At the high frequencies you can't look at the cable as a usual cable. On higher frequency it works as a waveguide. Characteristic impedance is specific resistance for electro-magnetic waves. So: It's the load the cable poses at high frequencies. The high frequency goes (dependent of cable of course) usually from 100kHz and up.
If you feed a sinusoidal electrical AC signal of reasonable frequency into one end of the cable, then the signal travels as an electrical wave down the cable. If the cable length is an extremely large number of wave-lengths at the frequency of that AC signal, and you measure the ratio of AC Voltage to AC current in that traveling wave, then that ratio is called the characteristic impedance of the cable.
In practical cables the characteristic impedance is determined by cable geometry and dielectric. The cable length has no effect of it's characteristic impedance.
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 ).
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. Pulses - in the form of a series of voltages and no voltages - can 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.)
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.
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.
If you'd like to read more about this feel free.
-Brad
A+, MCSE NT4, MCDBA SQL7
-Best cartoon of all time :-D 'Spongebob Squarepants'
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