2. Physical Layer

Question 1

What’s data

Answer 1

Raw or processed info with meaning

Question 2

What’s analog data

Answer 2

Continuous values (eg. human voice)

Question 3

What’s digital data?

Answer 3

Discrete values (eg. 0s & 1s)

Question 4

What is a carrier wave?

Answer 4

Wave used to move data; it propagates disturbances

Question 5

What’s the goal of data transmission?

Answer 5

To load data onto carrier waves

Question 6

What represents a sinusoidal wave?

Answer 6

Amplitude, Frequency & phase

Question 7

Formula for a sine wave

Answer 7

A • sin(2πft + p)

Question 8

What’s the purpose of modulation?

Answer 8

Load data onto sine waves (create signal waves)

Question 9

ASK (Amplitude Shift Keying) & FSK uses what?

Answer 9

• A: (binary/multilevel) wave if bit is 1, no wave if it’s 0.
• F: uses 2 carrier frequencies, 1st used if bit is 0, 2nd if it’s 1

Question 10

PSK (Phase Shift Keying) uses what?

Answer 10

2 signal elements, 1 with a phase of 0° & other with a phase of 180° (wave changes phase when bit is 1)

Question 11

What is a QAM (Quadrature Amplitude Modulation) ?

Answer 11

Combines ASK & PSK for higher bit rates

Question 12

What’s a signal?

Answer 12

A wave that varies to convey info

Question 13

What’s Analog transmission?

Answer 13

Requires Digital-to-Analog Conversion to transmit digital data via analog channels

Question 14

What’s digital transmission?

Answer 14

Uses Digital-to-Digital Conversion (digital data to digital signals) via line coding

Question 15

Digital-to-Digital conversion problem

Answer 15

When sender/receiver clocks not sync, receiver doesn’t know when 1 bit ends & next 1 starts.

Question 16

NRZ (Non-Return-to-Zero)

Answer 16

2 voltage levels of A with no level in between.
-NRZ-Level: level represents bit value
- NRZ-Invert:
- Change in level: 1
- No change: 0

Question 17

RZ (Return-to-Zero)

Answer 17

Uses 3 voltages for sync (+, 0, -)

Question 18

What’s bandwidth/ Potential Transmission Rate?

Answer 18

Range of frequencies (num of bits per sec) a channel can transmit

Question 19

What’s throughput/ Actual Transmission Rate?

Answer 19

How fast data is sent thru a network

Question 20

Latency/ Delay

Answer 20

Total time for message delivery

Question 21

What’s attenuation

Answer 21

Signal loses energy to overcome resistance of medium.
Compensated with amplification (both have diff signs)

Question 22

What’s Distortion?

Answer 22

Signal shape changes due to diff propagation speeds thru a medium & so its own delay

Question 23

What’s multipath distortion?

Answer 23

Interference when signal has >1 path between receiver & sender (signal reflected by many surfaces)

Question 24

What’s noise?

Answer 24

Unwanted signals (thermal, crosstalk)

Question 25

What’s Crosstalk?

Answer 25

Effect of 1 wire on the other. 1 acts as a sending antenna & other as receiving 1

Question 26

What’s SNR (Signal to Noise Ratio)

Answer 26

- SNR = Average Signal Power / Average noise power: - SNR db = 10 • log_10 (SNR)

Question 27

What affects data rate?

Answer 27

Bandwidth, number of levels & channel quality

Question 28

Total Bandwidth formula for TDM system

Answer 28

B = Channels * Rate Additional B = B * Overhead - **Total B** = B + Add B

Question 29

What’s Twisted-Pair cable?

Answer 29

2 wires twisted to reduce noise: - 1 carries signals to receiver - other used as a ground reference

Question 30

Give some Guided Media examples

Answer 30

Physical cables: - Twisted-pair - Coaxial - Fiber Optic

Question 31

Unguided media disadvantages

Answer 31

(Wireless media): - Noise - fading - mulipath effects causing distortion

Question 32

Describe Coaxial Cables

Answer 32

Inner conductor + metallic shield supports higher frequency ranges

Question 33

Describe Fiber-Optic cables

Answer 33

- Uses reflection to guide light thru channel - core covered by cladding of less density so light reflected off cladding instead of refracting into it

Question 34

Describe Multimode, Step Index & Graded Index.

Answer 34

Many beams from light source move thru core in different paths: - Step index: Density of core constant - Graded-index: Varying densities, highest at center of core & decreases gradually to its lowest at edge.

Question 35

Modes of propagating light along optical channels:

Answer 35

- Multimode - Single mode: 1 beam

Question 36

What are ISM bands?

Answer 36

(Unguided) Unlicensed frequencies used in WiFi Bluetooth, etc.

Question 37

What’s multiplexing?

Answer 37

- Transmitting many signals over 1 data link. - Combines low Bandwidth channels to make larger 1

Question 38

What’s FDM (Freq Division Multiplexing)?

Answer 38

- Each signal gets a unique carrier freq - signals combined into single 1 that can be transported by link - uses guard bands

Question 39

What’s WDM (Wavelength “ ”)

Answer 39

- FDM but optical signals transmitted thru fiber-optic channels - bands of light from diff sources combined to make 1. - At receiver, signals separated by demultiplexer

Question 40

What’s TDM (Time “ “)?

Answer 40

- Instead of sharing B as in FDM, time is shared. - Time divided into fixed duration frames during which channels have time slot to transmit data.

Question 41

CDMA (code division multiple access)

Answer 41

- Stations can transmit over entire frequency spectrum simultaneously. - Stations use unique chip seq - transmit 1 bit: send sequence - transmit 0 bit: send -sequence **Recover signals:** (Og sequence*received sequence) / length: -1 = 0 bit sent 1 = 1 bit sent 0 = nothing

Question 42

How does Manchester encoding work?

Answer 42

Synchronous line code combining data & clock signals into 1 bitstream. - Bit 0: high to low - Bit 1: low to high

Question 43

How does Multipath fading cause variations in signal strength & phase, causing signal degradation?

Answer 43

- signals suffer different amounts of attenuation causing distortion & interference. - causes phase dispersion leading to signal spreading in frequency domain

Question 44

What’s the **Length** of a bit with a propagation speed X and bandwidth Y?

Answer 44

Distance = speed * time: Length = X*Y