Cable Testing

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Understanding the electrical impulses of UTP cabling sheds light on cable performance.

With the exception of wireless systems, networks rely on cable to conduct data from one point to another. In the case of copper -wire-based, unshielded twisted-pair (UTP) cabling, data is conveyed in the form of electric, digital signals. Because these signals are essentially bursts of electricity, the electrical characteristics of the cable itself greatly affect the integrity of the signal being transmitted. A bad length of cable or a poor cable installation can result in signal loss or distortion, and consequently, network failure.

To minimize such occurrences, cable vendors test their cables to guarantee performance. However, this doesn't make their products fault-proof; bad cabling does exist. In some cases, the error lies in improper cable installation. Network managers can use cable testers to ensure that a cable can conduct signals correctly. They can also use cable testers to verify if a cable is properly installed and to troubleshoot faulty cable.

A solid grounding in the electrical properties of UTP is a good way to learn how cable can affect the performance of a network.

The Electrical Circuit

A network can be broken down in simplistic fashion into an electrical circuit metaphor. In this case, a network essentially comprises energy sources, conductors, and loads. An energy source is a network device that transmits an electrical signal (data). The conductors are the wires that the signal travels over to reach its destination, which is usually another network device. The receiving device is known as the load. In its entirety, the connected network is a completed circuit (see Figure).

click to expand
Figure 1: Simple Circuitry: When two devices communicate over a network, they form an electrical circuit. Cables serve as the transfer medium and are bound to the same electrical properties as normal conductors.

When an energy source transmits a signal, it is outputting an electric charge onto the conductor by applying voltage to the completed circuit. Voltage is measured in volts . The voltage propels the charge across the cable, and the flow of the charge is known as a current, which is expressed in amperes, or amps.

In the computer world, the electric signal transmitted by an energy source is a digital signal known as a pulse. Pulsesin the form of a series of voltages and no voltagescan be used to represent a series of ones and zeros. Digital pulses form bits, and a series of eight bits creates the almighty byte.

The key to a successful signal transmission is that when a load receives an electrical signal, the signal must have a voltage level and configuration consistent with what had been originally transmitted by the energy source. If the signal has undergone too much corruption, the load won't be able to interpret it accurately.

In short, a good cable will transfer a signal without too much fudging of the signal, while a bad cable will render a signal meaningless.

Property Limits

Due to the electrical properties of copper wiring, the signal will undergo some corruption during its transit. Obviously, signal corruption within certain limits is acceptable. Once the electrical properties exceed the limits prescribed to a certain cable type, the cable is no longer reliable and must be replaced or repaired.

As a signal propagates down a length of cable, it loses some of its energy. So, a signal that starts out with a certain input voltage will arrive at the load with a reduced voltage level. The amount of signal loss is known as attenuation, which is measured in decibels, or dB. If the voltage drops too much, the signal may no longer be useful.

The table lists the attenuation values allowable at the end of 100 meters of Category 3 through 5 UTP.

(Please note that the attenuation and near-end crosstalk [NEXT] values in the table are performance specifications detailed in Telecommunications Systems Bulletin [TSB] -67, written by the Electronics Industry Association/Telecommunications Industry Association [EIA/TIA]. All other values are suggested limits, not standards. In addition, the table shows limits for certain frequencies, although different frequencies can operate on each category of cabling. The limits for some properties vary according to frequency.)

Table 1: Standard Cable Limits

Cable

Frequency

Impedance

Capacitance

Attenuation

NEXT

Cat 3

10MHz

100ohms +/-15%

20pF/ft.

11.5dB

22.7dB

Cat 4

16MHz

100ohms +/-15%

17pF/ft.

9.9dB

33.1dB

Cat 5

100MHz

100ohms +/-15%

17pF/ft.

24dB

27.1dB

Attenuation has a direct relationship with frequency and cable length. The higher the frequency used by the network, the greater the attenuation. Also, the longer the cable, the more energy a signal loses by the time it reaches the load.

A signal loses energy during its travel because of electrical properties at work in the cable. For example, every conductor offers some resistance to a current. Resistance, which is measured in ohms, acts as a drag on the signal, restricting the flow of electrons through the circuit and causing some of the signal to be absorbed by the cable. The longer the cable, the more resistance it offers.

Due to its electrical properties, a cable not only resists the initial flow of the current, it opposes any change in the current. The property that forces this reaction is called reactance, of which there are two relevant kinds: inductive reactance and capacitive reactance.

In an inductive reaction, a current's movement through a cable creates a magnetic field. This field will induce a voltage that will work against any change in the original current.

Capacitance is a property that is exhibited by two wires when they are placed close together. The electrons on the wires act upon each other, creating an electrostatic charge that exists between the two wires. This charge will oppose change in a circuit's voltage. Capacitance is measured in farads or picofarads (see table).

Reactance can distort the changes in voltage that signify the ones and zeros in a digital signal. For example, if the signal calls for a one followed by a zero, reactance will resist the switch from voltage to no voltage, possibly causing the load to misidentify what the voltage represents.

Impeding Progress

When you combine the effects of resistance, inductance, and capacitance, the result is the total opposition to the flow of the current, which is known as impedance and is measured in ohms.

It's important for components of a circuit to have matching impedance. If not, a load with one impedance value will reflect or echo part of a signal being carried by a cable with a different impedance level, causing signal failures. For this reason, cable vendors test their cables to verify that impedance values, as well as resistance and capacitance levels, comply to standard cable specifications.

It's also important for the impedance of a cable to be uniform throughout the cable's length. Cable faults change the impedance of the cable at the point where the fault lies, resulting in reflected signals.

Cable testers use this trait to find cable faults. For example, a break in a wire creates an " open circuit," or infinitely high impedance at that point. When a high frequency signal emitted from a cable tester encounters this high impedance, it will reflect back towards the tester like an ocean wave bouncing off a seawall. Similarly, a short circuit represents zero impedance, which will also reflect a high frequency signal, but with an inverted polarity.

The cable testing device can then tell you approximately how far down the cable the fault lies. The formula for this feature uses a cable value known as nominal velocity of propagation (NVP), which is the rate at which a signal can flow through the cable, expressed as a percentage of light speed. The cable tester multiplies the speed of light by the cable's NVP and by the total time it takes the pulse to reach the fault and reflect back to the tester, and divides it by two, for the one-way distance.

The same concept is used to check the electrical length of a cable installation. In this case, you must make sure not to terminate one end of the cable. The open end will register as infinite impedance and reflect a pulse back to the tester. Again, this response time is plugged into the formula to estimate the overall electrical length of the wire.

As an aside, some cable testers can't check the first 20 feet or so of a cable. The reason for this blind spot is that a pulse transmitted by the tester will be reflected back to the device before it is entirely transmitted. Thus, the tester can't get an accurate reading.

Mixing Signals

Finally, the successful transmission of a signal can be jeopardized by noise, which can introduce false signals, or noise spikes, at different frequencies on a wire. A load may interpret a noise spike as part of a digital signal, distorting the original content of the signal. Common sources of noise spikes include AC lines, telephones, and devices such as radios, microwave ovens, and motors. Some cable testers test for noise, running tests at different frequencies.

Another type of interference is called crosstalk, or more specifically , near-end crosstalk (NEXT). As mentioned, when a current moves through a wire, it creates an electromagnetic field. This field can interfere with signals traveling on an adjacent wire. To reduce the effect of NEXT, wires are twistedthus the name twisted pair. The twisting allows the wires to cancel each other's noise.

The risks of NEXT are highest at the ends of a cable because wire pairs generally don't have twists at their ends, where they enter connectors. If the untwisted end length is too long, NEXT levels can rise to distorting levels.

Also, due to attenuation, signals are strongest when they are transmitted, and weakest when they arrive at their destination. So, the magnetic field of a signal being transmitted from a device on one wire may overwhelm a signal arriving at the same device on the wire's pair.

NEXT is measured in decibels, which represent a ratio of a signal's strength to the noise generated by crosstalk (see table). The stronger the signal and weaker the noise, the higher the NEXT value. For this reason, a high NEXT reading is good. Low NEXT readings , which indicate high crosstalk interference, can mean the cable is terminated improperly.

The Not-So-Final Word

In the past, the topic of cable testing and performance has been a contested one, due mostly to the absence of accepted testing and performance standards. Recently, the EIA/TIA finalized TSB-67, which defines what it calls "transmission performance specifications for field testing of unshielded twisted-pair cabling systems." These specifications cover Category 3, 4, and 5 UTP. TSB-67 also details specifications for cable tester performancesomething that has been noticeably absent.

Although the document is doing much to calm some pretty turbulent waters, other questions concerning cable performance and testing loom on the horizon. For example, TSB-67 addresses specifications for Category 5 cabling up to 100MHz. How does this relate to 155MHz ATM, which is supposed to run over Category 5?

This tutorial, number 93, by Lee Chae, was originally published in the May 1996 issue of LAN Magazine/Network Magazine.

 
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Network Tutorial
Lan Tutorial With Glossary of Terms: A Complete Introduction to Local Area Networks (Lan Networking Library)
ISBN: 0879303794
EAN: 2147483647
Year: 2003
Pages: 193

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