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Telecommunications trials study Flashcards

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1
Q

3 Guided media types

A
  1. Twisted pair cable
  2. Coaxial
  3. Fibre optical
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2
Q

Twisted Pair Cables

A

Cheapest, shortest range, greatest EM/noise interference

Twisting reduces crosstalk
4 pairs of wire with different twist lengths

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3
Q

USB cables

A

Provides power and supports data transfer
- red and black for power
- green and white for signal

Coaxial Cable

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4
Q

Ethernet cables

A

Twisted pair cable

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5
Q

Coaxial Cable

A

Has 4 layers:
1. outer plastic sheath
2. Woven copper shield
3. Inner dielectric layer (insulator)
4. Copper core (or copper-clad steel)

Better noise resistance

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6
Q

Skin effect

A

AC power flows mainly along the surface
- skin depth dereases as frequeny increases
Coaxial cables opeerate at a

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7
Q

Copper-clad steel (CCS)

A

Increases skin depth

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8
Q

Fibre Optic Cables, pros and cons

A

Transmits light pulses
Several 100x faster the coaxial (up to 10km)

More immunity to noise, low signal attenutation, not susceptible to EMI, cannot be tapped (Security)
ALSO: digital signals, normally encoded

BUT fragile, expensive

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9
Q

Unguided media and types

A

NOT physically connected e.g.
radio (broadcast radio, TV)
microwave (directional transmissions e.g. mobile phone networks and satellites)
Infrared (remote controls, short range)

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10
Q

Directionality

A

Point-to-point: direct
vs
Broadcasting: various receivers

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11
Q

Mircophones/speakers

A
  1. Diaphragm vibrates when sound hits it
  2. Coil of wire with a magnet inside: Faraday’s law of induction
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12
Q

Antenna

A

Long piece of metal with a running current
- creates a magnetic field
- makes EM waves
- propagates in 3 dimensions

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13
Q

4 Types of radio waves

A

Direct: line-of-sight
Wave reflected from ground/ground waves: affected by terrain and vegetation (e.g. mountains): due to diffraction
Sky wave: skip propagation (bounces off ionosphere) - long distances only

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14
Q

Amplitude vs Digital signals

A

Analogues are infinitely variable (VERY complex), digitals have set levels (‘steps’) e.g. binary only has 1 or 0

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15
Q

Digitising steps

A
  1. Sampled at regular intervals
  2. This is quanitsed e.g. rounded to closest digital level (Quantisation)
  3. Encoded as a binary signal before transmission.

SQE: Stupid Quokkas Eat

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16
Q

Why digitise?

A

Security, can send more information in digital (channel capacity utilisation - multiplexing), noise immunity (ability to REGENERATE), lower cost for equipment

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17
Q

Bandwidth

A

Capacity for a channel to convey info (measured in bits for digital transmission)

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18
Q

Attenuation

A

Decrease in signal intensity (decibels dB)

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19
Q

Signal-to-noise ratio (SNR)

A

Higher means more resiliant

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20
Q

Regenerative repeaters

A

Take signal, then amplify: increases range
- first regenerates it to remove the noise
- ONLY for digital

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21
Q

Modulation, why it is important

A

Taking a message (baseband signal) and modifying a property so the message is easier transmitted.
Has high frequency carrier signal
MODEM: does the modulation

WHY?
1. Reduce antenna size
2. Reduce interference (send things at different frequencies)
3. Allow Multiplexing

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22
Q

3 Analogue Continuous wave modulation types (shift keying)

Study These Flashcards
A
  1. Amplitude (very affected by noise, inefficient power use)
  2. Frequency (less noise interference as noise impacts amplitude, needs high bandwidth)
  3. Phase (ok for digital, not for analogue)
23
Q

LESS IMPORTANTL Pulse analogue modulation

Study These Flashcards
A

Used for brightness of a lightbulb, etc.
- PAM: amplitude matches samples amplitude
- PWM: checks the width as well: Motor and LED control
PPM: position matches sampled

24
Q

Digital Modulation (transmission of signals in binary)

Study These Flashcards
A

SHIFT KEYING
1. ASK: amplifude (high for 1, low for 0)
2. FSK: frequency (high for 1, low for 0)
3. PSK Phase (original is 1, shifted 180 out of phase is 0) - appears to flip

25
Demodulation
Opposite of modulation: getting rid of carrier 1. Antenna is tuned to pick up a specific frequency (by coil and variable capacitor) 2. Diode rectifies the signal (makes positive) 3. Fixed capacitor smooths it 4. Earphones vibrates according to that -> converts to sound.
26
Diode
Rectifies AC signal (only leaves positive)
27
Guided media + pros and cons
Guided: transmission media that carries signals with a conductor e.g. fibre optic cables: has a PHYSICAL LINK - short distance, but expensive for long distance/weird terrain
28
Power Cables
1. Low voltage - copper with PVC insulation 2. High voltage - Cu or Al, shielded with polyethylene 3. Extra high voltage - Al with steel core, not insulated. (wire is often stranded to increase flexibility)
29
Conductor Properies
Cu, Al Gold High conductivity/low resistivity High ductility High Tensile strength Good corrosion resistance Low cost
30
Copper propeties
High tensile strength good conductivity good ductility
31
Pure copper types
32
Why polymers?
Cheap Ductile -> needs to be extruded Low electrical conductivity (insulating) Flexible
33
Main polymers
Polyethylene Polyvinyl Chloride (PVC) Polypropylene Nylon ABS Polycarbonate
34
Polyethylene properties
retains insulation in humidity BUT low softening temperature Allows water vapour penetration Relatively expensive used for high-frequency interior cables (not used outside)
35
PVC
Not as good an insulates and polyethylene, but tougher higher softening temp Naturally rigid by can be made flexible thru plasticisers
36
Polypropelene
Silimar e properties to P.ethy but is tougher highter softening temp Harder: used in thin-wall insulation but less flexible, expensive
37
Nylon
Insect resistant outer jacket or sheath for underground use Abrasion resistant hard smooth: difficult for insect/termite to grip
38
ABS
Can be formed into complex shapes tough and shock resistant - commonly used in computer and phone casings (injection molding process)
39
Polycarbonate
Similar mechanical properties to ABS Good e insulation heat resistant and flame retardant Used in casings
40
Testing techniques
Multimeter Megger test Oscilloscopes
41
What needs to be tested (3)
Voltage Current Resistance Test for inconsistencies and disruptions in circuit
42
Multimeter
Combines several instuments in one e.g. voltmeter, anmeter. ohmmeter, etc
43
Volt vs anm set up
Volt in parallel, anmeter in series
44
Insulation testing why
PRevent e shoks ensure safety reduct equiptment downtime
45
Megger testing
test for insulation faults: megohmmeter applied high voltage, measures resistance over time high resistance = good, low = fault
46
Oscilloscope
Measure how signal changes over time Can show: Voltage/current changes over time frequency of oscillating signal Malfunctioning components How much of signal is DC and AC How much noise, whether noise is changing
47
Frequency Analysis
using a spectrum analyser - use Fast Fourier Transform (FFT) to covert signals from time domain to frequency domain
48
Multimode cables
Short distance High bandwidth support higher cable cost Lower electronics cost Easier to terminate due to large core size
49
Single mode cable
Short and long distance Highest bandwidth support Lower cable cost Higher electromics cost Harder to terminate with smaller core size
50
Structure of Fibre OC
1. high refractive index core 2. Low r.i. cladding (ensures light signals stay trapped in core even with bending)
51
Advantages of FO
Lighter Corrosion resistant Wide band width + high transmission capacity (light faster then electricity
52
Problems of Fo
Attenuation: atomic absorption of light by glass Scattering of light by flaws and impurities Reflection of light by splices and connectors DISPERSION: Spreading/overlapping of light pulses with distance caused by chromatic dispersion (different wavelengths, pink Floyd) and modal dispersion - fibres w large diameters: light travel along different modes/paths - limits bit rate.
53
Multimode fibre types
allows alight to travel along many modes/paths 1. Step index: high R.I core and low R.I cladding - high attenuation and dispersion 2. Graded index: cores with higher R.I in the middle - light travelling straight down travels slowly: reduces dispersion
54

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