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TM355 > Block 1 Part 1 > Flashcards

Block 1 Part 1 Flashcards

1
Q

What is the signal in copper cabling?

A
  • Varying electrical voltage
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2
Q

Describe an Analogue signal

A
  • Follows the air vibration
  • Voltage is analogous to fluctuating air pressure (rises and falls in same pattern)
  • Can take any value within continuous range
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3
Q

Describe Digital signal

A
  • Data represented by two different voltages representing 1s and 0s
  • These last for fixed period of time
  • digital quantities limited to discrete set of values
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4
Q

Why use digital?

A
  • regenerate digital signals

- This means the receiver knows it is receiving a digital signal and can regenerate the signal

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5
Q

What happens when you send a signal along communications channel?

A
  • it gets smaller (attenuates)

- it gets distorted (its shape changes)

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6
Q

What is a bipolar signal?

A
  • uses positive and negative voltages to represent 1s and 0s
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7
Q

What is threshold detection?

A
  • look at value of the signal at midpoint of interval
  • If above 0 then it’s assumed to be +1
  • If below it’s assumed to be -1
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8
Q

What are Analogue-to-digital (ADCs) and Digital-to-analogue (DACs) converters

A

-Electronic devices that convert between analogue and digital in each direction

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9
Q

How do you convert an analogue signal to digital?

A
  • First sampled by measuring value at regular intervals in time
  • To restrict measured values to discrete set, values are quantised
  • Quantisation levels (allowed values) not always evenly spaced
  • usually binary representation required
  • each quantisation level encoded with binary number
  • number of quantisation levels allowed normally power of 2
  • 4 bit number can represent 2 power 4 = 16 different levels
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10
Q

What does different range of variation give you?

A
  • large value of n improves accuracy of conversion because quantisation levels closer together
  • small n results in smaller amount of binary data at expense of conversion accuracy
  • ADCs referred to as having resolution of n bits
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11
Q

How is information lost when converting analogue to digital?

A
  • signal not measured at every instance of time but only at sampling point
  • Approximation has been made by rounding samples to nearest quantisation level
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12
Q

What is a sinusoid?

A
  • sine wave
  • turn up naturally in number of situations
  • example of periodic symbol, one that repeats at regular intervals
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13
Q

What is periodic signal?

A
  • repeats at regular intervals
  • section of periodic signal between two points called a cycle
  • duration of cycle is the period
  • number of cycles in one second is frequency
  • unit of frequency is hertz
  • amplitude is max value of sinusoid
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14
Q

What is another characteristic of a sinusoidal signal?

A
  • Its phase
  • relates to point sinusoid has reached at particular time
  • shifting signal to right or left changes its phase
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15
Q

Frequency domain

A
  • also known as the spectrum
  • any signal can be represented
  • sinusoid shown as single line as it represents single frequency of particular strength
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16
Q

Time domain

A
  • any signal can be represented

- shows sinusoid as it progresses in time

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17
Q

Sawtooth wave

A
  • made up of sum of sinusoids of decreasing amplitude
  • these sinusoids are exact whole number multiples of lowest frequency
  • higher frequency sinusoids called harmonics
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18
Q

square wave

A
  • binary signal with alternating 1s and 0s

- even multiples are missing in this wave

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19
Q

Non-periodic signals

A
  • also known as aperiodic signals
  • also have both time and frequency representations
  • no longer lines at particular frequencies
  • spectrum spread out over continuous range of frequencies
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20
Q

Modulation

A
  • message signal converted to suitable form for transmission
  • two signals combined
  • message signal, called modulating signal
  • signal of right frequency for transmission, called carrier signal
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21
Q

Resultant modulated signal

A
  • No longer periodic

- occupies range of frequencies not just one

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22
Q

Optical fibre

A
  • transmits large amount of info rapidly over long distances

- uses light signals

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23
Q

Three main components of optical fibre link

A
  • suitable source of light, controlled by input data in form of electrical signal
  • optical fibre itself, carries resulting pulses of lights
  • detector which converts pattern of light and dark back to electrical signal
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24
Q

Electromagnetic wave

A
  • electric and magnetic field both sinusoidal and are at right angles to each other
  • whole wave pattern moves forward at speed of light
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25
Q

Wavelength

A
  • distance between two consecutive peaks

- light waves have short wavelengths measured in nanometres

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26
Q

Frequency

A
  • number of cycles that pass given point in one second
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27
Q

Electromagnetic spectrum

A
  • chart is plotted on logarithmic scale
  • frequency increases left to right
  • wavelength increases from right to left
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28
Q

Refractive index

A
  • speed of light in medium such as glass found by dividing c be refractive index
  • depends on material
  • around 1.5 for most optical glasses
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29
Q

How optical fibre works

A
  • refractive index not same all the way across the fibre

- higher in central core than cladding around core

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30
Q

How can light change direction?

A
  • refraction: occurs in lenses, ray of light travels from one medium to another with different refractive index
  • reflected: occurs in mirrors
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31
Q

Total internal reflection

A
  • if light directed one medium to another with lower refractive index it can bounce back if angle low enough.
  • reflected into first medium
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32
Q

Multimode fibre

A
  • light travels along in variety of ways
  • commonly diameter of core larger than wavelengths typically used
  • two rays of light could set off at same time but arrive at different times
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33
Q

Graded-index fibre

A
  • refractive index varies smoothly from max in centre to min within cladding
  • means waves taking longer paths travel faster
  • solves problem of waves arriving at different times
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34
Q

Single-mode fibre

A
  • if core diameter reduced there comes a point where signals all travel along same path
  • provides best performance over long distance
  • useful for long haul transmissions
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35
Q

Attenuation

A
  • signal gradually loses power over distance
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36
Q

Decibels

A
  • way of comparing two powers

- logarithmic measure of ratio between two powers

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37
Q

multimode

A
  • preferred for short distance because of lower component costs and ease of use
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38
Q

problems with fibre

A
  • various effects distort signal
  • smears out transitions between light and dark
  • signal merges into one another
  • longer the fibre worse it gets
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39
Q

pulse spreading

A
  • different paths result in different timings

- this called multimode distortion, main cause of pulse spreading in multimode fibres

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40
Q

Other mechanisms that cause pulse spreading

A
  • Distortion

- Polarisation mode distortion

41
Q

Distortion

A
  • or chromatic dispersion

- caused by light of different wavelengths travelling at different speeds

42
Q

Polarisation mode distortion

A
  • affects single mode fibres
  • caused by another variation in speed of light
  • speed varies with orientation of light wave in fibre
43
Q

Optical transmitter

A
  • convert input data in form of electrical signal to light signal
  • two main types: LED and laser diode
44
Q

Light emitting diode (LED)

A
  • similar to LEDs seen in displays

- rather than emitting visible light, they emit in infrared region of spectrum

45
Q

Laser diodes

A
  • also found in CD, DVD

- where they read and write data from disc

46
Q

Where are LEDs used

A
  • multimode fibres
  • number of disadvantages
  • lower in power and emit over range of wavelengths leading to dispersion
  • emit broad cone of light
47
Q

How LED and laser diode work

A
  • beam of light modulated to convey useful signal
  • either varied in intensity or switched on or off
  • data rate depends on how quick beam can be modulated
48
Q

How to vary LED and Laser diode

A
  • beam can be modulated by varying electrical power supplied to them
  • Laser diode has advantage on how quick it can be switched
49
Q

Photodiode

A
  • detector at other end of fibre

- provides current output that varies with intensity of light it receives

50
Q

How to increase range of fibre

A
  • repeater or regenerator
  • devices that counteract effects of attenuation
  • restores signal back to original form
51
Q

Difference between repeater and regenerator

A
  • repeater tends to include simpler devices
  • regenerator does further processing; reshapes and retimed
  • regenerated pulse is copy of original signal with noise removed
52
Q

Optical amplifiers

A
  • developed as better solution for long haul links

- amplify signal directly without converting back to electrical signal

53
Q

Erbium doped fibre amplifier (EDFA)

A
  • consists of section of fibre that contains small proportion of erbium atoms
  • energy from pump combined with signal in device known as coupler
  • stimulated emission takes place within doped fibre and amplifies signal
54
Q

Distributed Raman amplifier

A
  • amplifies signal along whole of transmission path
  • pumping can be done from either end
  • usually done backwards from receiver end
55
Q

Semiconductor optical amplifier (SOA)

A
  • similar principle to semiconductor laser

- have advantages of low cost and compactness

56
Q

Optical switch

A
  • one technique relies on moving mirrors

- mirrors need to be small and lightweight to allow fast switching

57
Q

Directional couplers

A
  • consists of two fibres fused together along short length
  • two signals combined by feeding them in one end of coupler
  • more than two signals can be joined or split by joining couplers
58
Q

Wavelength division multiplexing (WDM)

A
  • for example; three transmitters send light of different colours
  • three receivers at other end only take certain light each
  • can all operate at same time
  • wavelength uses same principle
  • uses infrared radiation of different wavelengths instead of lights
59
Q

Demultiplexing

A
  • splits multiplexed signal into constituent wavelengths

- special filters used that only allow particular wavelength to pass through

60
Q

Receiver sensitivity

A
  • result must meet or exceed minimum power a receiver can detect
61
Q

Copper cable

A
  • operates with electrical signals
  • link consists of pair of conductors
  • voltage applied at one end appears at other end
62
Q

Resistance

A
  • all ordinary conductors have this
  • must be kept as low as possible to avoid wasting energy
  • two conductors in pair kept apart by insulating material (plastic)
63
Q

Dielectric loss

A
  • small part of energy wasted in electromagnetic fields
64
Q

Unshielded tested pair (UTP)

A
  • pair of conductors twisted together along length
  • any interference affects both conductors evenly
  • twisting gives some protection against crosstalk
65
Q

Coaxial cable

A
  • two conductors take form of central conductor with conducting shield around it
  • electric and magnetic fields confined in the shield
  • gives good immunity to interference
66
Q

Shielded twisted pair (STP)

A
  • combines advantages of shielding and twisting

- at expense of greater complexity

67
Q

Antennas

A
  • Radio wave electric and magnetic fields generated directly from electrical signals in structures known as antennas
68
Q

Filtering

A
  • A filter in receiver allows narrow band of frequencies through while attenuating all others
69
Q

Bandwidth

A
  • Amount of spectrum occupied by signal
  • equal to difference between highest and lowest frequencies
  • Larger the bandwidth, the more info signal can convey
70
Q

Response

A
  • measure of relative sensitivity of receiver to frequencies at and around the frequency it is tuned to
  • usually centre frequency of a wanted transmission
71
Q

Passband

A
  • Range of frequencies that receiver responds best to

- passband extends from lower cut-off frequency to higher cut-off frequency

72
Q

Selectivity

A
  • receiver that is good at rejecting signals outside the passband said to have high selectivity
73
Q

Inverse square law

A
  • describes reduction in power with distance from transmitter, due to spreading
  • applies ideally to free space
74
Q

Isotropically

A
  • A wave moving outwards from a transmitter, radiating equally well in all directions
75
Q

Specular reflection

A
  • light is reflected at shiny surfaces, like mirrors

- travels towards and away from such a surface at equal angles

76
Q

Scattering

A
  • occurs when reflecting objects are small compared to wavelength
  • scattering by intervening objects can result in loss of useful energy
  • reduces the received signal
77
Q

Ionosphere

A
  • Refraction important at lower frequencies

- lower layer of atmosphere called ionosphere can bend path of radio waves back towards ground

78
Q

Absorption

A
  • radio waves absorbed as they travel through atmosphere or buildings
  • measured in decibels per metre or kilometre
  • dependent on frequency
79
Q

Attenuation coefficient

A
  • The attenuation in decibels per kilometre of distance travelled
80
Q

Diffraction

A
  • spreading or bending of electromagnetic wave when it passes through gap or encounters sharp corner
81
Q

Dipole

A
  • simplest and most common antenna
  • consists of two conductors fed at midpoint with electrical signal
  • effective over narrow band of frequencies
82
Q

Near field

A
  • region close to antenna where inverse square law ceases to apply
  • extends for few wavelengths from the antenna
83
Q

omnidirectional

A
  • Radiates better in some directions than others

- typically broadcast antenna in middle of area would have omnidirectional antenna

84
Q

Beamwidth

A
  • angle of cone that contains predominant radiation

- cone is taken to include radiation above certain power level

85
Q

Antenna gain

A
  • compares performance of directional antenna in preferred direction with that of reference antenna
  • ratio of the power sent by directional antenna to power sent by reference antenna
  • usually measured in decibels
86
Q

Surface wave

A
  • At low frequencies, below 3 MHz, special form of propagation occurs
  • carries radio waves over long distances
  • wave interacts with ground and follows earths curvature
87
Q

Sky wave

A
  • when conditions right, ionosphere can refract radio waves back towards ground
  • allows transmissions over the horizon
  • wave reflects to the ground then back to sky in series of hops
88
Q

Multipath

A
  • Several paths a transmission can take to receiver
  • due to reflections from buildings and ground
  • propagation described as multipath
89
Q

Direct and reflected waves

A
  • Different factors affect whether in phase or not
  • positions of transmitters and receivers, determine relative paths
  • wavelength or frequency of transmission
90
Q

Fading

A
  • if receiver or transmitter moves around received signal can vary in strength
91
Q

Amplitude modulation (AM)

A
  • amplitude of carrier waveform altered in proportion to information signal, referred to as modulating signal
92
Q

Envelope

A
  • used to describe varying strength, or shape, of modulating signal
93
Q

Mixer

A
  • modulated signal can be created by multiplying modulating signal and carrier signal together using mixer
  • shifts power from one frequency to power at anther frequency
94
Q

Sidebands

A
  • situated either side of carrier frequency

- replicas of original modulating signals spectrum

95
Q

Frequency modulation (FM)

A
  • frequency of carrier waveform altered in proportion to envelope of modulating signal
  • amplitude and phase remain the same
96
Q

Voltage controlled oscillator

A
  • used to create modulated signal

- takes voltage signal as input and produces periodic electronic signal

97
Q

Frequency deviation

A
  • can be defined as max deviation of FM modulated frequency from carrier frequency
  • modulating signal that doesn’t vary much in amplitude results in smaller frequency deviation
98
Q

Phase modulation

A
  • phase of carrier waveform altered in proportion to amplitude of modulating signal
  • Bandwidth of PM signal can be approximated by Carson’s rule

TM355 (8 decks)

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