Most modems and routers have the ability to monitor line statistics. Some modems like the SpeedTouch Home, 2Wire, and Cayman have very detailed monitoring while others may only show basic information. If you run a third party router in conjunction with a modem you may have to connect the modem directly to the computer in order to access the modem interface. Good advice: Pull your line stats and save/print all of your previous modem logs before you change anything or troubleshoot. Reference the links below to learn how to check line statistics for your particular modem:

Motorola 2210
Westell 2100/2200/6100
Westell 327W
SpeedTouch Home
2Wire
Cayman

Although what is monitored and the exact name may be different depending on manufacturer, the overall information is pretty much the same. Most modems will provide upstream (from modem to DSLAM) and downstream (from DSLAM to modem) stats. Below are some common terms and measurements used to judge line quality. Remember these are not hard numbers but simply a generalization of line statistics:

Important MaxSync Information: Customers that are provisioned on the MaxSync Profile will show SN Margins that are typically lower than customers on a service profile that is hard set at the DSLAM.

SN Margin (AKA Signal to Noise Margin or Signal to Noise Ratio)
Relative strength of the DSL signal to Noise ratio. 6dB is generally the lowest dB manufactures specify in order for the modem to be able to synch. In some instances interleaving can help raise the noise margin to an acceptable level. Generally speaking, as overall bandwidth increases, your signal to noise ratio decreases. A customer that upgrades from 1.5 to 6.0 (or provisioned with the MaxSync Profile) will typically see a corresponding decrease in the signal to noise ratio. This is normal and nothing to worry about. The higher the number the better for this measurement.
6dB or below is bad and will experience no synch or intermittent synch problems
7dB-10dB is fair but does not leave much room for variances in conditions
11dB-20dB is good with no synch problems
20dB-28dB is excellent
29dB or above is outstanding

Important MaxSync Information: MaxSync customers will rarely have SN Margins above 20dB and should not be concerned unless it is below 9-10dB.

Line Attenuation
Measure of how much the signal has degraded between the DSLAM and the modem. Maximum signal loss recommendation is usually about 60dB. One of the biggest factors affecting line attenuation is distance from the DSLAM. Generally speaking, bigger distances mean higher attenuation. The lower the dB the better for this measurement.
20dB and below is outstanding
20dB-30dB is excellent
30dB-40dB is very good
40dB-50dB is good
50dB-60dB is poor and may experience connectivity issues
60dB or above is bad and will experience connectivity issues

DSL Rate (Westell)
Your provisioned ATM Rate (actual service rate) plus bandwidth to cover the overhead and provisioning of the service.

Attainable Line Rate (AKA Synch Rate)
This is the maximum rate at which your modem can connect to the DSLAM if there was no service provisioning limiting the bandwidth. Anything over 2,000Kbps is considered good. The higher the rate the better. Not all modems have the ability to provide attainable line rate.

Used Line Rate (Speed Touch Home)
Your Used ATM Rate (actual service rate) plus bandwidth to cover the overhead and provisioning of the service. Not all modems have the ability to provide this statistic.

Fast Used ATM Rate
Actual bandwidth at which your service has been provisioned. The actual number can vary a little depending how you are physically serviced. If there is a number here that also means your connection is "fastpath". Not all modems have the ability to provide this statistic.

Interleaved Used ATM Rate
Actual bandwidth at which your service has been provisioned. The actual number can vary a little depending how you are physically serviced. If there is a number here that also means your connection is "interleaved". Not all modems have the ability to provide this statistic.

Relative Capacity (AKA Line Capacity)
Percentage of your overall available bandwidth used to obtain your service ATM rate. For example; if your max line synch rate was 5888Kbps and you were provisioned on a 1472Kbps service you would be using 25% capacity. 1472/5888=25% capacity. The lower the relative capacity the better, but you can still get maximum speeds (although a less stable connection) even with a very high relative capacity. In other words you could be synching at 1472Kbps with 98% relative capacity and achieve maximum speeds, but you may experience more disconnects. Not all modems have the ability to provide line capacity.

Output or TX Power
How much power modem (upstream) or DSLAM (downstream) is using. Maximum recommended is about 15dB. The lower the power the better for this measurement. Not all modems have the ability to provide output power.

CRC Errors (Cyclic Redundancy Check)
CRC is a method of detecting errors in data transmission. A high CRC count in itself is not really cause for alarm. However, any increase in CRC errors after your initial connection is established is a problem and usually points to a physical issue somewhere between the modem and the DSLAM. Isolate your inside wiring as a cause by testing from the NID and troubleshoot from there.
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Andy Houtz DSL