Chapter 3: Physical Layer (2)

Question 1

3.4: What are the three types of transmission impairment?

Answer 1

1. Attenuation
2. Distortion
3. Noise

Question 2

3.4: What is attenuation?

Answer 2

A signal’s loss of energy as it travels through, and overcomes the resistance of, a transmission medium.

Question 3

3.4: What are decibels?

Answer 3

Measure the relative strengths of two signals, or one signal at two different points in time.

Decibels are a measure of relative power.

Question 4

3.4: What’s the equation for decibels?

Answer 4

dB = 10 * log(10) [ P2 / P1 ]

P1 and P2 are powers of a signal at Point 1 and 2.

If the result is positive, the signal is amplified; if negative, attenutated.

Question 5

3.4: What is distortion?

Answer 5

The change in form or shape of SIMPLE signals in a COMPOSITE signal.

Different simple signals have different propagation speeds. If these waves arrive at different times, the PHASE of the signal between sender and receive will be different.

Question 6

3.4: What is noise?

Answer 6

A catch-all term for forms of interference that affect the signal – that is, what’s sent is different from what’s received.

Question 7

3.4: What are the four categories of “noise”?

Answer 7

1. Thermal Noise
2. Induced Noise
3. Crosstalk Noise
4. Impulse Noise

Question 8

3.4: What is thermal noise?

Answer 8

The random motions of electrons in a transmission medium, which can create extra signal(s).

Question 9

3.4: What is induced noise?

Answer 9

Signals that are produced by sources such as motors and appliances. Transmission mediums will act as “receiving antennas” to these transmissions, which can affect the intended signal, causing noise.

Question 9

3.4: What is induced noise?

Answer 9

Signals that are produced by sources such as motors and appliances. Transmission mediums will act as “receiving antennas” to these transmissions, which can affect the intended signal, causing noise.

Question 10

3.4: What is crosstalk noise?

Answer 10

The effect of one wire on another, where one wire acts as a “sender”, the other as a “receiver”, and what’s received can cause noise in the intended signal.

Question 11

3.4: What is impulse noise?

Answer 11

The result of a high energy spike in a short period of time, such as lightning, that creates noise.

Question 12

3.4: What is the signal-to-noise ratio (SNR)?

Answer 12

The ratio of what’s wanted (signal) to what’s not wanted (noise).

A high SNR means the signal is LESS corrupted by noise. We WANT high SNR.

A low SNR means the signal is MORE corrupted by noise. We do NOT WANT low SNR.

Question 13

3.4: What is the signal-to-noise ratio (SNR) equation?

Answer 13

SNR = (average signal power / average noise power)

Question 14

3.4: What is the signal-to-noise ratio (SNR) equation when measuring in decibels?

Answer 14

SNR(dB) = 10 * log(10) [SNR]

Question 15

3.5: What are the three factors that impact data rate limits (how fast we can send data, in bits-per-second, over a channel)?

Answer 15

1. Bandwidth available
2. Signal levels used
3. Channel quality

Question 16

3.5: What is the Nyquist formula? What does it measure?

Answer 16

Defines the maximum bit rate of a channel.

BitRate = 2 * bandwidth * log(2) [Levels]

Question 17

3.5: What do “levels” refer to? What’s their relationship to bit rate?

Answer 17

Levels refer to different states of a signal. In a digital signal, each level corresponds to a specific number of bits.

The more levels you have, the more data you can transmit per unit of time, which results in a higher bit rate.

Question 18

3.5: What is the Shannon Capacity? What does it measure?

Answer 18

Determines the highest data rate for a noise channel.

Capacity = bandwidth * log(2) [1 + SNR]

Question 19

3.6: What are the two types of bandwidth?

Answer 19

1. Bandwidth in Hertz
2. Bandwidth in bits-per-second (bps)

Question 20

3.6: What is bandwidth in bits-per-second (bps)?

Answer 20

Refers to the amount of data that can be sent over a network in one second.

Question 21

3.6: What is throughput?

Answer 21

Measures how fast we can send data through a network.

Question 22

3.6: What’s the difference between bandwidth and throughput?

Answer 22

Bandwidth is the maximum capacity of the channel. Throughput is how fast we actually send data through the network.

Throughput is always equal to or less than bandwidth; it can NEVER exceed bandwidth.

Question 23

3.6: What is latency?

Answer 23

How long it takes for an ENTIRE MESSAGE to completely arrive at the destination, from the time the first bit is sent from the source.

Question 24

3.6: What four things make up latency?

Answer 24

1. Propagation time
2. Transmission time
3. Queuing time
4. Processing delay

Question 25

3.6: How is propagation time calculated?

Answer 25

Propagation time = distance / propagation speed

Propagation time is in seconds.

Question 26

3.6: How it transmission time calculated?

Answer 26

Transmission time = message size / transmission rate

Transmission time is in seconds.