lecture 4

Question 1

what are the two types of transmission media

Answer 1

• guided (physical)

- unguided (wireless)

Question 2

what are the three cables that fall under the guided criteria

Answer 2

• twisted pair cables
• coaxial cables
• fibre optic cables

Question 3

what are the performance measures for transmission

Answer 3

• bandwidth
• delay
• cost
• ease of installation

Question 4

why are the twisted pair twisted

Answer 4

• to minimize interference

Question 5

features of the twisted pair cables (4)

Answer 5

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

Question 6

disadvantage of the twisted pair cables

Answer 6

• distance is short

Question 7

2 advantages of the twisted pair cables

Answer 7

1. thick wire = shielding from noise

2. low cost

Question 8

describe how the baseband coaxial is composed

Answer 8

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

Question 9

2 advantages of the baseband coaxial

Answer 9

1. higher bandwidth and noise immunity (1GHz)

2. span longer distances at higher speed

Question 10

features of the baseband coaxial

Answer 10

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

Question 11

features of the broadband coaxial

Answer 11

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

Question 12

3 components of optical transmission

Answer 12

• light source
• transmission medium
• detector

Question 13

why is a light source needed

Answer 13

to be converted to bits

Question 14

what is the transmission medium (optical transmission)

Answer 14

• ultra thin fibre
  OR
• glass

Question 15

why is a detector needed

Answer 15

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

Question 16

features of the fibre optics

Answer 16

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

Question 17

5 advantages of the fibre optics

Answer 17

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

Question 18

2 main disadvantages of the fibre optics

Answer 18

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

Question 19

how does wireless transmission work (hint: not magic)

Answer 19

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

Question 20

what is wavelength

Answer 20

the distance between two consecutive maxima or minima

Question 21

relationship between wavelength and frequency

Answer 21

• higher frequency, smaller wavelength

- lower frequency, bigger wavelength

Question 22

relationship between frequency and information

Answer 22

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

Question 23

wavelength * frequency =

Answer 23

speed of light

Question 24

change in frequency=

Answer 24

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

Question 25

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

Answer 25

- frequency band - 3*10^8/wavelength - = analog data rate

Question 26

who gets the frequencies?

Answer 26

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

Question 27

why do we use narrow frequency?

Answer 27

- it will not interfere with others because its so narrow | - unlike a wide frequency that would overlap with others

Question 28

spread spectrum

Answer 28

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

Question 29

radio waves (3)

Answer 29

- omnidirectional - frequency dependent - easy to generate

Question 30

radio waves at high and low frequencies

Answer 30

``` 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 ```

Question 31

micro waves (5)

Answer 31

- 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.

Question 32

3 difficulties with micro waves

Answer 32

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)

Question 33

satellite (4)

Answer 33

- 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

Question 34

how does satellite stay constant relative to the ground station

Answer 34

earth is moving

Question 35

microwaves are used for __ communication

Answer 35

unicast communication | such as cellular telephones, satellite networks, and wireless LANs

Question 36

infrared (4)

Answer 36

- 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.

Question 37

infrared use

Answer 37

line-of-sight propagation

Question 38

line-of-sight propagation

Answer 38

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