Copper Cable Transmission Characteristics 1 ET2080 Jaringan Telekomunikasi.

Copper Cable Transmission Characteristics

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ET2080 Jaringan Telekomunikasi

Crosstalk

Whenever a current flows through a conductor, a magnetic field is set up around the conductor in a direction given by the right-hand corkscrew rule

Because the signal on a transmission line is an electro-magnetic wavefront propagating along the line, current flowing in one direction in one conductor flows in the opposite direction in the other conductor.

The magnetic field around the right hand conductor flows anti-clockwise. At some distance from the line the effects of the fields cancel out, but near to the

conductors the fields re-inforce, and are in the same direction throughout the length of the transmission line


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Source:

magnetic fields (crosstalk)

If we now bring another pair of conductors close to the first, the re-inforcing field created by the currents flowing in the first line cuts through the plane of the new line, and this has the effect of inducing current into the new line.

We have created a very long narrow transformer, and have caused a coupling between the two lines which is crosstalk


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Now let us see what happens if we twist the interfering pair.


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The magnetic fields still rotate around the conductors in the same directions, and they still re-inforce near to the pair. But now they are not pointing in the same direction all along the length of the line. At every twist, the direction is reversed, so the net effect on the adjacent pair is cancelled out.

If the second pair is twisted as well, the crosstalk is reduced still further, provided that the twists in the two pairs are not in phase with each other. This is an important consideration when designing and manufacturing cables.

By twisting the pairs we have reduced the potential for crosstalk.

External noise pickup is ideally the same on the two conductors (zero difference) and ignored by the receiver resulting in total noise suppression

Only the differential (opposite on the two conductors) data signal is let into the receiver +2.5V1 + N - ( -2.5V2 + N ) = +5.0VDiff


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External noise pickup on either of the two conductors cannot be neutralized and will interfere with desired signal received

The differential (opposite on the two conductors) data signal and Noise is let into the receiver +2.5V1 + N - ( -2.5V2 ) = +5.0VDiff + N


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Crosstalk Measurement

Crosstalk is measured in two ways and resulted in NEXT (Near-end crosstalk) and FEXT (Far-end crosstalk)


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NEXT

Near End Crosstalk (NEXT) is measured at one end only of a cable, by transmitting a signal into one pair and measuring the resulting signal power on an adjacent pair at the same end.

The NEXT is given by: NEXT = PIN/POUT = 10 log (PIN/POUT) dB

To be sure that the measured signal is really due to crosstalk, and not to some other source of interference, the receiver is tuned to the same frequency as the transmitter. This can be a single frequency or it can be swept across the frequency

spectrum The desired outcome for the NEXT measurement is a dB value as

large as possible NEXT is used as indicator for quality of components and

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PSNEXT

Power sum NEXT (PSNEXT) is actually a calculation, not a measurement

PSNEXT is a measure of difference in signal strength between disturbing pairs and a disturbed pair

A larger number (less crosstalk) is more desirable than a smaller number (more crosstalk)


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FEXT

Far End Crosstalk (FEXT) is measured at both ends of a cable, by transmitting a signal into one pair at one end and measuring the resulting signal power on an adjacent pair at the other end.

The FEXT is given by FEXT = PIN/POUT = 10 log (PIN/POUT) dB

A higher FEXT values correspond to better cable performance

FEXT is highly influenced by the length of the cable, since the signal strength inducing the crosstalk is affected by how much it has been attenuated from its source


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Equal Level Far End Crosstalk (ELFEXT) ELFEXT is a calculated result, rather than a

measurement It is derived by subtracting the attenuation of the

disturbing pair from the Far End Crosstalk (FEXT) this pair induces in an adjacent pair.

ELFEXT loss is critical when two or more wire-pairs carry signals in the same direction.

50 m link example: For FEXT = 45 dB and Attenuation = 11 dB, then ELFEXT

= 45 - 11 = 34 dB


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Power Sum ELFEXT (PSELFEXT) PSELFEXT is the computed effect of

disturbing pairs upon the disturbed pair with respect to the far end of the cable


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Nominal velocity of propagation (NVP) NVP refers to how quickly signals travel

in a cable expressed as a percentage relative to the speed of light in vacuum


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Propagation Delay

Propagation Delay is the time required for data signal to travel from its source to its destination over a single pair

If we have more than one pair, for example bellow we have four pairs, then since each pair has different twist rates, each pair length is different Therefore, the propagation delay in a 4 pair cable is different

for each pair This variance (delay skew) should not exceed 50 nS on any

link segment up to 100 meters


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Delay Skew

Delay skew is a calculation, derived from the propagation delay

Delay Skew is the difference in Propagation Delay time between the fastest (shortest) and slowest (longest) pairs within the same cable sheath

Well-constructed and properly installed structured cabling should have a skew less than 50 nanoseconds (nSec) over a 100-meter link

Lower skew is better Anything under 25 nSec is excellent. Skew between 45 and 50 nanoseconds is marginally

acceptable


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Insertion Loss (Attenuation)

Attenuation is the loss of electrical power as signal travels along a cable

Any passive device inserted in a circuit, such as a cables, has an attenuation and so it is also called insertion loss

Insertion loss (expressed in dB) measures the amount of energy that is lost as the signal arrives at the receiving end of the cabling link

Insertion loss also increases with the length of the link

The smaller insertion loss measurement values (expressed in dB) are better than larger values.


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Depending upon the gauge of wire used in constructing the pairs, 24 gauge wires will have less attenuation than the same length 26 gauge (thinner) wires. American Wire Gauge (AWG) is a standardized method of

measuring wire diameter As the AWG number increases, the wire diameter

decreases


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Also, stranded cabling will have 20-50% more attenuation than solid copper conductors

If the power transmitted by the source is PT and the power received by the load is PR, then the insertion loss in dB is given by


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Attenuation to Crosstalk Ratio (ACR)/Headroom The difference between NEXT (in dB) and Attenuation (in dB) ACR is a very good indicator of the real transmission quality of the link The higher the ACR the better as it implies that the desired signal is not

being so severely attenuated that the effect of crosstalk noise will become too significant

In communication channels it is generally considered that a positive value of ACR is required for successful error free transmission

Minimum value of ACR of the cabling system in the applicable bandwidth should be greater than 10 dB


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Cable engineering for local area networks By Barry J. Elliott

Return Loss

Return Loss (RL) is a measure of the reflected energy from a transmitted signal at all locations along the link and is expressed in decibel (dB).

A higher RL values correspond to better cable performance Mismatches predominantly occur at locations where connectors are

present, but can also occur in cable where variations in characteristic impedance along the length of the cable are excessive

Other RL factors include manufacturing tolerances, installation and termination methods such as kinks in the cable, poor cable construction, improper termination or a compressed cable


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Return lossdB = 20 log(Zn + Z0)/(Zn − Z0)Zn = line impedanceZ0 = characteristics impedance

Example: Consider the following network’s output port and its termination.

The characteristic impedance (Z0) of the output of the network is 600 . W We have terminated this network in its characteristic impedance (Z0). Let us assume for this example that it is 600 W. How well does the network’s output port match its characteristic impedance?

Return loss tells us this First let us suppose that Zn is exactly 600 W. If we substitute that

in the equation, what do we get? We have then in the denominator 0. Anything divided by zero is infinity. Here we have the ideal case, an infinite return loss; a perfect match.

Suppose Zn were 700 W . What would the return loss be? We would then have:

Return lossdB = 20 log(700 + 600)/(700 − 600) = 20 log(1300/100) = 20 log 13 = 22.28 dB.


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Good return loss values are in the range of 25 dB to 35 dB


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DC loop resistance

DC loop resistance for copper conductors, the following formula is applicable

RDC = 0.1095/d2

RDC = loop resistance (W/mi)

d = diameter of the conductor (inches) Example: If we want a 17-mile loop, allowing 100

W per mile of loop (for the 1700-W limit), what diameter of copper wire would we need?

Ans: 100 = 0.1095/d d = 0.1095/100 = 0.001095 d = 0.0331 inches or 0.84 mm or about 19 gauge


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Twisted Pair Connectors24

Kabel twisted pair untuk komputer menggunakan konektor RJ45 (8 pin)

Kabel twisted pair untuk telepon menggunakan konektor RJ11


Cable Fire Rating


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Cable Comparison


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Copper Cable Transmission Characteristics 1 ET2080 Jaringan Telekomunikasi.

Documents

nearend crosstalk

near end crosstalk

far end crosstalk fext

pair of conductors

end crosstalk elfext

adjacent pair

disturbed pair

measured signal