adsl connection status by the numbers

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.

description value meaning
Modem Status
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
LOS Errors 0
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
DSL Statistics
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.


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