In-depth analysis of ADSL access network faults

Users of the access network have encountered many problems or faults. Here we will introduce the solutions to common problems of ADSL access network faults. The application of the ADSL access technology greatly increases the user's Internet access speed and solves the network bottleneck in the past. However, with the rapid growth of ADSL users, user fault complaints have also greatly increased. In these user fault complaints, line faults account for a considerable proportion.

The main reason for this is that ADSL directly uses the User Access Network of the original PSTN to carry out broadband access services, some problems existing on the original wired road that are not suitable for high-frequency signal transmission cause frequent line faults of ADSL users, which plague the majority of Internet users and line maintenance personnel.

There are two main causes of ADSL user faults: attenuation line faults and interference line faults. Attenuation line faults are mainly manifested in: user line passing length, line material transmission performance is poor, line short-circuit and open circuit, joint oxidation, poor contact, excessive bridging head, line adding coil, etc. The main manifestations of interference-type ADSL access network faults are: high background noise, broadcast interference, Signal Crosstalk between different cable pairs, electromagnetic interference of household appliances, electromagnetic interference caused by motor or electric locomotive, and weather such as lightning). User faults are often caused by the combination of several factors listed above. Next we will further analyze the fault security of some routine maintenance of ADSL.

ADSL Access Network Fault Security 1: poor contact, poor connection Oxidation

[Symptom] an ADSL user calls normally, but cannot access the Internet. The Modem activation indicator is always flashing. the user can enable the Modem activation indicator only when the phone is put off the server.

[Test] in the local measurement room, the user line mixing line is about to short connect user A and line B), and the ring resistance of the measurement line on the user end is 4653 Ω, the Ring Resistance of the measurement line at the user's off-floor distribution box is 673 Ω.

[Analysis] the user can call normally to determine that there is no open circuit or short circuit on the user's external line. The results of the ring resistance test at the wire box are normal, but the ring resistance indicators tested on the user end are not normal. After the inspection, it is found that the user's sub-household line is connected by three lines in a string, the joints are oxidized seriously, and the fault disappears after the user is offline. the user can call and access the Internet normally, and no disconnection occurs.

[Conclusion] The user's ring resistance test is an important test for maintaining ADSL lines. The line ring resistance indicators suitable for activating ADSL Services should be less than 900 Ω, if the line length calculated based on the tested user-line loop resistance value is divided by the loop resistance value by the DC resistance on the line unit length) A large deviation from the actual line length of the user indicates that there are high-resistance faults such as poor contact and oxidation of joints on the line. It should be noted that when the resistance value of the outdoor wire ring is automatically tested using the 112 measuring unit, due to the DC current applied on the line during the test, the oxide film of some non-serious oxidized films will be broken down to obtain the normal test results, this is also the reason why the Modem can be activated when the user places the phone number in the off-hook state ).

ADSL Access Network Fault Security 2: excessive attenuation of user introduced lines

[Symptom] a adsl user applied for a downstream bandwidth of 2 Mbit/s, but it is slower than other users with 2 Mbit/s bandwidth.

The test results show that the maximum activation rate is 1.3 Mbit/s and the ring resistance is 820 Ω; test the maximum activation rate of the user port in the downstream direction of the user's external wire box is 2.9 Mbit/s, the ring resistance is 795 Ω, the user's offline line length is 100 m, and there is no connection.

[Analysis] According to the test results, the main reason for the low activation rate is that the signal attenuation of the user's household line is too high, the maximum downstream activation rate measured on the user end is 2.6 Mbit/s after the next user line is replaced with Copper Twisted Pair wires, increasing the user's Internet access rate.

[Conclusion] most of the current user-introduced lines are commonly referred to as "sub-household lines") use parallel single-strand lines, and most of the materials are ferroalloy materials, its high-frequency signal transmission performance and anti-interference performance are much lower than that of Copper Twisted Pair wires. Therefore, excessive line attenuation may cause low activation rate or difficulty. We recommend that you use Copper Twisted Pair wires, and the length should not exceed 100 m.

ADSL Access Network Fault security 3: unreasonable offline cabling causes serious signal interference.

[Symptom] an ADSL user reports that the Internet access speed is slow, the download speed is only 7 kbit/s, and the download speed is often dropped.

[Test] The maximum activation rate of the instrument on the user end is 232 kbit/s, and the actual activation rate is only 96 kbit/s. The user's next household line is 150 m in length and uses parallel lines. In order to fix the line, it is wound in multiple places, such as the outdoor pole and the heating pipe in the room. Change the user's offline cable to twisted pair wires and minimize the length to 100 mb. If the user's fault disappears, the maximum downstream activation rate reaches 2.6 Mbit/s, the actual downstream speed is 512 kbit/s, and the download speed is also increased to 60 kbit/s.

[Analysis] the background noise spectral density of the user line suitable for activating ADSL Services within 10 kHz to 1.1 MHz in the ADSL band should be less than-94dBm/Hz. The background noise tested on the user end is too high, and the test result is normal after the sub-box is opened. The main cause of the user's fault is that the sub-line is too long and the wiring is unreasonable, too many interference signals are introduced, resulting in low activation rate and frequent disconnection.

[Conclusion] too long and too many winding lines will cause external electromagnetic interference signals to be introduced like an antenna, and the anti-interference capability of parallel lines is poor, further aggravating this fault.

ADSL Access Network Fault case 4: high background noise on the device Port

[Symptom] users accessing a certain type of ADSL local device generally have a low activation rate, and it is difficult for users with a line ring resistance exceeding 500 Ω to activate the device.

[Test] Judging from the common characteristics of the fault, the problem may be caused by local equipment. The measuring station disconnects the user's external line and directly tests the port of local equipment, it is found that the maximum activation rate in the downstream direction measured at this position is only 3.4 Mbit/s, and the background noise power spectral density and bandwidth noise indicators in the ADSL band are further tested.

[Analysis] The maximum downstream activation rate of the user port tested on the measurement station is only 3.4 Mbit/s. The test point is very close to the device port, and the line attenuation can be ignored, therefore, the main cause of this fault is signal interference. If you choose to disconnect the user's external line from the measurement station for testing, you can queue the external line for interference to verify the noise leakage of the local device port. The reference value of the bandwidth noise indicators of user lines suitable for activating the ADSL service in the ADSL band is: the upstream direction is less than-50.23dBm, and the downstream direction is less than-40.23dBm. In this example, although the power spectral density of the background noise of the port does not exceed the reference value-94dBm/Hz given to the specified point, the noise power in the entire ADSL band, especially in the downstream direction, is high, as a result, the bandwidth noise indicator in the downstream direction is unqualified.

[Conclusion] G. the maximum downstream activation rate of the ADSL user line operated in DMT mode should be greater than 7 Mbit/s after the measuring station disconnects the user's external line, when the tested data is smaller than this value, the background noise of the local device's user port is high or the local wiring attenuation is high. Therefore, during maintenance of users with ADSL access network faults, we should first ensure that the indicators tested on the measurement station are normal, thus avoiding blind external testing. To sum up, in order to ensure the normal use of ADSL users and reduce the fault complaint rate, users should pay attention to the rationality of users' wiring during initial installation, so as to avoid some wiring methods that may cause users' faults in the future.

On the other hand, with the large-scale development of ADSL Services, the methods and testing methods of PSTN line maintenance in the past are insufficient to find out the causes of ADSL access network faults for broadband users. Some new testing methods need to be introduced, for example, background noise power spectrum density test, broadband noise test, line vertical balancing performance test, cable crosstalk indicator test, etc. Only through comprehensive tests can we find out the real cause of ADSL user barriers.