Block 1 Part 1 Flashcards
What is the signal in copper cabling?
- Varying electrical voltage
Describe an Analogue signal
- Follows the air vibration
- Voltage is analogous to fluctuating air pressure (rises and falls in same pattern)
- Can take any value within continuous range
Describe Digital signal
- 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
Why use digital?
- regenerate digital signals
- This means the receiver knows it is receiving a digital signal and can regenerate the signal
What happens when you send a signal along communications channel?
- it gets smaller (attenuates)
- it gets distorted (its shape changes)
What is a bipolar signal?
- uses positive and negative voltages to represent 1s and 0s
What is threshold detection?
- 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
What are Analogue-to-digital (ADCs) and Digital-to-analogue (DACs) converters
-Electronic devices that convert between analogue and digital in each direction
How do you convert an analogue signal to digital?
- 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
What does different range of variation give you?
- 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
How is information lost when converting analogue to digital?
- signal not measured at every instance of time but only at sampling point
- Approximation has been made by rounding samples to nearest quantisation level
What is a sinusoid?
- sine wave
- turn up naturally in number of situations
- example of periodic symbol, one that repeats at regular intervals
What is periodic signal?
- 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
What is another characteristic of a sinusoidal signal?
- Its phase
- relates to point sinusoid has reached at particular time
- shifting signal to right or left changes its phase
Frequency domain
- also known as the spectrum
- any signal can be represented
- sinusoid shown as single line as it represents single frequency of particular strength
Time domain
- any signal can be represented
- shows sinusoid as it progresses in time
Sawtooth wave
- made up of sum of sinusoids of decreasing amplitude
- these sinusoids are exact whole number multiples of lowest frequency
- higher frequency sinusoids called harmonics
square wave
- binary signal with alternating 1s and 0s
- even multiples are missing in this wave
Non-periodic signals
- 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
Modulation
- 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
Resultant modulated signal
- No longer periodic
- occupies range of frequencies not just one
Optical fibre
- transmits large amount of info rapidly over long distances
- uses light signals
Three main components of optical fibre link
- 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
Electromagnetic wave
- electric and magnetic field both sinusoidal and are at right angles to each other
- whole wave pattern moves forward at speed of light
Wavelength
- distance between two consecutive peaks
- light waves have short wavelengths measured in nanometres
Frequency
- number of cycles that pass given point in one second
Electromagnetic spectrum
- chart is plotted on logarithmic scale
- frequency increases left to right
- wavelength increases from right to left
Refractive index
- 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
How optical fibre works
- refractive index not same all the way across the fibre
- higher in central core than cladding around core
How can light change direction?
- refraction: occurs in lenses, ray of light travels from one medium to another with different refractive index
- reflected: occurs in mirrors
Total internal reflection
- if light directed one medium to another with lower refractive index it can bounce back if angle low enough.
- reflected into first medium
Multimode fibre
- 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
Graded-index fibre
- 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
Single-mode fibre
- 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
Attenuation
- signal gradually loses power over distance
Decibels
- way of comparing two powers
- logarithmic measure of ratio between two powers
multimode
- preferred for short distance because of lower component costs and ease of use
problems with fibre
- various effects distort signal
- smears out transitions between light and dark
- signal merges into one another
- longer the fibre worse it gets
pulse spreading
- different paths result in different timings
- this called multimode distortion, main cause of pulse spreading in multimode fibres
Other mechanisms that cause pulse spreading
- Distortion
- Polarisation mode distortion
Distortion
- or chromatic dispersion
- caused by light of different wavelengths travelling at different speeds
Polarisation mode distortion
- affects single mode fibres
- caused by another variation in speed of light
- speed varies with orientation of light wave in fibre
Optical transmitter
- convert input data in form of electrical signal to light signal
- two main types: LED and laser diode
Light emitting diode (LED)
- similar to LEDs seen in displays
- rather than emitting visible light, they emit in infrared region of spectrum
Laser diodes
- also found in CD, DVD
- where they read and write data from disc
Where are LEDs used
- multimode fibres
- number of disadvantages
- lower in power and emit over range of wavelengths leading to dispersion
- emit broad cone of light
How LED and laser diode work
- 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
How to vary LED and Laser diode
- beam can be modulated by varying electrical power supplied to them
- Laser diode has advantage on how quick it can be switched
Photodiode
- detector at other end of fibre
- provides current output that varies with intensity of light it receives
How to increase range of fibre
- repeater or regenerator
- devices that counteract effects of attenuation
- restores signal back to original form
Difference between repeater and regenerator
- repeater tends to include simpler devices
- regenerator does further processing; reshapes and retimed
- regenerated pulse is copy of original signal with noise removed
Optical amplifiers
- developed as better solution for long haul links
- amplify signal directly without converting back to electrical signal
Erbium doped fibre amplifier (EDFA)
- 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
Distributed Raman amplifier
- amplifies signal along whole of transmission path
- pumping can be done from either end
- usually done backwards from receiver end
Semiconductor optical amplifier (SOA)
- similar principle to semiconductor laser
- have advantages of low cost and compactness
Optical switch
- one technique relies on moving mirrors
- mirrors need to be small and lightweight to allow fast switching
Directional couplers
- 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
Wavelength division multiplexing (WDM)
- 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
Demultiplexing
- splits multiplexed signal into constituent wavelengths
- special filters used that only allow particular wavelength to pass through
Receiver sensitivity
- result must meet or exceed minimum power a receiver can detect
Copper cable
- operates with electrical signals
- link consists of pair of conductors
- voltage applied at one end appears at other end
Resistance
- 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)
Dielectric loss
- small part of energy wasted in electromagnetic fields
Unshielded tested pair (UTP)
- pair of conductors twisted together along length
- any interference affects both conductors evenly
- twisting gives some protection against crosstalk
Coaxial cable
- 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
Shielded twisted pair (STP)
- combines advantages of shielding and twisting
- at expense of greater complexity
Antennas
- Radio wave electric and magnetic fields generated directly from electrical signals in structures known as antennas
Filtering
- A filter in receiver allows narrow band of frequencies through while attenuating all others
Bandwidth
- Amount of spectrum occupied by signal
- equal to difference between highest and lowest frequencies
- Larger the bandwidth, the more info signal can convey
Response
- measure of relative sensitivity of receiver to frequencies at and around the frequency it is tuned to
- usually centre frequency of a wanted transmission
Passband
- Range of frequencies that receiver responds best to
- passband extends from lower cut-off frequency to higher cut-off frequency
Selectivity
- receiver that is good at rejecting signals outside the passband said to have high selectivity
Inverse square law
- describes reduction in power with distance from transmitter, due to spreading
- applies ideally to free space
Isotropically
- A wave moving outwards from a transmitter, radiating equally well in all directions
Specular reflection
- light is reflected at shiny surfaces, like mirrors
- travels towards and away from such a surface at equal angles
Scattering
- occurs when reflecting objects are small compared to wavelength
- scattering by intervening objects can result in loss of useful energy
- reduces the received signal
Ionosphere
- Refraction important at lower frequencies
- lower layer of atmosphere called ionosphere can bend path of radio waves back towards ground
Absorption
- radio waves absorbed as they travel through atmosphere or buildings
- measured in decibels per metre or kilometre
- dependent on frequency
Attenuation coefficient
- The attenuation in decibels per kilometre of distance travelled
Diffraction
- spreading or bending of electromagnetic wave when it passes through gap or encounters sharp corner
Dipole
- simplest and most common antenna
- consists of two conductors fed at midpoint with electrical signal
- effective over narrow band of frequencies
Near field
- region close to antenna where inverse square law ceases to apply
- extends for few wavelengths from the antenna
omnidirectional
- Radiates better in some directions than others
- typically broadcast antenna in middle of area would have omnidirectional antenna
Beamwidth
- angle of cone that contains predominant radiation
- cone is taken to include radiation above certain power level
Antenna gain
- 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
Surface wave
- 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
Sky wave
- 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
Multipath
- Several paths a transmission can take to receiver
- due to reflections from buildings and ground
- propagation described as multipath
Direct and reflected waves
- Different factors affect whether in phase or not
- positions of transmitters and receivers, determine relative paths
- wavelength or frequency of transmission
Fading
- if receiver or transmitter moves around received signal can vary in strength
Amplitude modulation (AM)
- amplitude of carrier waveform altered in proportion to information signal, referred to as modulating signal
Envelope
- used to describe varying strength, or shape, of modulating signal
Mixer
- 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
Sidebands
- situated either side of carrier frequency
- replicas of original modulating signals spectrum
Frequency modulation (FM)
- frequency of carrier waveform altered in proportion to envelope of modulating signal
- amplitude and phase remain the same
Voltage controlled oscillator
- used to create modulated signal
- takes voltage signal as input and produces periodic electronic signal
Frequency deviation
- 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
Phase modulation
- phase of carrier waveform altered in proportion to amplitude of modulating signal
- Bandwidth of PM signal can be approximated by Carson’s rule
