Module 4: Cable Testing
4.2.2 Attenuation and insertion loss on copper media (Core)
Attenuation is the decrease in signal amplitude over the length of a link.
Long cable lengths and high signal frequencies contribute to greater signal attenuation. For this reason, attenuation on a cable is measured by a cable tester with the highest frequencies that the cable is rated to support.
Attenuation is expressed in dBs with negative numbers. Smaller negative dB values are an indication of better link performance.
The resistance of the copper cable converts some of the electrical energy of the signal to heat.
The normal impedance of a Category 5 cable is 100 ohms. If a connector is improperly installed on Category 5, it will have a different impedance value than the cable. This is called an impedance discontinuity or an impedance mismatch.
Impedance discontinuities cause attenuation because a portion of a transmitted signal is reflected back, like an echo, and does not reach the receiver.
When the reflected signal strikes the first discontinuity, some of the signal rebounds in the original direction, which creates multiple echo effects. The echoes strike the receiver at different intervals. This makes it difficult for the receiver to detect data values. This is called jitter and results in data errors.
The combination of the effects of signal attenuation and impedance discontinuities on a communications link is called insertion loss.
4.2.3 Sources of noise on copper media (Core)
Noise is any electrical energy on the transmission cable that makes it difficult for a receiver to interpret the data sent from the transmitter.
TIA/EIA-568-B certification now requires cables to be tested for a variety of types of noise.
Crosstalk involves the transmission of signals from one wire to a nearby wire.
When voltages change on a wire, electromagnetic energy is generated. This energy radiates outward from the wire like a radio signal from a transmitter. Adjacent wires in the cable act like antennas and receive the transmitted energy, which interferes with data on those wires.
Crosstalk can also be caused by signals on separate, nearby cables. When crosstalk is caused by a signal on another cable, it is called alien crosstalk(外部串音).
Crosstalk is more destructive at higher transmission frequencies.
Cable testing instruments measure crosstalk by applying a test signal to one wire pair. The cable tester then measures the amplitude of the unwanted crosstalk signals on the other wire pairs in the cable.
In twisted-pair cable, a pair of wires is used to transmit one signal. The wire pair is twisted so that each wire experiences similar crosstalk. Because a noise signal on one wire will appear identically on the other wire, this noise be easily detected and filtered at the receiver.
Higher categories of UTP require more twists on each wire pair in the cable to minimize crosstalk at high transmission frequencies.
When connectors are attached to the ends of UTP cable, the wire pairs should be untwisted as little as possible to ensure reliable LAN communications.
4.2.4 Types of crosstalk (Core)
The three types of crosstalk:
Near-end crosstalk (NEXT) is computed as the ratio of voltage amplitude between the test signal and the crosstalk signal when measured from the same end of the link.
NEXT needs to be measured from each pair to each other pair in a UTP link, and from both ends of the link.
Due to attenuation, crosstalk occurring further away from the transmitter creates less noise on a cable than NEXT. This is called far-end crosstalk, or FEXT. FEXT is not as significant a problem as NEXT.
Power Sum NEXT (PSNEXT) measures the cumulative(蓄積的) effect of NEXT from all wire pairs in the cable. PSNEXT is computed for each wire pair based on the NEXT effects of the other three pairs. TIA/EIA-568-B certification now requires this PSNEXT test.
Some Ethernet standards such as 10BASE-T and 100BASE-TX receive data from only one wire pair in each direction. However, for newer technologies such as 1000BASE-T that receive data simultaneously from multiple pairs in the same direction, power sum measurements are very important tests.
4.2.5 Cable testing standards (Core)
The ten primary test parameters that must be verified for a cable link to meet TIA/EIA standards are:
A Ethernet NIC transmits signals on pins 1 and 2, and it receives signals on pins 3 and 6. The wire map test insures that no open or short circuits exist on the cable. An open circuit occurs if the wire does not attach properly at the connector. A short circuit occurs if two wires are connected to each other.
There are several different wiring faults that the wire map test can detect. The reversed-pair fault occurs when a wire pair is correctly installed on one connector, but reversed on the other connector.
Example: If the white/orange wire is terminated on pin 1 and the orange wire is terminated on pin 2 at one end of a cable, but reversed at the other end, then the cable has a reversed-pair fault.
A split-pair wiring fault occurs when one wire from one pair is switched with one wire from a different pair at both ends. Look carefully at the pin numbers in the graphic to detect the wiring fault. A split pair creates two transmit or receive pairs each with two wires that are not twisted together. This mixing hampers the cross-cancellation process and makes the cable more susceptible to crosstalk and interference.