VoIP Active Reading Queries - Sampling Voice
A common question for the CCNA Voice candidate is - "Just how can we translate our analog voice waveform into the digitized form that is required for Voice over IP transmission through the converged network?" These active reading questions tell the story - enjoy!
Questions Only
In its natural form, what signal type is the human voice?
To send voice as a series of zeros and ones is known as what type of encoding?
Converting analog voice into digital data begins with taking “snapshots” of voice very frequently. This is called what?
If you sample voice too infrequently, the listener might hear a different sound. This is called what?
What is the reason that you would not want to oversample voice?
What theorem teaches that the sample rate needs to be twice as high as the highest frequency being sampled?
Based on the Nyquist Theorem, how often should we sample voice?
What is the initial process of voice sampling called?
What is the process of taking PAM amplitudes and assigning them a number?
What type of scale is used in voice quantization?
Rounding off during quantization can cause a “hiss” on the line. This is called what?
When is quantization error more noticeable?
What approach to the quantization logarithmic scale is commonly used in North America?
What approach to the quantization logarithmic scale is commonly used outside North America?
If VoIP equipment connects from different countries, what quantization logarithmic scale is used?
What is the breakdown of the 8 bits in a voice sample?
When considering voice sampling only- how much bandwidth is required to send voice and how is the value arrived at?
The process of encoding and decoding a wave form to save bandwidth is accomplished by what type of technology?
What type of codec does not actually compress the waveform and what is an example?
What type of codec sends the difference in the current sample versus the previous sample and what is an example?
What type of codec dynamically builds a codebook based on speech patterns and what is an example?
What is the most popular codec in the Cisco VoIP environment for sending voice over the WAN and why?
What is the bandwidth for a voice call required under the G.711 codec?
What is the bandwidth required for a voice call under the G.729 codec?
What type of codec is very similar to CS-ACELP but uses a smaller codebook and what is an example?
What are the bandwidth and delay characteristics of LDCELP?
What codec is typically used in the LAN in a Cisco VoIP environment?
What variation of G.729 uses a less complex algorithm?
What variation of G.729 enables VAD?
What is VAD?
What are four other factors that impact the size of a voice packet?
What VoIP quality measurement uses a trained ear to rate quality on a scale of 1 to 10?
What quality measurement digitally measures the difference in the original signal and the signal after it passes through the codec?
What variation of PSQM attempts to match the measurement with MOS?
Calculating the number of telephone calls during the busiest time of day is referred to as what?
What is an Erlang?
What is the formula for calculating the number of call minutes a corporate phone system uses during the busiest hour of the day?
What is the percentage of call to reject during the busiest hour of the day referred to as?
Questions and Answers
In its natural form, what signal type is the human voice?
Analog
To send voice as a series of zeros and ones is known as what type of encoding?
Binary
Converting analog voice into digital data begins with taking “snapshots” of voice very frequently. This is called what?
Sampling
If you sample voice too infrequently, the listener might hear a different sound. This is called what?
Aliasing
What is the reason that you would not want to oversample voice?
Requires too much bandwidth
What theorem teaches that the sample rate needs to be twice as high as the highest frequency being sampled?
The Nyquist Theorem
Based on the Nyquist Theorem, how often should we sample voice?
Every 125 ms
What is the initial process of voice sampling called?
PAM (Pulse Amplitude Modulation)
What is the process of taking PAM amplitudes and assigning them a number?
Quantization
What type of scale is used in voice quantization?
Logarithmic
Rounding off during quantization can cause a “hiss” on the line. This is called what?
Quantization error
When is quantization error more noticeable?
At lower volumes
What approach to the quantization logarithmic scale is commonly used in North America?
Mu-Law
What approach to the quantization logarithmic scale is commonly used outside North America?
a-Law
If VoIP equipment connects from different countries, what quantization logarithmic scale is used?
a-Law
What is the breakdown of the 8 bits in a voice sample?
1 Polarity Bit; 3 Segment Bits; and 4 Step Bits
When considering voice sampling only- how much bandwidth is required to send voice and how is the value arrived at?
8000 samples per second * 8 bits per sample = 64 Kbps
The process of encoding and decoding a wave form to save bandwidth is accomplished by what type of technology?
A Codec
What type of codec does not actually compress the waveform and what is an example?
Pulse Code Modulation (PCM); an example is G.711
What type of codec sends the difference in the current sample versus the previous sample and what is an example?
Adaptive Differentiated PCM (ADPCM); an example is G.726
What type of codec dynamically builds a codebook based on speech patterns and what is an example?
Conjugate Structure Algebraic Code Excited Linear Prediction (CS-ACELP); G.729 is an example
What is the most popular codec in the Cisco VoIP environment for sending voice over the WAN and why?
G.729; decent quality with lower bandwidth requirements
What is the bandwidth for a voice call required under the G.711 codec?
64 Kbps
What is the bandwidth required for a voice call under the G.729 codec?
8 Kbps
What type of codec is very similar to CS-ACELP but uses a smaller codebook and what is an example?
Low-Delay Conjugate Excited Linear Prediction (LDCELP); G.728
What are the bandwidth and delay characteristics of LDCELP?
It reduces delay, but at the cost of higher bandwidth
What codec is typically used in the LAN in a Cisco VoIP environment?
G.711
What variation of G.729 uses a less complex algorithm?
G.729a
What variation of G.729 enables VAD?
G.729b
What is VAD?
Voice Activity Detection – devices will not send “the sound of silence” with VAD
What are four other factors that impact the size of a voice packet?
Media, Tunneling, Header Compression, Codec used
What VoIP quality measurement uses a trained ear to rate quality on a scale of 1 to 10?
Mean Opinion Score (MOS)
What quality measurement digitally measures the difference in the original signal and the signal after it passes through the codec?
PSQM (Perceptual Speech Quality Measurement)
What variation of PSQM attempts to match the measurement with MOS?
PESQ (Perceptual Evaluation of Speech Quality)
Calculating the number of telephone calls during the busiest time of day is referred to as what?
Traffic Engineering
What is an Erlang?
One solid hour of phone usage
What is the formula for calculating the number of call minutes a corporate phone system uses during the busiest hour of the day?
[Monthly_Call_Minutes/22] * .15
What is the percentage of call to reject during the busiest hour of the day referred to as?
GOS (Grade of Service)
