A DSL subscriber reports that the CPE modem untrains and retrains several times each hour, but not at regular intervals. The subscriber has a DMT modem operating on 10 kft of #26 cable (3 km of 0.4 mm cable). The provisioned downstream rate is 2 mbps, and the provisioned upstream rate is 256 kbps. When the modem retrains, the downstream DSL may be as low as 512 kbps. Sometimes manually retraining the modem allows it to return to a 2 mbps line rate, and sometimes manually retraining the modem does not improve the DSL line rate. Which options would cause the
intermittent train/untrain symptom as described? (multiple answer)
A.
A telephone on the same phone line was installed without a microfilter. When the telephone handset is taken off-hook, the phone causes excessive attenuation of the DSL frequencies and the resulting high error rate results in a retrain. Because of the degraded signal levels while the phone is off-hook, the modem can not retrain at 2 mbps.
B.
There is an interfering signal in the same cable. When the interfering signal is present, an excessive error rate results. The modem retrains at a lower line rate in order to recover an acceptable noise margin and error rate.
C.
The noise margin was incorrectly set too high. Reducing the noise margin will correct the symptom.
D.
ADSL DMT will not permit 2 mbps downstream rates at 10 kft of #26 cable (3 km of 0.4 mm cable). ADSL 2 megabit service will always be unreliable and intermittent on this cable length and wire size.
E.
The DMT profile is using interleaved mode. Changing to fastpath will correct the symptom.
F.
The ATM QoS is UBR, and the subscriber’s traffic is yielding to VBR or CBR traffic.