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DSL Terminology

René Jorissen on October 20, 2008 0 Comments • Tags: #atm #attenuation #downstream #dsl #interface #margin #noise #ratio #show #signaltonoise #snr #upstream

When configuring DSL or other analog connection, I sometimes have problems with the specific terminologies used in these technologies.  I found a post explaining the terminology used for understanding Cisco DSL statistics. Reading this post helps me remember the terminology.

Taken from the post:

To troubleshoot Layer 1 problems, you can use the show dsl interface atm 0 command to verify that the Cisco 877 router is trained to the DSLAM. If the Cisco 877 router is successfully trained to the DSLAM, this command will also display the trained upstream and downstream speed in kbps.


Noise Margin (also signal-to-noise ratio)
When DSL service is provisioned in a DSLAM, the minimum acceptable noise margin is usually specified. CAP DSL service is typically provisioned with a downstream margin of 3 dB and an upstream margin of 6 dB. Research has shown that the optimum margins for DMT service are 6 dB downstream and 6 dB upstream.


Avoiding configuring a DSL service with more noise margin than appropriate is important because the system will train to an unnecessarily low DSL rate to provide the specified margin. It is also important to avoid specifying an exceptionally low margin, such as 1 dB downstream and 1 dB upstream because a small increase in noise level on the transmission line would probably result in excessive errors and a subsequent retraining to a lower DSL rate.


Increasing the transmit power levels will also improve the noise margin but at the cost of interfering with other services in the same cable.


Most DSLAMs and CPE report both the provisioned and actual noise margins for each DSL line. If the actual margin is higher than the provisioned margin, the line should provide an acceptable error rate at the present DSL line rate. As the actual margin drops below the provisioned margin, there is a high probability of an excessive error rate and subsequent retrain to a lower DSL rate.


Attenuation generally refers to any reduction in the strength of any type of signal, whether digital or analog. More precisely in the case of DSL, attenuation is the normal loss of signal strength over distance. Attenuation specifically is a logarithmic function of the power setting. As power increases, attenuation increases logarithmically. Also called simply loss, attenuation is a natural consequence of signal transmission over long distances. The extent of attenuation is usually expressed in units called decibels (dB).


Capacity Used
Percentage of the capacity that is being used.


Here are ranges for these values that I received from an AT&T provisioning engineer.


For Noise Margin: (the higher this value, the better)
8-13 Average
14-22 Very Good
23-28 Excellent


For Attenuation: (the lower this value, the better)
20-30 Excellent
30-40 Very Good
40-60 Average

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René Jorissen

Co-owner and Solution Specialist at 4IP Solutions
René Jorissen works as Solution Specialist for 4IP in the Netherlands. Network Infrastructures are the primary focus. René works with equipment of multiple vendors, like Cisco, Aruba Networks, FortiNet, HP Networking, Juniper Networks, RSA SecurID, AeroHive, Microsoft and many more. René is Aruba Certified Edge Expert (ACEX #26), Aruba Certified Mobility Expert (ACMX #438), Aruba Certified ClearPass Expert (ACCX #725), Aruba Certified Design Expert (ACDX #760), CCNP R&S, FCNSP and Certified Ethical Hacker (CEF) certified. You can follow René on Twitter and LinkedIn.

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