DSL Internet circuits may experience problems due to a many of reasons. Most are related to the individual end user’s connection. And most can be detected by a simple check of Sync speed, 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 advisable to run a packet loss test on any Internet connection before an improvement project starts. The packet loss test should confirm that there is trouble at the end user’s connection and that the Internet Service Provider’s (ISPs) network is not the source of the trouble. In addition, the packet loss test will provide baseline statistics to compare future results against.
Assuming a packet loss test has revealed trouble at the end user’s IP address, it is time to look at the DSL modem and examine it for trouble:
According to Wikipedia.org, “signal-to-noise ratio (SNR) is a measure 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 the noise power corrupting the signal. A ratio higher than 1:1 indicates more signal than noise.”
An everyday example of SNR is listening to music in your car over road noise or other people talking. The louder the radio compared to the other noise in the car, the more clearly you hear the music. The same is true for the DSL modem “hearing” the signal transporting the Internet traffic. When the signal is loud compared to the noise, Internet communication happens at a faster speed with less packet loss. When the signal is not loud enough compared to the noise, speeds slow down causing latency. Packets may be lost or discarded, creating retransmissions of data packets and trouble with real-time applications such as Voice over Internet Protocol (VoIP) used by Vonage, Skype and Hosted PBX providers. In fact, any real-time application will suffer including video streaming from Netflix, Blockbuster and others or any type of online gaming.
DSL Parameter Values
The values to check in a DSL modem are Sync, Attenuation and Signal-to-Noise Ratio.
Sync is described in downstream and upstream 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 is not just dependent on distance. It also depends on cable type and gauge (which can differ over the length of the cable), the number and location of other connection points on the cable. 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 may experience connectivity issues
60db and above = Bad and will experience connectivity issues
Like Sync and Attenuation, SNR has downstream and upstream measurements.
6db or below = Bad and will experience no line synchronization and frequent disconnections 7db-10db = Fair but does not leave much room for variances in conditions.
11db-20db = Good with little or no disconnection problems
20db-28db = Excellent
29db or above = Outstanding
SNR will sometimes be displayed in margin or SNRM. This is the difference between the current SNR value and the SNR that is obligate to keep a reliable circuit at the connection speed. If the SNRM is minimal, the circuit is more likely to suffer intermittent connection faults and slowdowns. High margins are required to prevent bursts of interference from causing connection losses. The target SNRM is usually 6db but could be as high as 12db.
Improving the DSL Modem’s Values
Cabling and connectors are the most common cause of DSL problems. Internal wiring can easily be eliminated by simply plugging the modem into the Network Interface Device (NID) and unplugging everything else. The NID is usually located outside on a house 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 and look for:
1. Replace the modem.
2. Have the ISP verify that there are no load taps or bridge coils in their cabling.
3. Have the ISP verify that there are no T1 circuits grouped in the cable bundle serving the DSL (not likely to apply to residential).
4. If problems seem to be weather related especially during rain, have the ISP inspect and bypass any weather worn cabling or find better cabling pairs.
5. If none of the above is successful, have the ISP change the ports on the DSLAM.
If plugging the modem into the NID improved the values, then the problem is in the internal wiring. Check for the following:
1. Cabling from NID to modem is in good condition without any cable splices.
2. All telephone devices pass through a DSL filter.
3. Telephone jacks are in good condition and connectors behind the wall plates are solidly connected.
4. If the modem is plugged into a DSL filter, replace the filter.
5. If none of above is successful, unplug all telephone devices from the wall except the modem and check its values. If the values are acceptable, start plugging the other telephone devices back into the wall one at a time. Check the values as each device is plugged into the jack. When the modem values change back to an unacceptable range, the culprit has been found.