DSL Internet circuits can experience problems for a variety of reasons. Most are related to the connection of the individual end user. And most can be detected through a simple check of the sync rate, attenuation, and signal-to-noise ratio (SNR) statistics on the DSL modem. This article will explain why these parameters are important, how to check them, and how to improve the values.
It is always recommended to perform a packet loss test on any Internet connection before starting an upgrade project. The packet loss test should confirm that there is a problem with the end user’s connection and that the Internet Service Provider (ISP) network is not the source of the problem. Additionally, the packet loss test will provide baseline statistics to compare future results.
Assuming that a packet loss test has revealed problems with the end user’s IP address, it’s time to look at the DSL modem and examine it for problems:
signal to noise ratio
According to Wikipedia.org, “Signal-to-Noise Ratio (SNR) is a measurement used in science and engineering to quantify how much a signal has been corrupted by noise. It is defined as the ratio of signal power to noise power that corrupts the signal. A ratio greater than 1:1 indicates more signal than noise.”
An everyday example of SNR is listening to music in your car over the noise of the road or other people talking. The louder the radio is compared to the rest of the noise in the car, the more clearly you will hear the music. The same is true of the DSL modem that “listens” for the signal carrying Internet traffic. When the signal is loud compared to the noise, Internet communication occurs at higher speeds with less packet loss. When the signal is not strong enough compared to the noise, speeds slow down and cause latency. Packets can be lost or dropped, creating data packet retransmissions and problems with real-time applications such as Voice over Internet Protocol (VoIP) used by Vonage, Skype and hosted PBX providers. In fact, any streaming application will be affected, including streaming video from Netflix, Blockbuster, and others, or any type of online gaming.
DSL parameter values
Values to check on a DSL modem are Sync, Attenuation, and Signal-to-Noise Ratio.
Timing is described in the downstream and upstream directions and is the connection speed in each direction. Downstream is from the ISP to the modem. Upstream is from the modem to the ISP.
Attenuation is the loss of signal over distance. The db loss does not only depend on the distance. It also depends on the type and gauge of the wire (which can vary depending on the length of the wire), the number and location of other connection points on the wire. The attenuation is listed with both downstream and upstream values.
20db and below = Outstanding
20db-30db = Excellent
30db-40db = Very good
40db-50db = Good
50db-60db = Poor and you may experience connectivity issues
60db and above = Bad and you will experience connectivity issues
Like Sync and Dimming, SNR has downstream and upstream measurements.
6db or less = Bad and you won’t experience line sync and frequent disconnections 7db-10db = Acceptable but doesn’t leave much room for variances in conditions.
11db-20db = Good with little to no disconnection issues
20db-28db = Excellent
29db or higher = Outstanding
SNR will sometimes show up in the margin or SNRM. This is the difference between the actual SNR value and the SNR that is required to maintain a reliable circuit at the connection speed. If the SNRM is minimal, the circuit is more likely to experience intermittent connection failures and slowdowns. High margins are required to prevent interfering bursts from causing connection loss. The SNRM target is typically 6 db, but could go as high as 12 db.
Determining the values of a DSL modem
Many DSL modem configuration pages can be viewed at the internal address of 192.168.1.254. Refer to http://broadband.modemhelp.net/dsl_modem_info/index.shtml and look up the specific make and model of your DSL modem or the manufacturer’s manual. The manufacturer’s website or manual will give you the address of the modem’s setup page. Just enter the address into a web browser on a PC connected to the modem and the page should load. Remember, these values can fluctuate. If intermittent problems are experienced, the values should be checked when all is well and compared to the values when problems are experienced.
Improved DSL modem settings
Cabling and connectors are the most common cause of DSL problems. Internal wiring can be easily eliminated by simply connecting the modem to the Network Interface Device (NID) and disconnecting everything else. The NID is usually located on the outside of a home or in an equipment room for a business. If the values do not improve to acceptable levels, then the problem is with the modem or the ISPs infrastructure. Here are some things to try to look for:
1. Replace the modem.
2. Have the ISP verify that there are no load taps or jumper coils in your wiring.
3. Have the ISP verify that there are no bundled T1 circuits in the cable package serving the DSL (not likely to apply to residential).
4. If the problems appear to be weather related, especially during rain, have the ISP inspect and reroute any weather-worn cabling or find better cable pairs.
5. If none of the above is successful, have the ISP change the ports on the DSLAM.
If connecting the modem to the NID improved the values, then the problem is with the internal wiring. Check the following:
1. Wiring from the NID to the modem is good with no wire splices.
2. All phone devices go through a DSL filter.
3. Telephone jacks are in good condition and jacks behind wall plates are securely connected.
4. If the modem is connected to a DSL filter, replace the filter.
5. If none of the above is successful, unplug all telephone devices from the wall except the modem and check their settings. If the values are acceptable, start plugging the other telephone devices into the wall one at a time. Check the values as each device is connected to the connector. When the modem values change back into an unacceptable range, the culprit has been found.
In most cases, the problem has been corrected by one of the items above. If the ISP has been involved, persistence may be required until a knowledgeable technician or customer support is found to fix the problem. Keep in mind that almost all Internet problems can be solved with determination and perseverance.