David’s Cisco Networking Blog

Nov 14 2007   3:39PM GMT

Understanding Cisco DSL Statistics

David Davis David Davis Profile: David Davis

Recently, a user on my website, GaryV, had a question about how to understand Cisco DSL statistics. In his troubleshooting process, he was able to gain a lot of valuable information from the Cisco TAC. He wanted to share that information with everyone else so perhaps it would help someone else, the next time you are trying to understand Cisco DSL statistics.

The full message text can be found here.

Below, is the post from GaryV, which details how to understand and interpret Cisco DSL statistics:

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I did have some degree of success with AT&T, and I actually got a response from the ATM group at Cisco. Hopefully this will be useful to other readers as well…

First, here is what I have so far from Cisco TAC, and they indicate there is more to come.

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.

Now, the definitions:

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
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.

Now something actually useful. Shocked
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

I will append any additional information I get from Cisco.

Regards,
Gary

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-David
Personal Website: HappyRouter.com – home of Cisco how-to articles & videos
David Recommends:
HappyRouter Cisco VMware Workstation & Server Video Training Series
HappyRouter Cisco CCNA & CCNP Video Training Series

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