DCN286Introduction to Data Communication

Technology

Session 5

Objectives

NoiseCable testing

What measurements can we take from this?

How about?

3

Waves

1 second

1 second

Many types of noise:White NoiseImpulseCrosstalkJitterDelay DistortionEcho

Description?Cause?

4

Noise

Inducing Noise:

5

Noise

Electrical signal being transmitted

Weaker, induced signal on parallel wire

Magnetic field created by electrical signals

Inducing Noise:

Grounded shielding protects from some noise types

Shield absorbs energy, therefore blocks the field from crossing it

6

Noise

Causes:Resistance in copper converts electrical energy into heat

Defective connectors (impedance)

Electrical energy lost as it leaks through the cable's insulation

High frequencies attenuate more than low frequencies

Impurities in glass weaken the signal in fibre optics

7

Attenuation

Impedance Discontinuity or Impedance Mismatch:If there is a mismatch on the receiving cable, some of the signal will be reflected back along the cable (echo)

JitterSignals reflecting off both ends cause interpretation problems with receiver- which is the real signal, which is not?

Insertion lossCombination of attenuation and impedance values

8

Connector Installation Issues

What is it? How does it work?

Poorly installed connectors – too much untwisting. and exposed wiring

Higher transmission frequencies cause more crosstalk

9

Crosstalk

NEXT – Near End Crosstalk

FEXT – Far End Crosstalk

PSNEXT – Power Sum Near End Crosstalk

10

Types of Crosstalk

NEXT – Near-End CrosstalkSend a signal down one pair of wires, hear the crosstalk on the other pairs

Measured in dB, the smaller the dB, the better.

-30dB or -40dBWhich has less NEXT? 11

Types of Crosstalk

Send signal on these wires

Measure crosstalk on these wires

crosstalk current induced near the transmitter

FEXT – Far-End CrosstalkSend a signal down one pair of wires, hear the crosstalk on the other pairs

FEXT causes less noise:Transmission has attenuated, therefore less induced currentFEXT has farther to travel so the noise also attenuates

12

Types of Crosstalk

Send signal on these wires

Measure crosstalk on these wires

crosstalk current induced at the far end of the cable

PSNEXT – Power Sum Near-End CrosstalkSend a signal down three pairs of wires, hear the combined crosstalk on the remaining pair

A measure of crosstalk from combined sources – this test is now a requirement for EIA-568-B certification.

13

Types of Crosstalk

Send signal on these wires

Measure crosstalk on these wires

crosstalk current induced at the far end of the cable

Wire mapTests that the right wires are connected to the right pins

Insertion LossMeasure attenuation over the length of the cable

NEXTMeasure crosstalk at the end of the cable nearest the transmitter

PSNEXTNEXT calculation when transmitting on all pairs

Equal Level Far-End Crosstalk (ELFEXT)Calculated number to compare cables of different lengths

14

Cable Testing Standards

Power Sum Equal Level Far-End Crosstalk (PSELFEXT)Combined ELFEXT when transmitting on all pairs

Return LossMeasures attenuation of signal (noise) reflected back to transmitter

Propagation delayThe time required for a signal to travel from one end of a cable to the other

Cable lengthA test that uses Time Domain Reflectometer (TDR) to determine the length of a cable (kinks, splices, loose connections will reduce the effective length of a cable)Works like radar.

15

Cable Testing Standards

Delay SkewDifference in propagation delay between fastest and slowest wire pairs

16

Cable Testing Standards

Open CircuitThe connection between two pins is absent:Broken wirePoorly installed wire (not connected to pin)

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17

Wire Map Test

Short CircuitTwo wires are connected, when they should be isolatedstripped or cracked insulationcontamination inside connector (wire or metal fragments)

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18

Wire Map Test

Reversed-pair FaultOne pair of wires are mis-wired

Normally PIN1 on one end of the cable is connected to PIN1 at the other end.

In Reversed-pair fault, PIN1 is connected to PIN2

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19

Wire Map Test

Split-pair Wiring FaultA wire from one pair is cross with a wire from a different pair (at both ends)

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20

Wire Map Test

Transposed-pair Wiring FaultA wire pair is connected to the wrong pins (at both ends)

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21

Wire Map Test