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127 notes > lecture 4 > Flashcards

lecture 4 Flashcards

1
Q

what are the two types of transmission media

A
  • guided (physical)

- unguided (wireless)

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2
Q

what are the three cables that fall under the guided criteria

A
  • twisted pair cables
  • coaxial cables
  • fibre optic cables
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3
Q

what are the performance measures for transmission

A
  • bandwidth
  • delay
  • cost
  • ease of installation
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4
Q

why are the twisted pair twisted

A
  • to minimize interference
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5
Q

features of the twisted pair cables (4)

A
  • made up of two copper wires
  • distances of several km
  • carry analog+digital signals
  • data rate in Mbps
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6
Q

disadvantage of the twisted pair cables

A
  • distance is short
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7
Q

2 advantages of the twisted pair cables

A
  1. thick wire = shielding from noise

2. low cost

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8
Q

describe how the baseband coaxial is composed

A
  • innermost copper core
  • copper core covered in insulating material
  • insulator is covered in braided conductor
  • finally, everything is enveloped with a plastic covering
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9
Q

2 advantages of the baseband coaxial

A
  1. higher bandwidth and noise immunity (1GHz)

2. span longer distances at higher speed

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10
Q

features of the baseband coaxial

A
  • transmits digital signals only (analog signals would have to be converted)
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11
Q

features of the broadband coaxial

A
  • transmits analog signals (digital signals would have to be converted)
  • covers a MAN
  • amplifiers used to boost
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12
Q

3 components of optical transmission

A
  • light source
  • transmission medium
  • detector
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13
Q

why is a light source needed

A

to be converted to bits

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14
Q

what is the transmission medium (optical transmission)

A
  • ultra thin fibre
    OR
  • glass
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15
Q

why is a detector needed

A

because it generates an electrical pulse when the light falls on the cable

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16
Q

features of the fibre optics

A
  • uses hair-width silicon or glass
  • signals are pulses (1=pulse, 0=no pulse)
  • use reflection and refraction principles
17
Q

5 advantages of the fibre optics

A
  1. huge data rates
  2. low error rates
  3. not affected by power loss (it uses photons, not electrons)
  4. hard to tap = very secure+light leak is unlikely
  5. thinner than coax
18
Q

2 main disadvantages of the fibre optics

A
  1. hard to tap = hard to add elements/scalability is difficult
  2. unidirectional (simplex) = two fibre optics needed for duplex communication = expensive
19
Q

how does wireless transmission work (hint: not magic)

A
  1. electrons move, which generates electromagnetic waves that can propagate through free space
  2. antennas are used to capture those waves
  3. waves are broadcasted
20
Q

what is wavelength

A

the distance between two consecutive maxima or minima

21
Q

relationship between wavelength and frequency

A
  • higher frequency, smaller wavelength

- lower frequency, bigger wavelength

22
Q

relationship between frequency and information

A
  • the higher the frequency, the bigger the amount of data that can be carried (more signals carried)
23
Q

wavelength * frequency =

A

speed of light

24
Q

change in frequency=

A

speed of light (3x10^8)*change in wavelength
/
wavelength^2

25
Q

if we are given the width of of a wavelength band, what can we compute?

A
  • frequency band
  • 3*10^8/wavelength
  • = analog data rate
26
Q

who gets the frequencies?

A

FCC (Federal Communication Commission) allocates the spectrum
- to eliminate interference with other frequencies

27
Q

why do we use narrow frequency?

A
  • it will not interfere with others because its so narrow

- unlike a wide frequency that would overlap with others

28
Q

spread spectrum

A

hops from one frequency to another in a regular pattern (hard to detect and impossible to jam)
- good for security

29
Q

radio waves (3)

A
  • omnidirectional
  • frequency dependent
  • easy to generate
30
Q

radio waves at high and low frequencies

A 
high:
- travel in a straight line
- bounce off obstacles
- absorbed by rain
- e.g satellite; bad weather, buffering
low:
- follow the ground
- pass through obstacles
31
Q

micro waves (5)

A
  • above 100MHz
  • the wave travel in straight lines
  • it can be blocked by walls, absorbed by rain
  • inexpensive to build
  • used for cordless phones, garage doors etc.
32
Q

3 difficulties with micro waves

A
  1. weather interferes with signals
  2. easy to tap
  3. signals bouncing off of structures may lead to out-of-phase signals that the receiver must filter out (multi path fading)
33
Q

satellite (4)

A
  • communication is based on line-of-sight.
  • satellite never ``moves’’ relative to ground stations (constant)
  • acts as a big microwave repeater, receiving signals from earth on one channel and rebroadcasting them on another
  • rebroadcast data
34
Q

how does satellite stay constant relative to the ground station

A

earth is moving

35
Q

microwaves are used for __ communication

A

unicast communication

such as cellular telephones, satellite networks, and wireless LANs

36
Q

infrared (4)

A
  • wavelength is shorter. so, they can’t go through objects.
  • infra-red devices are relatively cheap
  • used for short distances, within same
    room e.g. remote controls, communication between notepad PC and printer.
  • they can’t be used outdoors.
37
Q

infrared use

A

line-of-sight propagation

38
Q

line-of-sight propagation

A

a type of propagation that can transmit and receive data only where transmit and receive stations are in view of each other without any sort of an obstacle between them

127 notes (11 decks)

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