Main factors affecting the quality of ADSL lines
The ADSL signal and basic audio telephone signal (below 4KHz) are passively coupled to the ordinary telephone line through the ordinary telephone service splitter. Even if the ADSL system fails, it will not affect the ordinary telephone service.
ADSL (Asymmetric Digital Subscriber Line) is a new technology for transmitting high-speed asymmetric digital signals on existing user twisted-pair copper wires. Among them, the asymmetry is from the office end to the user end, and the downlink rate can reach 1.5-8Mb / S; the upstream channel from the user end to the office end includes the control overhead signal to the network, the rate is generally between 64k ~ 1Mb / s.
ADSL can transmit such a high rate, mainly using advanced modulation technology, DMT modulation is the current international standard of ADSL. The main principle is to divide the transmission channel into 256 orthogonal sub-channels (that is, use multiple carrier frequencies to transmit data simultaneously).
The ideal ADSL line should have no induction coils, no change in wire gauge, no bridge taps, good insulation, and transmission distance is not too long. In fact, the end-to-end performance of the line is affected by the following factors: loop length, induction coil, end-to-end parallel connection quality of the wire segment, complex changes in wire gauge, home wiring, time, corrosion, beam group, crosstalk Interference and bridging taps. In addition, home decoration, corrosion, internal bridging joints and aging insulators damage telephone lines. Most lines bypass physical obstacles indoors and outside and lengthen. These more or less affect the quality of ADSL lines.
Decay
In the transmission system, after the signal sent by the transmitter is transmitted over a certain distance, its signal strength will be weakened. The high-frequency component of the ADSL transmission signal has a severe attenuation history when passing through the subscriber line. For example, when a 2.5V transmission signal reaches the ADSL receiver, the amplitude can only reach the millivolt level. This weak signal is difficult to ensure the signal-to-noise ratio required for reliable reception, so additional coding is necessary. In the ADSL system, the signal loss is also closely related to the transmission distance, the transmission line diameter and the frequency point where the signal is located. The higher the frequency, the greater the attenuation. Since the frequency used by ADSL is between 20KHz and 1.1MHz, its attenuation is greater than that of pure voice transmission. And another technology called HomePNA uses a frequency of up to 10MHz, so the effective transmission distance is shorter than ADSL.
The thickness of the wire diameter is another factor affecting the attenuation. At the same frequency, a cable with a thicker wire diameter can transmit a longer distance. But the thicker the cable, the more copper is consumed, and the greater the investment. Now some telecommunications departments have begun laying 0.6mm or larger copper wires to provide higher speed data transmission.
In the actual application of ADSL, the attenuation value has become a content that must be tested, and it is also an important factor to measure the quality of the line. The increase in the attenuation caused by the increase in the distance between the user equipment and the office equipment will directly cause the transmission rate to decrease. In actual measurement, the loop resistance between lines is undoubtedly an important parameter to measure the transmission distance. For example, in the same situation, the actual measurement: when the line resistance is 245Ω, the attenuation value is 18dB, and when the line resistance is 556Ω, the attenuation value will increase to 33dB.
Attenuation is inevitable, but we cannot simply increase the transmit power to ensure the strength of the receiving signal. Because with the increase of power, the influence of crosstalk and other interference on the transmission quality will increase, and it may also interfere with adjacent radio communications. For each ADSL manufacturer, generally the Modem's attenuation adaptation range should be between 0dB-55dB.
Intersymbol interference
ISN (Intersymbol Interference) is the most important interference in digital communication systems except for noise interference. It is different from additive noise interference and is a multiplicative interference. There are many reasons for ISI. Both channel attenuation and group delay distortion may cause signal waveform distortion, resulting in inter-channel interference (ICI) and ISI. In fact, as long as the frequency band of the transmission channel is limited, it will inevitably cause a certain amount of ISI. The waveform sequence transmitted at a certain speed is affected by the non-ideal channel as the duration of the waveform of each symbol is prolonged, so that the adjacent symbol waveforms overlap, resulting in a decision error. When the linear distortion is serious, ISI will be more serious. In order to eliminate inter-symbol interference, there are usually two ways: First, the transmission system has a uniform and infinite bandwidth, so that the transmitted signal will not produce any distortion, but it is actually impossible. Second, it only guarantees that the signal has no inter-symbol interference at the sampling time, and does not require sampling values ​​at non-sampling points.
Reflection interference
A bridge tap is a short wire that extends somewhere. A non-terminated tap emits energy, reduces the strength of the signal, and becomes a source of noise. From the central office equipment to the user, there are at least three connectors (bridging points), and the wire diameter of each connector will change accordingly. In addition, the sudden change in impedance caused by cable loss will cause power reflection or reflected wave loss. In voice communication, the performance is echo, while in ADSL, the complex modulation method is susceptible to interference from the reflected signal. At present, there is no test equipment capable of measuring the reflection source in the user distribution cable at 300 kHz or higher. At present, most of them use echo cancellation technology. Although the effect of eliminating echo from one source is very good, when the signal passes through multiple reflections, echo cancellation becomes almost ineffective.
Crosstalk
Crosstalk interference occurs when the pair of wires is twisted in a bundle. Due to the coupling of capacitance and inductance, the transmission signal of the twisted pair transmitter in the same backbone cable may cross into other transmitters or receivers, causing crosstalk. Generally divided into near-end crosstalk (NEXT) and far-end crosstalk (FEXT).
For ADSL lines, the far-end crosstalk will not have a great impact, because as the distance increases, the far-end crosstalk will produce greater attenuation through the channel transmission, and the impact on the line is less, and the near End-to-end crosstalk interferes with the sending end from the beginning and has a greater impact on the line. But when the distance is short, the distortion caused by the far-end crosstalk is also very large, especially when many users in a cable transmit this high-speed signal, the interference is particularly significant, and it will limit the role of this system's echo cancellation device Scope, this kind of crosstalk interference is particularly serious for DMT ADSL equipment using echo cancellation technology. In addition, crosstalk interference, as a function of frequency, grows rapidly as the frequency increases. Since voice belongs to a relatively low-speed service, crosstalk interference will not be very serious, but ADSL uses high frequencies, which will have serious consequences. Therefore, it is best not to have multiple ADSL lines or lines with similar frequencies in the same trunk.
Noise interference
The circuit lines may be affected by several forms of noise and interference. In order to achieve effective data transmission, it is necessary to ensure that the received signal is strong, the dynamic range, and the signal-to-noise ratio are within an acceptable range.
In addition, ADSL is a technology based on twisted pair transmission. Twisted pair twists two insulated copper wires to each other with a certain rule, which can effectively resist the external electromagnetic field interference. But at present, many lines from the telephone company's junction box to the user's phone are parallel lines, which is very unfavorable for ADSL transmission and greatly reduces the Internet access rate. For example, under the same circumstances, the use of twisted pair downlink rate can reach 852kb / s, while the use of parallel line downlink rate is only 633kb / s.
There are many reasons for the noise, which may be the switch of household appliances, the off-hook and on-hook of the telephone, and the movement of other electric equipment. These sudden electromagnetic waves will be coupled into the ADSL line, causing sudden errors. Because ADSL is enough to superimpose high-frequency digital signals on the low-frequency voice of ordinary telephone lines, adding any equipment to the connection from the telephone company to the ADSL splitter will affect the normal transmission of data, so do not connect the phone before the ADSL splitter , Telephone anti-theft devices and other equipment.
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