ADSL service subscriber line test and test instrument selection
The ADSL service uses existing copper wire resources to achieve high-speed data, voice, and image transmission, which enhances the advantages of traditional operators in data access, but also brings new challenges. Since the transmission rate of the ADSL service is higher than that of the old ordinary telephone, it puts forward higher requirements on the quality of the subscriber line. The specifications, length and quality status of copper wires used in existing user networks are relatively complicated. In order to ensure the true realization of ADSL services, the user lines must be tested first.
Under this background, Beijing Telecom R & D Center undertook the testing of Beijing Telecom subscriber line quality and ADSL service opening rate, and successively conducted systematic subscriber line and ADSL service opening rate tests in Wuluju and Clothing College. And cooperated with Wangjing Residential Area ADSL trial network to carry out the test of the subscriber line of the trial user. Based on the above actual work, the specific items of the ADSL service subscriber line test are summarized.
2 Introduction to ADSL Technology
ADSL has two different modulation techniques: carrier-free amplitude-phase modulation (CAP) and discrete multitone modulation (DMT). CAP is based on QAM modulation technology. Compared with DMT, CAP uses a carrier, which is more sensitive to noise interference and requires more power, but the implementation technology is relatively simple. The main principle of DMT modulation technology is to divide the frequency band (0 ~ 1.104 MHz) into 256 orthogonal sub-channels indicated by frequency (each sub-channel occupies a 4 MHz bandwidth), the input signal undergoes bit allocation and buffering, and the input data is divided The bit block is encoded by TCM and then 512-point inverse discrete Fourier transform (IDFT) is used to transform the signal into the time domain. At this time, the bit block is converted into 256 QAM sub-characters. Then add a cyclic prefix to each bit block (used to eliminate inter-symbol interference), and send the signal to the channel through the data analog conversion (DA) and transmission filter. At the receiving end, receive and decode in the reverse order. As DMT technology is recommended in the ASNI T1.413 standard, but there is no standard for CAP, it is expected that ADSL will use DMT technology in the future.
3 Test objectives
The main objective of the subscriber line test is to verify the copper wire's ability to transmit high frequency signals. Only when the ADSL service is launched on the subscriber line that meets certain indicators can the quality of the ADSL service be guaranteed.
Another objective of the subscriber line test is to determine the quality of service that the ADSL service can ultimately provide to customers, usually referring to the highest upstream / downstream rate.
Finally, the test should also be able to locate certain faults that cause poor line quality in order to eliminate them.
4 Factors affecting ADSL transmission
The factors affecting ADSL transmission mainly include line length, bridge taps, induction coils and noise.
4.1 Line length
ADSL technology uses the high-frequency characteristics of copper wire for data transmission. The longer the copper wire, the greater the attenuation of the high-frequency part. The typical value of the longest line that can be applied with ADSL transmission is 4 ~ 6 km. The maximum length of accurate transmission depends on the copper wire specifications and line conditions.
4.2 Bridge tap
Bridge taps are unused branch lines that are connected across copper wires. The reflected signal on the bridge tap near the ADSL Modem accessory has a certain power, which may cancel out useful signal pulses from the far end. ADSL requires the total length of bridge taps on the line to be less than 800 m, and a single bridge tap to be less than 650 m. The bridge taps near both ends have the most obvious effect on transmission.
4.3 Induction coil
When the copper wire is only used for ordinary telephone services, if the line is too long, in order to ensure the flatness of the line spectrum within the 3KHz bandwidth, one or more induction coils are often connected. The induction coil greatly attenuates high-frequency signals, so the induction coil should be removed when the ADSL service is opened.
4.4 Crosstalk and background noise
Crosstalk from other pairs of transmitted digital signals in the same wire harness, especially near-end crosstalk, may be due to poor copper shielding, excessive signal power or line imbalance will affect the signal-to-noise ratio on the line. In addition, for 50Hz The induced voltage of the power line or radio will also affect the signal-to-noise ratio of the line, thus affecting the transmission rate of ADSL data.
5 Test steps and content
From the different process of opening ADSL service, the test can be divided into: line quality test before opening, transmission quality test and fault location test.
5.1 Line quality test before opening
In order to ensure the quick and effective opening of the ADSL service, the quality of the copper wire should be tested before the opening. The items that should be tested are: line length measurement, frequency response test, sensing coil and bridge tap detection.
5.1.1 Measurement of line length
· Determine the line length by measuring capacitance. When there is no bridge tap on the line and there is no water immersion, the length of the line can be known through the capacitor. The line specifications set on the instrument are the same as the line under test.
· Judge the line length by time domain reflectometer (TDR). Disconnect the two ends of the line and connect the meter to one end; the length of the line can be known according to the situation of receiving the reflected signal;
· Determine the line length by measuring the resistance. Use the meter to measure the loop end-to-end resistance. For example: If the resistance value is 1 127Ω, for a copper wire of 0.4 mm, the resistance is 1Ω for every 4m, so that the total length of the line is 4 508 m.
If the test results of several methods are significantly different from the expected values, it may indicate a problem with the line.
5.1.2 Frequency response test
Since ADSL uses a spectrum from 20 kHz to 1.1 MHz or 1.6 MHz, it must be clear whether the signal attenuation on this spectrum meets the requirements. ANSI T1.413 gives requirements for the insertion loss of various CSA loops.
5.1.3 Detection of induction coil and bridge tap
Through the test instrument, it can be determined whether there is the presence and location of the induction coil on the line, and it can also be determined whether the bridge plug and the location are present on the line.
5.2 Transmission quality test
Use the test instrument to simulate the ADSL Modem to determine whether the line meets the service transmission requirements. After activating the circuit, measure the upstream and downstream rates of the line and the number of bits transmitted on each subchannel, as well as the noise margin and bit error rate (BERT).
5.3 errata test
When there is a problem with the ADSL transmission in the service, such as the ADSL Modem can not activate the circuit or fail to meet the service requirements, you need to find the cause of the fault through measurement to find the fault location.
(1) Once the ADSL line cannot be activated, replace the ATU-R with a test instrument to activate the circuit with the central office ATU-C.
· If it can be activated at this time, check whether the minimum bit rate of the NMS is set too high. If it is too high, the ATU-R cannot activate the circuit;
· If the minimum bit rate is set correctly, the ATU-R on the user side should be replaced. You can also use the test instrument with ATU-C module to simulate ATU-C to determine whether the user ATU-R is faulty;
(2) If the instrument cannot activate the circuit at this time, check whether there is a dial tone. Determine whether the circuit is connected to the central office.
· If there is no dial tone, use the TDR function to check whether the line is open;
· If there is a dial tone, check if there is a sense coil on the line, and if there is, exclude it.
(3) If the meter can activate the circuit, but the performance is slightly worse, use a test meter to measure the line background noise, signal-to-noise ratio, and bit number distribution of each subchannel.
If the number of bits on a sub-channel is very small, then the noise at this frequency has a large impact. The possible reasons are:
· AC induction or crosstalk near digital lines. Such as HDSL impact at 196 kHz, ISDN impact at 40 kHz;
· Power line or radio interference;
· There are bridge taps on the line or some lines are soaked, and the position can be determined by the TDR function.
· The insertion loss function of the instrument can also be used to observe whether there is a bridge tap on the line, and its length can be estimated.
If the number of bits on all sub-channels decreases, the most likely cause is a DC problem on the line, such as short circuit, grounding, etc.
(4) Because there are transfer boxes or junction boxes on the line from the office end to the user end, in order to accurately locate the fault, technicians may be required to repeat the above tests with test instruments at various points of the line until the fault is eliminated.
6 Requirements for test instruments
6.1 Functional requirements of the test instrument
In order to meet the test objectives, the ADSL test instrument should have two functions: line quality test and ADSL service quality test.
ADSL service quality test refers to that after the installation of the ADSL central office equipment is completed, the test instrument serves as an emulation terminal to test the highest uplink / downlink rate of the actual service. Therefore, ADSL service quality testing is related to specific ADSL equipment. At present, on the ATU-C side, most manufacturers are compatible with ALCATEL, while on the ATU-R side, in addition to ALCATEL, there are other manufacturers.
Line quality tests include:
(1) Frequency response test
The main factor restricting the development of ADSL services is the transmission capacity of local loops to high-frequency signals. The frequency response test is to determine the attenuation change of the signal transmitted on the subscriber line in the frequency range from 2K to 2.0 MHz.
(2) Noise test
Noise is an important indicator for judging the transmission quality of communication lines, and an important factor that determines the transmission speed of ADSL signals in subscriber lines. The noise in the subscriber line usually includes impulse noise, crosstalk noise and induced noise. Before starting the actual business, it is necessary to quantitatively measure the noise on the line.
(3) Fault location
Use the time domain reflectometer to determine the specific location of the fault in the line.
(4) Detection of load coil and bridge tap
The existence of the load coil and the bridge tap has a great damage to the transmission of high-frequency signals, so first of all, the sense coil and bridge tap in the line should be detected in order to clear it.
(5) Metal characteristic loopback test
Used to measure the metal characteristics of the line, including voltage, current, capacitance and impedance.
6.2 Performance indicators of test instruments
(1) Frequency response
Frequency response testing should include single-ended testing and double-ended testing. The distance range should be 4 000 m for single-ended testing and 8 000 m for double-ended testing. The frequency range is from 0 to 2 MHz. The noise level ranges from 0 ~ -90 dBm / Hz.
(2) Noise test
Noise testing includes power spectral density and impulse noise testing. With IEEE-743 / 1995 E, F and G filters. The frequency range is from 0 to 2 MHz. Noise level range: 0 ~ -156 dBm / Hz.
(3) Time domain reflectometer (TDR)
The distance range should be 8 000 m.
(4) Detection of load coil and bridge tap
The distance ranges from 0 to 5 000 m.
(5) Electrical characteristics loopback test
The test content includes voltage, current, capacitance and resistance. The maximum DC voltage is 400V, the AC voltage is 250V, 50Hz, the resistance is from 0 to 9.9 MΩ, the capacitance is from 100 pF to 10 μF, and the maximum current is 200 mA.
(6) Measurement of the highest rate
The distance range should be 8 000 m, and the frequency range is 4.3 k ~ 1.104 M.
6.3 Other requirements for test equipment
· Test instruments must be lightweight and easy to transport;
· The test instrument needs an RJ-45 connector connected to the subscriber line;
· The operation manual should include a description of the operation of the test set and background knowledge related to the measurement of the time domain reflectometer;
· The test instrument has self-protection function against harmful voltage and current to meet the internationally recognized electrical safety requirements;
· Test instrument with LCD graphic display;
· The test equipment must be easy to operate and can store and print measurement results independently.
7 Conclusion
Through the analysis of the field test data, it can be seen that most of the current lines can meet the transmission requirements of the ADSL service, but some old lines with a length of more than 3 km cannot open the ADSL service. Therefore, in order to ensure the effective provision of ADSL services, the testing of line quality and transmission rate is indispensable. In addition, the test work should also consider possible errata tests, and the choice of test instrument is an important factor that affects the test efficiency.
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