Integrated Wiring concepts

In view of the uneven quality of practitioners in the field of structured integrated cabling in China, there are deviations in the understanding of testing methods and testing standards, this newspaper will give a brief introduction to the parameters involved in the cabling system testing and their testing principles in several phases, in order to promote the development of the cabling industry.
If the new test standards for category 5 and Category 6 can be released in the first quarter of next year as scheduled by TIA, domestic cabling product providers will face a severe test. The new test standard will add several parameters, such as level remote crosstalk ELFEXT), Return Loss (Return Loss), latency difference Delay Skew) and so on. These new parameter requirements will make the previous cabling method that relies only on one person for supervision and several people for construction history, and the original one-way five-category Cable Testing Instruments in the cabling vendor will also be eliminated. Testing will become an indispensable part of a cabling project.
According to the requirements of TSB-67 standard, in the structured Integrated Wiring System Validation Test indicators need to contain wiring diagram, length, attenuation, near-end crosstalk and other four parameters. ISO also requires the addition of a parameter, I .e., ACR attenuation to crosstalk ratio ). In view of the current development trend of the network and the gradual popularization of cat6 lines, this year, TIA added the testing standards and parameters for the Integrated Wiring System. The added test parameters include:
Wiring Diagram
Length Measurement
NEXT)
　
Accumulated power NEXT (PowerSum NEXT, PSNEXT)
　
Attenuation)
　
Attenuation to Crosstalk ratio (Attenuation Crosstalk Rate, ACR)
　
Remote crosstalk (FEXT) and other level remote crosstalk (ELFEXT)
　
Propagation Delay)
　
Latency difference (Delay Skew)
　
Structured Return Loss and Return Loss
Bandwidth
Impedance)
DC loop resistance
Miscellaneous
■ Wiring diagram
The wiring diagram is used to indicate the wrong wiring method. The four-to-eight-core wiring diagram of each cable can be expressed as follows:
Correct line position at each end point
Whether it is connected to the remote connection
Two-core or multi-core Short Circuit
Staggered line pair
Reverse line pair
Split line pair
Other wiring errors
Reverse refers to the opposite polarity of one end of the line pair. Crossover refers to the peer adjustment of two remote lines. Split refers to the one-to-one connection of each core line, but the physical line is separated from each other. Note that the split-line pair is a frequent wiring fault that cannot be accurately identified using a simple disconnection instrument. In the 10Base-T network, this wiring fault does not have much impact on the overall operation of the network because of its loose requirements on the cabling system. However, high-speed Ethernet testing equipment, for example, the wiring diagram test function of 100Base-TX testing instrument must be able to detect such errors. Because the five types of verification instruments are expensive, users can choose to use the local area network detector microbench produced by Microtest in the United States, the instrument can comprehensively detect various wiring problems, the price is cheap and convenient.
■ Length Measurement
A tdr (time-domain reflection measurement) technology is used to measure the length of copper cables. The tester emits a pulse wave from one end of the copper cable. When the pulse wave is traveling, if the impedance changes, such as open circuit, short circuit, or abnormal wiring, some or all of the pulse wave energy will be reflected back to the tester. Based on the delay time of the back and forth pulse wave and the NVP (rated transmission rate) Rate of the known signal transmitted on the copper cable, the tester can calculate the length of the pulse wave receiving end to the Return Point of the pulse wave. NVP is expressed by the percentage of light speed (c), such as 0.75c or 75%.
The amplitude of the returned pulse wave is proportional to the degree of impedance change. Therefore, when the impedance changes greatly, such as an open or short circuit, a relatively large ECHO is returned. Impedance changes (impedance exceptions) caused by poor contact will produce a small echo.
Whether the length of the measurement is accurate depends on the NVP value. Therefore, a known length data (must be above 15 meters) should be used to correct the NVP value of the tester. However, the accuracy of TDR is hard to reach less than 2%. At the same time, the NVP values of various pairs of the same cable also have 4-6% differences. In addition, the actual length of a twisted pair cable is longer than that of a cable. The pulse wave running in a long cable changes to form a tooth shape, which also produces several nanoseconds of error. These are the reasons that affect the accuracy of TDR measurement.
The pulse wave width emitted by the tester is about 20 nanoseconds, and the propagation rate is about 3 nanoseconds/meter. Therefore, when the pulse wave travels to 6 meters, it is the time when the pulse wave leaves the tester. This is the blind zone of the tester during the measurement length. Therefore, the possible wiring problems within the six meters cannot be found during the measurement length (because there is no echo ).
The tester must also be able to display the length of each line. If only the length of one cable can be obtained, it does not mean that the line pairs are of the same length.
Some early testers did not use the TDR principle to measure the length, but used the frequency domain method to measure the reflux loss to measure the impedance change so as to calculate the length, this method may lead to misjudgment when the length of each line is different.
■ NEXT)
When the current flows through a wire, it will produce a certain electromagnetic field and interfere with the signal on the adjacent wire. The higher the frequency, the greater the impact. Twisted Pair wires are twisted together to offset mutual interference due to a phase difference of 180 degrees. The tighter the twisting distance, the better the offset effect, and the higher the data transmission rate.
Near-end crosstalk refers to the crosstalk signal that is sensed from the sending line pair at the receiving end of the same side as the sending end. When the crosstalk signal is too large, the receiver cannot determine whether the signal is a weak signal sent from the remote end or a crosstalk message.
Note that the greater the value of NEXT (such as 45dB), the greater the amplitude difference between the sent signal and the crosstalk signal, and the smaller the value of NEXT (such as 20 dB ), this is something to avoid.
To meet Category 5 specifications, the length of the non-stranded part at the end of the cable cannot exceed 13 meters. Generally, the reason for excessive NEXT generation is as follows:
Use a patch cord that is not a twisted pair.
No crimping terminal as required.
Use 66 old-fashioned wiring blocks.
Use a non-data-level connector.
Use voice-level cables.
Use the connector for the socket.
In addition, when measuring the NEXT value at both ends of the link, especially when the length is greater than 40 meters, the remote crosstalk will be offset by the attenuation of the link, however, the NEXT value cannot be measured in the near-end. The NEXT value measured at both ends of the link is different. Therefore, all test standards require that the NEXT value be measured at both ends of the link.
■ ATTENUATION)
The electrical signal strength gradually decreases with the length of the cable, which is called attenuation. It is represented by negative dB. A larger value indicates a larger reduction, that is, the signal of-10 dB is weaker than that of-8 dB. The difference of 6 dB indicates that the signal strength of the two is twice different. For example, the-10dB signal is twice as strong as the-16dB signal, and four times stronger than the-22dB signal. The factors that affect attenuation are skin effect and insulation loss.
When the frequency is high, the current density in the conductor is no longer evenly distributed in the whole conductor, but concentrated on the conductor surface, thus reducing the current loss caused by the conductor section. The skin effect is proportional to the square root of the frequency. Therefore, the higher the frequency, the larger the attenuation. This is why single-strand cables are more conductive than multi-strand cables.
Temperature may also affect the attenuation of some cables. Some insulation materials will absorb the current flowing through the conductor, especially the PVC material used by the three types of cables, because the chlorine atoms of the PVC will generate bipolar atoms in the insulation material, however, the fluctuation of the bipolar sub will cause the electrical signal to lose part of the electric energy. This situation deteriorates even when the temperature is high. The higher the temperature, the larger the attenuation. This is why the temperature specified in the standard is 20 ℃.
When measuring the attenuation, it must be determined that the measurement is conducted in one direction, instead of measuring the attenuation of the loop and dividing it by 2.
■ Attenuation to crosstalk ratio (ACR)
Due to the attenuation effect, the signal received by the receiver is the weakest, but the receiver is also the most crosstalk signal. For unshielded cables, crosstalk is the most important noise generated from the sender. The so-called ACR refers to the difference between crosstalk and attenuation. ACR represents the performance of the cable, that is, the richness of the signal at the receiving end. Therefore, the larger the ACR value, the better. ACR indicators are defined in ISO and IEEE standards, but not in TIA/EIA 568A.
Because the NEXT values of each line pair are different, the ACR values of each line pair are also different. The ACR value of the cable is the worst ACR value during measurement. If it is compared with PSNEXT, it is expressed by PSACR value.
■ Remote crosstalk (FEXT) and other level remote crosstalk (ELFEXT)
FEXT is similar to NEXT, but the signal is sent from the near end, while the crosstalk noise is measured at the far end. FEXT must also be measured from both ends of the link.
However, FEXT is not a very effective test indicator. The length of the cable has a great impact on the measured FEXT value, because the signal strength is related to its crosstalk and signal attenuation at the sending end. Therefore, two identical cables have different FEXT values due to different lengths. Therefore, the measurement of the FEXT value must be replaced by the measurement of the ELFEXT value. The EXFEXT value is actually the FEXT value minus the attenuation value. ELFEXT can also be understood as the ACR of the far end. Of course, like PSNEXT, the value corresponding to ELFEXT is the value of PSELFEXT.
To measure ELFEXT, the dynamic range (sensitivity) of the tester must be 20 dB lower than the measured signal.
■ Accumulate Power NEXT (Power Sum NEXT)
PSNEXT is actually a computing formula, rather than a measurement step. The PSNEXT value is derived from the algebra and derivation of the crosstalk between the three pairs of lines and the other pair of lines. Measurement of PSNEXT and ELFEXT is a very important test parameter for networks that require four-wire transmission of signals, such as Gigabit Ethernet. Each link has four PSNEXT values.
■ Propagation Delay)
Propagation delay refers to the time required to transmit a signal from one end of the cable to the other end. It is also proportional to the NVP value. Generally, the latency of Class 5 UTP ranges from 5 to 5 bytes per meter ~ About 7 nanoseconds (ns. ISO specifies that the worst time delay of the 100-meter link is 1 microsecond (us ). Latency is one of the main reasons why the LAN has a length limit.
■ Latency difference (Delay Skew)
Latency difference is the transmission time difference between the greatest latency and the smallest line pair in the UTP cable. Some cable manufacturers consider the shortcomings of copper materials, one or two pairs of cables into other materials, this will produce a large time difference. In particular, when a Gigabit Ethernet application is running, too much time difference will cause the signal sent from both the four-line pairs to the receiving end at the same time. Generally, the maximum time difference between 100-meter links is 50 nanoseconds, but it is best to be less than 35 nanoseconds.
■ Structured Return Loss
Structured Return Loss (SRL, Structural Return Loss) measures the consistency of the cable impedance. Because the structure of the cable cannot be completely consistent, the impedance may change a little. Changes in impedance may cause loss of signals. Structured return loss is related to the design and manufacture of cables, and is often affected by the construction quality unlike NEXT. SRL is represented by dB. The higher the value, the better.