What exactly triggers a "DSL Link Retrains" error on a dsl modem?

Question

My AT&T DSL connection has been extremely flaky of late. After a lot of hair pulling I figured out that every time the modem logs either a 'DSL Link Retrains' or a 'DSL Training Errors' the internet connection is dropped. This becomes a major source of frustration because my work RDP fails and I have to re-connect.

After much effort, I was able to initiate a communication channel with ATT but last night someone from their support called up and said since there were no 'line drop' errors in the logs, the only other reason is the modem has gone bad.

I really don't expect $100 DSL modems to expire in a year (right after warranty expired).

Google isn't helping much. If someone can throw light on what causes the above error I will feel better when I buy the next modem and set some expectations for myself. In other words I can't take ATT support's word that the modem has gone bad!

Following is the complete Error Report

Statistics
Collected for 1 day 1:38:37

                        Since Reset    Current 24-Hour  Current 15-Minute  Time Since Last Event

                                       Interval         Interval
ATM Cell Header Errors:         0      0                0              0:00:00 

ATM Loss of Cell Delineation:   0      0                0              0:00:00 

DSL Link Retrains:             29      8                0              0:16:46 

DSL Training Errors:           20      3                0              1:11:10 

DSL Training Timeouts:          0      0                0              0:00:00

DSL Loss of Framing Failures:   0      0                0              0:00:00

DSL Loss of Signal Failures:    0      0                0              0:00:00

DSL Loss of Power Failures:     0      0                0              0:00:00

DSL Loss of Margin Failures:    0      0                0              0:00:00

DSL Cumulative Errored Seconds: 0      0                0              0:00:00

DSL Severely Errored Seconds:   0      0                0              0:00:00

DSL Corrected Blocks:           0      0                0              0:00:00

DSL Uncorrected Blocks:         0      0                0              0:00:00

ISP Connection Establishment:   1      1                1              0:01:31

@sawdust Thanks a ton for the info on Line Noise. On further digging I pulled up this report from the router. Anyway to tell for sure if this looks healthy or not?

Both the answers deserve an upvote.

DSL                Down              Up
Current Rate:       6016 kbs        768 kbs
Max Rate:          10448 kbs        1060 kbs 

Current Connection:
Current Noise Margin:       21.0 dB     14.0 dB
Current Attenuation:        20.8 dB     12.0 dB
Current Output Power:       19.7 dBm        11.9 dBm 

ATM              Cells      Errors      %
Transmit:       4412167         0       0
Receive:        67168824        0       0
IP               Bytes      Packets     Errors      %
Transmit:       172853405       1427956     0       0
Receive:        3160277814      2535418     0       0

Answer 1

Training errors are problems with the syncing process to the DSLAM at the CO. DSL Link Retrains increment each time the modem disconnects and must retrain and training errors are issues it encounters in the training process.

As far as possible reasons for these errors, that can vary. You could have too much noise on the line, you could be too far from the CO for your current speed, you could have some sort of voltage on the line or your modem could be bad. Those are really the most common, there are still many more (like bridge taps and NID splitters, or even problems with the DSLAM port that you are on or your cross-connect at the CO, but those issues are much rarer). If you are behind the times and are still on lineshare, there could still be other reasons.

The first step would be to swap out the phone cable you are using. If that doesn't work, go out to the NID (phone box) and plug into the interface there (depending on the NID style, you may need to strip an end of the phone cord). If you still see the same problem, try a different modem. If still nothing changes, it is time to call AT&T and have them run a set of loop tests on the line.

Answer 2

The ADSL modem at the phone company's central office is the line or master unit, and the ADSL modem at your home is the remote or slave unit. In order to establish a bi-directional ADSL link, each end of the ADSL link performs a prescribed sequence of transmission and receive tests. One modem will transmit, while the other is in receive mode, and then use the results to configure transmission power levels, equalization and other parameters. This "handshaking" sequence to setup the link is called training. The signal processing to transmit and then receive an attenuated ADSL signal after thousands of feet of small gauge copper wire is quite sophisticated. The training phase is required to setup the signal processing before the link goes active.

The criteria for a "bad link" or "loss of link" (that would initiate a retraining) could vary by modem and/or carrier. Too many bit errors in the last frame, or too many cumulative bit errors or loss of frame sync are possible cause.

You should check your line quality or transceiver stats at you modem (through its web page, use your PC's web browser). Low (signal to noise) margin and/or high line attenuation could indicate a possible problem (especially if the numbers have changed).