| Date of Issue: 01-01-2023 | Rate this Study Guide |
Question 1.
H.323 was originally designed for:
| a) | Controlling voice calls over WANs |
| b) | Controlling voice calls over LANs |
| c) | Controlling voice calls over circuit-switched networks |
| d) | Replacing SS7 |
| e) | Replacing ISDN |
Question 2.
What is voice activity detection (VAD) used for in Voice over IP networks?
| a) | To suppress the transmission of silence packets |
| b) | To activate the speaker in the handset when someone starts to speak |
| c) | To transmit background noise when no person is speaking |
| d) | To activate the microphone in the handset when someone starts to speak |
| e) | None of the above |
Question 3.
What is the reference bandwidth for pre-VoX digitized voice channels?
| a) | 8 Kb |
| b) | 16 Kb |
| c) | 56 Kb |
| d) | 64 Kb |
| e) | 1.544 Mb |
Answers
Question 1.
H.323 was originally designed for:
| a) | Controlling voice calls over WANs |
| b) | Controlling voice calls over LANs |
| c) | Controlling voice calls over circuit-switched networks |
| d) | Replacing SS7 |
| e) | Replacing ISDN |
| b) | Controlling voice calls over LANs |
Explanation
H.323 was originally designed and intended as a control protocol for voice calls over Local Area Networks. Work since then has expanded the protocol to control calls over WANs as well.
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Question 2.
What is voice activity detection (VAD) used for in Voice over IP networks?
| a) | To suppress the transmission of silence packets |
| b) | To activate the speaker in the handset when someone starts to speak |
| c) | To transmit background noise when no person is speaking |
| d) | To activate the microphone in the handset when someone starts to speak |
| e) | None of the above |
| a) | To suppress the transmission of silence packets |
Explanation
VAD is used in Voice over IP networks to suppress the transmission of silence packets. This is a bandwidth-saving mechanism, which means that rather than send packets with no voice payload (or rather, voice payload that is silent), the router will suppress sending voice packets when no one is talking. This obviously reduces the bandwidth necessary when no one is speaking. Choices b and d are not functions that are implemented in Voice over IP networks, and choice c refers to comfort noise.
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Question 3.
What is the reference bandwidth for pre-VoX digitized voice channels?
| a) | 8 Kb |
| b) | 16 Kb |
| c) | 56 Kb |
| d) | 64 Kb |
| e) | 1.544 Mb |
| d) | 64 Kb |
Explanation
64 Kbps was the bandwidth of the original PCM digitized voice channels. Consequently, the digital transmission systems used by carriers historically are designed to transmit multiples of 64 Kbps, making it the standard channel for ISDN, etc. More modern techniques of digitizing use less bandwidth, but 64 Kbps is evenly divisible by their rates; thus the concept of "standard" or "reference" bandwidth.
The 64-Kbps rate is derived from the fact that the voltage of an analog signal must be sampled in time to be accurately recreated digitally. Specifically, an analog signal is sampled at 256 different voltage levels, 8000 times a second. The resulting signal is then transmitted as a 64-Kb digital bit stream, defined by the DS0 specification.
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