DCN286 Introduction to Data Communication Technology

1
DCN286Introduction to Data Communication
Technology

• Session 5

2

• Objectives
• Noise
• Cable testing

3
Waves
What measurements can we take from
this? How about?
1 second
1 second
3
4
Noise
Many types of noise White Noise Impulse Crosst
alk Jitter Delay Distortion Echo Description?
Cause?
4
5
Noise
Inducing Noise
Magnetic field created by electrical signals
Electrical signal being transmitted
Weaker, induced signal on parallel wire
5
6
Noise
Inducing Noise Grounded shielding protects
from some noise types Shield absorbs energy,
therefore blocks the field from crossing it
6
7
Attenuation
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
8
Connector Installation Issues
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
9
Crosstalk
What is it? How does it work? Poorly installed
connectors too much untwisting. and exposed
wiring Higher transmission frequencies cause
more crosstalk
9
10
Types of Crosstalk
NEXT Near End Crosstalk FEXT Far End
Crosstalk PSNEXT Power Sum Near End Crosstalk
10
11
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?
crosstalk current induced near the transmitter
Measure crosstalk on these wires
Send signal on these wires
11
12
Types of Crosstalk

• 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

crosstalk current induced at the far end of the
cable
Measure crosstalk on these wires
Send signal on these wires
12
13
Types of Crosstalk
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.
crosstalk current induced at the far end of the
cable
Measure crosstalk on these wires
Send signal on these wires
13
14
Cable Testing Standards
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
15
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
16
Cable Testing Standards
Delay Skew Difference in propagation delay
between fastest and slowest wire pairs
16
17
Wire Map Test

• Open Circuit
• The connection between two pins is absent
• Broken wire
• Poorly installed wire (not connected to pin)
• 1 1
• 2 2
• 3 3
• 4 4
• 5 5
• 6 6
• 7 7
• 8 8

17
18
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)
• 1 1
• 2 2
• 3 3
• 4 4
• 5 5
• 6 6
• 7 7
• 8 8

18
19
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
PIN2 1 1 2 2 3 3 4
4 5 5 6 6 7
7 8 8
19
20
Wire Map Test
Split-pair Wiring Fault A wire from one pair is
cross with a wire from a different pair (at both
ends) 1 1 2 2 3
3 4 4 5 5 6 6 7
7 8 8
20
21
Wire Map Test
Transposed-pair Wiring Fault A wire pair is
connected to the wrong pins (at both
ends) 1 1 2 2 3
3 4 4 5 5 6 6 7
7 8 8
21