Chapter 3: Physical Layer (2) Flashcards
3.4: What are the three types of transmission impairment?
- Attenuation
- Distortion
- Noise
3.4: What is attenuation?
A signal’s loss of energy as it travels through, and overcomes the resistance of, a transmission medium.
3.4: What are decibels?
Measure the relative strengths of two signals, or one signal at two different points in time.
Decibels are a measure of relative power.
3.4: What’s the equation for decibels?
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.
3.4: What is distortion?
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.
3.4: What is noise?
A catch-all term for forms of interference that affect the signal – that is, what’s sent is different from what’s received.
3.4: What are the four categories of “noise”?
- Thermal Noise
- Induced Noise
- Crosstalk Noise
- Impulse Noise
3.4: What is thermal noise?
The random motions of electrons in a transmission medium, which can create extra signal(s).
3.4: What is induced noise?
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.
3.4: What is induced noise?
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.
3.4: What is crosstalk noise?
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.
3.4: What is impulse noise?
The result of a high energy spike in a short period of time, such as lightning, that creates noise.
3.4: What is the signal-to-noise ratio (SNR)?
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.
3.4: What is the signal-to-noise ratio (SNR) equation?
SNR = (average signal power / average noise power)
3.4: What is the signal-to-noise ratio (SNR) equation when measuring in decibels?
SNR(dB) = 10 * log(10) [SNR]
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)?
- Bandwidth available
- Signal levels used
- Channel quality
3.5: What is the Nyquist formula? What does it measure?
Defines the maximum bit rate of a channel.
BitRate = 2 * bandwidth * log(2) [Levels]
3.5: What do “levels” refer to? What’s their relationship to bit rate?
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.
3.5: What is the Shannon Capacity? What does it measure?
Determines the highest data rate for a noise channel.
Capacity = bandwidth * log(2) [1 + SNR]
3.6: What are the two types of bandwidth?
- Bandwidth in Hertz
- Bandwidth in bits-per-second (bps)
3.6: What is bandwidth in bits-per-second (bps)?
Refers to the amount of data that can be sent over a network in one second.
3.6: What is throughput?
Measures how fast we can send data through a network.
3.6: What’s the difference between bandwidth and throughput?
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.
3.6: What is latency?
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.
3.6: What four things make up latency?
- Propagation time
- Transmission time
- Queuing time
- Processing delay
3.6: How is propagation time calculated?
Propagation time = distance / propagation speed
Propagation time is in seconds.
3.6: How it transmission time calculated?
Transmission time = message size / transmission rate
Transmission time is in seconds.
