How healthy is my DSL link?

An underperforming DSL link may cause any one of the following:

Unexpected pauses in a video or movie
Fuzzy image quality in a movie or video
Delays when accessing a new Web site or Web page
A slowdown in sending or receiving information over the Internet, such as email or electronic files
Poor response time for action games
Speeds that are significantly lower than those advertised for the DSL service
Variations in the perceived performance of the Internet connection over time
Inability to access the Internet

Unfortunately, these symptoms are not restricted to problems with the DSL connection or any other single source.

You may also experience similar symptoms because of a poor Wi-Fi connection, congestion on the Internet, or problems with your computer, such as a virus or malware. For tips to ensure optimal DSL performance, please see Getting the Best DSL Speed.

The following guidelines will help provide a better snapshot of whether the problems may relate to your DSL link:

1. Measure the throughput capacity of your DSL connection using a service such as www.speedtest.net. Refer to Getting Accurate Results for guidelines on measuring DSL throughput.

If your performance test results fall below the minimum speed advertised by your service provider, then DSLPro recommends that you contact your service provider to test the DSL line. The minimum advertised speed represents the slowest speed that you should experience on your DSL line. (Depending on the type of DSL service you receive, your provider may not publish a minimum performance level.)

You may also wish to compare your performance results with those of any neighbors who subscribe to the same type of DSL service.

DSL performance depends very strongly on the “loop length,” meaning the distance between your house and the phone company’s DSL equipment.

Typically, the loop lengths for your home and for the homes of your neighbors are roughly the same, and so your DSL performance should be very similar.

If you find that your performance falls significantly below that of your neighbor, then DSLPro recommends contacting your service provider. (Be sure to confirm that your neighbor is receiving the same type and level of DSL service as you.)

If you think you may be experiencing variations in DSL performance, DSLPro also recommends that you run the performance test multiple times throughout the day or week. NOTE: Speedtest.net provides a convenient way to store and compare test results over time.

By collecting results over time, you may be able to pinpoint the cause of the problem more accurately.

Certain electronic devices such as microwave ovens may generate noise that can interfere with the DSL signal. By tracking performance over time, you may notice that the DSL line performs worse when the electronic device is operating. In this case, you may be able to eliminate the interference by relocating the modem to a phone jack that is farther away from the “noisy” device.

2. Measure the stability of your DSL connection using a service such as www.pingtest.net

For a comprehensive measure of your DSL health, DSLPro recommends running additional tests to determine the latency and stability of your broadband link.

Latency represents the delay in sending information over the broadband connection. If you are using an Internet telephone service such as Skype, you will notice latency as a delay or lag hearing the person at the other end of the line.

Unreasonably high latency may also cause problems for online gaming – specifically for what are called “twitch games” such as combat games that require reflex responses.

For best DSL performance, the latency of a ping should fall under 100ms (milliseconds).

When measuring latency using pingtest.net, you will also want to consider the results for “jitter.”

Basically, jitter tells you how much your latency varies over time. If you add the values for latency and jitter from pingtest.net, the sum should not be greater than 150ms for a healthy DSL line.

DSLPro recommends that you notify your service provider if you have an unstable line.

Pingtest.net will also indicate any packet loss over the DSL connection. Packet loss refers to data that does not arrive at the intended destination – similar to a lost letter in the mail.

Packet loss can be most critical for streaming video applications, resulting for example in pixilation of the video images (blocky images on the screen).

You should not expect to see any packet loss on your DSL line. If you do, DSLPro recommends contacting your service provider to report an unstable DSL line.

3. Check the DSL light on your modem

Check the DSL light on your modem.

Another measure of your DSL health involves monitoring the “DSL” light on your modem or gateway. This light provides an indication of how well your modem can maintain a connection to the telephone company’s DSL equipment at the other end of the line.

If you see the DSL light blink from time to time throughout the day, then you may have an unstable DSL line configuration.

Each time that the light turns off, the modem runs through a series of steps to re-establish the connection. During this time, you will not have any access to the Internet, and so you may experience a delay or interrupt in any Internet applications you may be running.

In this case, contact your service provider to report an unstable connection.

If the DSL light is continually dark (unlit), then check the phone line that connects the modem to the phone jack (in the wall) and make sure that the modem is turned on. Unlike the DSL light, the Ethernet and Wi-Fi lights will blink regularly during normal operation.

4. Check the DSL diagnostic statistics that are available on your modem

Several DSL modems and gateways include a “diagnostics” page for the DSL link with a variety of technical measurements. At a minimum, most models will provide you with a basic set of data to gauge the health of the DSL connection. Refer to the user manual of your DSL modem for details on accessing this information. If you do not have a printed copy of the user manual, you can often find an electronic copy on the modem manufacturer’s Web site.

The diagnostics page usually displays the sync rate for the DSL connection (downstream and upstream) in either Megabits per second (Mbps) or kilobits per second (kbps).

(NOTE: To convert kilobits per second to Megabits per second, divide by 1,000.)

The sync rate represents the maximum possible throughput capacity of the line based on the DSL configuration. The actual throughput typically measures less because of the overhead of the DSL connection and other factors.

The diagnostics data may also indicate a “maximum” sync rate (downstream and upstream). The maximum sync rate represents that highest possible throughput that the DSL link can support. For instance in tiered service plans (DSL service offered at multiple rates such as 1.5Mbps, 3Mbps, and 6Mbps), the maximum rate may fall significantly higher than your current service tier.

Example of the diagnostics page from a DSL modem/gateway.

In this case, you may be able to request faster service from your provider (typically for a higher monthly fee). The maximum rate can vary over time, so you may wish to check this value several times throughout the day or week for the most accurate results.

“Noise margin,” measured in decibels (dB), represents another important diagnostic measure that many modems/gateways may provide.

Noise margin measures how well your DSL connection can tolerate unexpected noise or interference on the line (caused by radio transmissions, electronic appliances, neighboring DSL lines, and other factors).

A healthy DSL connection maintains a noise margin above 6dB. A very stable reports a noise margin above 10-15dB.