to stay connected so that the ISP may perform necessary maintenance which is often scheduled for the middle of the night. Apart from that the process of reconnecting a turned-off modem is not as simple as one might think.


When an ADSL modem is powered on it must perform a programmed connection sequence with the Digital Subscriber Line Access Multiplexer (DSLAM) at the phone company. This sequence is referred to as “training” which involves handshaking, negotiation to establish power levels, equalization, and the all-important connection speed. This training period configures the transmit and receive speeds for the fastest reliable speed and can be time-consuming when trouble is detected. This can cause the modem to not operate at its maximum speed potential until it has been continuously connected for hours or days.


If you unplug your modem frequently, the “training” could mistakenly identify your house as experiencing significant attenuation (reduction in signal level). The DSLAM might then conclude the connection to your house is over low-quality wire and reduce your speed accordingly. That would not happen if you leave your modem turned on 24 hours a day as the ISP suggests.


When reconnecting your modem “training” can employ different methodologies. All systems try to compensate for attenuation by reducing speed. Most systems try to restore that lost speed by “re-training” after a period of time to see if your wiring can handle a faster connection. The time it might take for that retraining is the big unknown. It could possibly take days but it will not be required at all if the modem is left on continuously.


So if you have been turning off your modem from time to time stop doing that! Leave it turned on and you just might find it works faster and more reliably.


Charles Miller is a freelance computer consultant, a frequent visitor to San Miguel since 1981 and now practically a full-time resident. He may be contacted at 415 101 8528 or email