Physical Layer

Question 1

Twisted Pair

Answer 1

Used in telephone lines, LANs
Twisting reduces noise as the magnetic fields induce currents in opposite directions
Two types: Unsheilded(UTP), Sheilded (STP)

Question 2

Coaxial Cable

Answer 2

Used for Ethernet LANs, cable tv, and MANs
Good shielding reduces noise
More bandwidth for longer distances and higher rates than twisted pair (capable of 1Gbps over 1km)

Question 3

Power Lines

Answer 3

A network that uses household electrical wiring
Convenient to use, but not great for sending data

Question 4

Fiber Optics

Answer 4

Used in long-distance ISP links
Common for high rates and long distance
Enormous bandwidth and tiny signal loss
Light carried in very long, thin strand of glass
Two types:
Multi-mode: Light can bounce(wider core). Used with LEDs for cheaper, shorter-distance links
Single-mode: Core so narrow so that light cannot bounce around. Used with lasers. Capable of carrying 100s of Gbps over 100s of kilometres

Question 5

Radio transmission

Answer 5

Radio signals penetrate buildings well and propagate for long distances

Question 6

Microwave Transmission

Answer 6

Microwaves have a lot of bandwidth
Used indoors(wifi) and outdoors (between masts of 4G, 5G networks, between satellites)
The signal is attenuated/reflected by everyday objects
Strength varies with mobility due to multipath fading

Question 7

Infrared transmission

Answer 7

Used for short-range communication, does pass through objects

Question 8

Transmission impairment

Answer 8

A signal is affected by
Attenuation: loss of signal strength
Noise: The random or unwanted signal that mixes up with the original signal
Distortion: Alteration of signal shape or form(induced by attenuation, noise or delay)

Question 9

Simplex transmission

Answer 9

One-way communication e.g. keyboard to computer

Question 10

Half-duplex transmission

Answer 10

Two-way communication link, but only one system can talk at a time. e.g. walkie talkie

Question 11

Full duplex

Answer 11

Transmit data in both directions simultaneously. E.g. telephone.
Most communication systems work in full duplex(extra cost is marginal)

Question 12

Parallel transmission

Answer 12

Transmits a group of bits simultaneously - high bit rate
Expensive over long distances because of the need for thicker cable
Synchronisation problem over long distance (bit skew)

Question 13

Serial transmission

Answer 13

Transmits bits one after another - slower than parallel transmission
More reliable and cheaper for transmission over longer distances
Incurs additional complexity for sending and receiving devices (the sender must determine the order bits are sent, the receiver must determine the order to reconstruct a byte)

Question 14

Multiplexing

Answer 14

Allows the carrying of multiple signals on a single medium(share channel among users).
Allows for more efficient use of the transmission medium(better utilisation)

Question 15

Frequency division multiplexing

Answer 15

Shares the channel by placing users on different frequencies.
Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a signal
Filters used at receiver to separate channels

Question 16

Time-division multiplexing

Answer 16

Shares a channel over time
Takes advantage of the fact that achievable bit. rate of the medium exceeds the required data rate of a digital signal

Question 17

Code division multiple access

Answer 17

Shares the channel by giving specific code to users.
Widely used as part of 3G networks

Question 18

Manchester encoding

Answer 18

Read from middle of the bit
High to low transmission (negative edge) = 1
Low to high transmission (positive edge) = 0
Will be 8 numbers to read

Question 19

NRZI encoding

Answer 19

Read from start of the bit(on the line)
A change in voltage = 1
No change in voltage = 0