Figure 3

Cable quality may be poor if a cable manufacturer has substituted some material because another material is in short supply, as happened several years ago during the worldwide shortage of FEP (Fluorinated Ethylene-Propylene Teflon). According to Anixter, there now exist more than 45 plenum and nonplenum cable designs that exhibit varying electrical performance characteristics but are still labeled Category 5 compliant. Moral: Have all existing cable installations tested and certified for use with the networking technology you plan to use. You may find that some existing cable runs will support high-speed networks, while others may need connector replacements, and still other require complete replacement.

The following sections discuss various cable characteristics and environmental conditions that affect performance and how those characteristics affect cable and network design.

Signal transmissions over long distances are subject to attenuation, a loss of signal strength or amplitude. Attenuation is also caused by broken or damaged cables. Attenuation is the main reason why networks have various cable-length restrictions. If a signal becomes too weak, the receiving equipment will interpret it incorrectly or not at all. This causes errors, which require retransmission, and loss of performance.

The following shows the weakening of signal due to attenuation:

Attenuation is measured in dB (decibels) of signal loss. For every 3dB of signal loss, a signal loses 50 percent of its remaining strength. Attenuation can be measured by cable testers that inject signals with a known power level at one end of the line and measure the power level at the other end of the line. A typical readout from a Fluke tester is shown in the following illustration. Note that the cable is tested at increasing frequencies and the margin between the tested attenuation and the maximum allowable attenuation (based on Category 5 specifications) is listed.

Attenuation is fairly easy to understand. The illustration is primarily meant to show how cable testers graphically display the test values of cable over the entire frequency range against the TIA-rated values.

Attenuation increases with frequency, so 100Base-TX at 80 MHz has higher attenuation than 10Base-T at 10 MHz. Attenuation also increases with temperature, so cable installers may need to plan shorter cable runs in hot environments. Metal conduit also increases attenuation and should be considered when planning cable length. Cable vendors should provide you with technical specifications for their cables.

Capacitance is the ability of a material to store a charge. Copper cables have capacitance that can distort signals by storing some of the energy of a previous signal bit. Capacitance is a measure of the energy that a cable and its insulator can store. Adjoining wires in wire bundles also contribute to the capacitance of a wire. Cable testers can check capacitance values to determine if a cable has kinks or has been stretched. All cable has known capacitance values that are measured in pF (picofarads). Twisted-pair wire used for network cabling is rated at 17-20 pF.

Test equipment from Fluke, Agilent, and other vendors can test for capacitance in order to identify cable link faults or installation problems. However, a TDR (time-domain reflectometer) is usually a better tool for finding such problems. Fluke testers use capacitance to determine if wiring problems like split pairs exist by testing at only one end of a cable (an adapter is not required at the other end of the cable).

A signal is prone to delay distortion caused by impedance, which is resistance that changes at different frequencies. It can cause the different-frequency components within a signal to arrive out of step at the receiver. The effect is more problematic on high-data-rate networks that use high frequencies. Impedance may change abruptly due to kinks and excessive bends in the cable, which cause signal reflections that distort data. That leads to retransmissions and a loss in network performance. In the worst case, the network may not operate.

Different types of cable should not be mixed along the same signal path, since a change of impedance at the junction causes a signal reflection back to the source. On high-speed networks, a connector used to join two cables will almost certainly cause an impedance problem because of the untwisting of the wire pairs at the connector. Such connectors should never be used when high-performance networking is implemented over Category 5 cable.

Decreasing the cable length and/or lowering the transmission frequency may solve these problems. Note that the impedance value of a cable can be measured to detect breaks or faulty connections. Data-grade cable should have an impedance value of 100 ohms at the frequency used to transmit data. Category 5 cable meant to support Gigabit Ethernet must be tested for delay distortion.

Delay skew is a problem in networks that transmit on multiple pairs in the same direction at the same time, such as Gigabit Ethernet. It is caused when signals travel at different speeds in each of the wire pairs of a cable. A Gigabit Ethernet transmitter will send signals in parallel across all pairs, which must be reassembled by the receiver. Delay skew desynchronizes these signals, which may prevent reassembly. Category 5 cable meant to support Gigabit Ethernet must be tested for delay skew.

Transmission lines are susceptible to background noise generated by external sources. This noise combines with and distorts a transmitted signal. While noise may be minor, attenuation can enhance its effects. As shown here, the signal is higher than the noise level at the transmitter but is equal to the noise level at the receiver due to attenuation:

Ambient noise on digital circuits is caused by florescent lights, motors, microwave ovens, and office equipment such as computers, phones, and copiers. Technicians can certify wire by testing for noise levels. If noise is a persistent problem in some areas, it can be avoided by running wire away from sources of noise, by using shielded cable, or by using fiber-optic cable.

As mentioned, twisted-pair cable is supposed to form a balanced circuit where one wire in a pair is equal in amplitude but opposite in phase to the wire it is twisted with. If this characteristic changes due to cable distortion or other factors, the cable becomes unbalanced and starts acting like an antenna, picking up noise from all over, including machines, fluorescent lights, radio stations, and alien transmissions.

Inductance occurs when current flows on two adjacent metallic conductors. Electromagnetic fields created by the current flows can create signal distortions in adjoining wires. The biggest problem this creates is near-end cross talk (NEXT), which is basically the crossing over of a signal on one wire pair to another wire pair (electromagnetic disturbance). NEXT occurs near the transmitter and creates distortions that typically affect signals on adjacent receive pairs, as shown here:

Note that strong fields from the transmit line may overwhelm the weak (attenuated) signal arriving on the receive line, which can lead to intermittent problems, lockups, or complete failure of the system. NEXT should be measured at both end of the cable.

A typical NEXT measurement is shown in the following illustration. This illustration is derived from a Fluke tester. NEXT is measured in dB, with higher values being better. Note that NEXT is measured for all frequencies between 0 and 100 MHz and that crosstalk varies across the spectrum. The lower line indicates the TIA minimum allowed value across the spectrum.

Twisting wire pairs is the primary method for reducing the effects of inductance, but the type of conductor and insulation also play a role. Twisting wire pairs cancels the positive and negative energy on the cable. Because of this, twists in cable must be preserved all the way up to the connection, especially on high-performance networks. In addition, wires cannot be untwisted more than one-half inch from their connection points.

NEXT is measured by injecting a signal on a wire pair and measuring its crosstalk on another wire pair. Every pair must be tested in this way. Fortunately, cable testers make this job easy and automatic. Keep in mind that NEXT refers to crosstalk at the near end, as the name implies. Crosstalk lessens down the cable as the signal strength weakens due to attenuation. However, this also implies that NEXT should be measured at both ends of a link.

FEXT is a relatively new cable measurement requirement. It is a measure of the crosstalk noise that exists at the opposite end of a cable (at the receiver) and is only relevant on network technologies that transmit on multiple pairs in the same direction at the same time-that is, Gigabit Ethernet (1000Base-T). What happens is that crosstalk occurs between transmitters as signals are transmitted down the line.

FEXT can be tested by putting a test signal on one pair and measuring how much of that signal crosses over to the other pairs at the far end of the cable. You will often see FEXT discussed in terms of ELFEXT (equal-level far-end crosstalk). ELFEXT provides a standard way to measure far-end crosstalk no matter what the cable length, so that all cabling can be tested to the same certification levels. The ELFEXT test is required for Gigabit Ethernet cable certification.

ACR is the ratio at which crosstalk affects an attenuated signal. In other words, how much does the noise on the cable distort the signal I am receiving? If the noise is high, and the signal being received is attenuated, the bit error rate (BER) will be high and retransmissions will be necessary, which leads to a loss in network performance. ACR is important because it provides a useful indication of a cable's performance and is helpful when making purchasing decisions. High ACRs indicate high-capacity cables.

The ratio is calculated by dividing attenuation by NEXT. Most cable testers gather all the information about a cable and perform this calculation automatically. Figure C-4 illustrates the relationship between NEXT and attenuation.

Table 2: Cable parameters