The section below is copied from the status output page of a Aztech ADSL modem/router. The particular concern of the user was slow upload performance.
Modem Status Connection Status Connected Us Rate (Kbps) 512 Ds Rate (Kbps) 5056 US Margin 6 DS Margin 24 Trained Modulation ADSL_G.dmt LOS Errors 0 DS Line Attenuation 5 US Line Attenuation 20 Peak Cell Rate 1207 cells per sec CRC Rx Fast 0 CRC Tx Fast 0 CRC Rx Interleaved 0 CRC Tx Interleaved 4 Path Mode Interleaved DSL Statistics Near End F4 Loop Back Count 0 Near End F5 Loop Back Count 0
The meaning of each item is explained in the chart below,
the items affecting the problem are bolded.
|Connection Status||Connected||ppp session is active|
|Us Rate (Kbps)||512||current upstream rate|
|Ds Rate (Kbps)||5056||current downstream rate|
|US Margin||6||upstream carrier signal quality|
|DS Margin||24||downstream carrier signal quality|
|Trained Modulation||ADSL_G.dmt||adsl carrier is G.DMT|
|DS Line Attenuation||5||downstream carrier signal attenuation|
|US Line Attenuation||20||upstream carrier signal attenuation|
|Peak Cell Rate||1207 cells per sec||atm cell rate|
|CRC Rx Fast||0||crc errors in fast path receives|
|CRC Tx Fast||0||crc errors in fast path transmits|
|CRC Rx Interleaved||0||crc errors in interleaved transmits|
|CRC Tx Interleaved||4||crc errors in interleaved receives|
|Path Mode||Interleaved||using interleaved transmission|
|Near End F4 Loop Back Count||0||testing counter|
|Near End F5 Loop Back Count||0||testing counter|
The numbers generally under consideration when dealing with reliablility problems in ADSL at the signal carrier level are the line attenuation and signal to noise ratios.
Line attenuation is the signal strength loss that is experienced by the carrier while traversing the wire. Lower numbers are better. The upper limit is usually considered to be 60db of signal loss. Factors affecting signal loss include distance, wire gauge, number and quality of joints, cable construction, and bridge taps. The line attenuation in this particular case is excellent. The subscriber is probably very close to the telco dslam location. However, the upstream and downstream numbers are quite different. This is cause for investigation.
Signal to noise ratios represent the ability of the signal to overcome spurious noise on the communication line. Higher numbers are better. The lower limit is usually considered to be 6db. 12db is quite sufficent for most conditions. The negotiated data rates depend on the signal to noise ratios observed during the initial communications session setup phase. The data rate may change over time to accomodate line conditions. In the particular case, while the downstream margin is excellent, the upstream margin is at the very limit of usability. This is confirmed by the presence of crc errors in the upstream transmits. The user is on an interleaved error correction profile. Interleaving includes bit level error correction that can overcome many spurious noise events. Thus, the presence of crc errors indicates retransmissions of cell superblocks that could not be corrected via the in flight error correction.
The subscriber should investigate the reasons for the imbalances in line attenuation and line margin noted above. Possible causes include: poor wiring, missing adsl filters, improperly placed adsl filters, wiring close to sources of interference, poor connector quality. The goal is to present the least resistant path to the adsl signal at the phsysical level.