DCN286 Introduction to Data Communication Technology Session 5

Objectives Noise Cable testing

What measurements can we take from this? How about? 3 Waves 1 second

Many types of noise: White Noise Impulse Crosstalk Jitter Delay Distortion Echo 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) Jitter Signals reflecting off both ends cause interpretation problems with receiver- which is the real signal, which is not? Insertion loss Combination 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 Crosstalk Send 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 -40dB Which 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 Crosstalk Send a signal down one pair of wires, hear the crosstalk on the other pairs FEXT causes less noise: Transmission has attenuated, therefore less induced current FEXT 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 Crosstalk Send 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 map Tests that the right wires are connected to the right pins Insertion Loss Measure attenuation over the length of the cable NEXT Measure crosstalk at the end of the cable nearest the transmitter PSNEXT NEXT 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 Loss Measures attenuation of signal (noise) reflected back to transmitter Propagation delay The time required for a signal to travel from one end of a cable to the other Cable length A 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 Skew Difference in propagation delay between fastest and slowest wire pairs 16 Cable Testing Standards

Open Circuit The connection between two pins is absent: Broken wire Poorly installed wire (not connected to pin) Wire Map Test

Short Circuit Two wires are connected, when they should be isolated stripped or cracked insulation contamination inside connector (wire or metal fragments) Wire Map Test

Reversed-pair Fault One 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 PIN Wire Map Test

Split-pair Wiring Fault A wire from one pair is cross with a wire from a different pair (at both ends) Wire Map Test

Transposed-pair Wiring Fault A wire pair is connected to the wrong pins (at both ends) Wire Map Test