Application and test of ADSL Technology

ADSL line physical layer indicators and Test Methods

Currently, ADSL technology has been widely used. ADSL is a technology that enables high-speed data transmission over traditional telephone networks. Therefore, we have encountered many maintenance and testing problems. This series of articles attempts to introduce the ADSL technology and its maintenance from different perspectives, such as the physical layer, transmission layer, protocol layer, and network structure layer. In this article, the testing thresholds and requirements for each indicator are mainly composed of three aspects: direct or indirect reference to international or national standards, and actual testing experience. An actual ADSL user access network structure is shown in Figure 1 ".

1. Insulation Performance of user lines and equipment

The insulation performance of the user line mainly affects whether the original telephone services on the line can work normally. In severe cases, the transmission performance of ADSL can be affected. This indicator requires testing from the "U-C2 interface", "U-R2 interface", "PSTN interface" and "POTS interface" in "Figure 1: the insulation resistance between line A and line B is greater than 5 MΩ.

2. Loop impedance and Loop Length

The user line ring resistance suitable for activating ADSL Services should be less than 900 ohm, and the line length should be less than 3.7 kilometers. If the ring resistance is too high or the Loop Length is too long, the rate of activation for ADSL users is very low or cannot be activated at all.

The user line length corresponds to the line diameter as follows:
User line length = loop resistance/resistivity) limit 2
Common User line routes are as follows: 0.3, 0.4, and 0.5.

3. Line Voltage

The DC voltage of the ADSL user line should be within ± 1V after the PSTN switch is disconnected. If the voltage value measured after the PSTN switch is disconnected does not meet the requirements, it indicates that the tested line has a short-circuit fault with other lines with voltage. This phenomenon is usually caused by faults such as poor Line Insulation, short circuits between the wire core and the distribution frame. In addition, when the PSTN switch is normally connected and the phone is on the hook, the voltage measured from the POTS interface should be around 45 V to ensure the normal operation of the phone.

4. frequency-domain attenuation Index

The larger the line attenuation, the lower the electrical flat of the received useful signals and the smaller the signal-to-noise ratio, the fewer bits the sub-carrier frequency modulated, as a result, the rate of the ADSL line decreases, the error rate increases, or even "dropped" or cannot be activated at all.

The longer the line, the greater the attenuation. In addition, because the line from DLSAM to the client device usually has 4 ~ Because of oxidation, moisture, poor insulation, and other factors, the attenuation of the five connectors usually increases. Therefore, the attenuation of the detection line is an important means of ADSL fault repair.

The attenuation of the ADSL user line at each frequency in the ADSL band should be less than 35 dB.

The commonly used unit for measuring the attenuation of a line is "decibels". It corresponds to the ratio of bandwidth and power:

5. Spectrum Density PSD of background noise)

Through the "spectrum density" test, we can effectively display the frequency distribution of background noise that affects ADSL communication, these noises mainly come from Signal Crosstalk of the same group of cables and strong interference sources in the band, such as AM radio station, motor, and switching power supply.

Actual testing experience shows that when the background noise of the ADSL user line on each DMT sub-channel is less than-94dBm/Hz, the line can adapt to various ADSL applications within 4 kilometers. For a line whose noise level is higher than this value, the method of limiting the downlink speed or reducing the loop attenuation is usually to reduce the Loop Length) should be adopted to improve the signal-to-noise ratio, so as to ensure the communication quality of the line.

The modulation depth of DMT sub-channels in frequencies with high background noise will be reduced, and the data rate will be reduced, if the background noise is high for the whole frequency, the user's availability rate may be low or frequent disconnection.

6. Vertical balancing

ADSL transmission uses twisted pair wires. The two core wires have the same physical characteristics theoretically, and the interference signal has the same effect on the two core wires, this "Common Mode Interference" can be easily removed at the receiving end of the signal. However, in the actual environment, the physical characteristics of the two wires of twisted pair wires are not identical, resulting in inconsistent effects of Interference Signals on each line, resulting in "Differential Mode Interference ", this type of differential mode interference cannot be distinguished from useful signals, which directly affects the communication quality. Lines with poor vertical balancing indicators have weak anti-interference capabilities, and are prone to disconnection or cannot be activated at all.

Vertical equilibrium attenuation = 20Log (Vout/Vin) (figure)

The vertical equilibrium attenuation value of the user line at each frequency in the ADSL band should be greater than 40 dB.

7. Near-end crosstalk NEXT)

Near-end crosstalk mainly refers to the interference of signals sent by devices on the same side of the cable to receive signals on other lines.

The near-end crosstalk indicators of the user cables that activate the ADSL service in each frequency in the ADSL band should be less than-40 dB. The maximum number of users of cables with poor near-end crosstalk indicators is small.

8. Remote crosstalk FEXT)

Remote crosstalk mainly refers to the interference of signals sent by devices at both ends of the cable to receive signals on other lines.

The user cable used to activate the ADSL service should have a remote crosstalk indicator of less than-50dB for each frequency in the ADSL band. The maximum number of users of cables with poor remote crosstalk indicators is small.

9. Pulse Noise)

Pulse noise may be caused by some unexpected events, such as Motor Startup and telephone dismounting. Pulse noise may increase the ADSL line code, and even interrupt the ADSL communication in severe cases. At the same time, the out-of-band leakage of ADSL signals can also form audio pulse noise, thus affecting the quality of telephone communication.

During the initialization or normal operation of the ADSL system, within 15 minutes of the test time, the PSTN or POTS interface receives a pulse noise count that exceeds-43dBmp for less than 15 times.

10. signal attenuation of the splitter In the ADSL band

The signal attenuation in the ADSL band should be greater than-1 dB. If this indicator is unqualified, the user may be disconnected from the Internet or the activation rate is low.

11. Bridge joint Bridge Tap)

Load Coil)

Adding a sense coil will cause the high-frequency part of the signal to be compensated accordingly, thus improving the quality of long-distance calls. The inductive coil should be removed from the ADSL line. However, there is basically no inductive coil in the domestic telephone network.