CN notes 前情提要：傳送門
目錄：
• Compare the bit rate for video, photos, and audio.
• What are the characteristics of streaming stored video?
• What are the characteristics of streaming live audio and video?
• What are the characteristics of conversational voice and video over IP?
• How does the encoding of analog audio work (in simple terms)?
• What are the three major categories of VoIP encoding schemes?
• What are the functions that signaling protocols are responsible for?
• What are three QoS VoIP metrics?
• What kind of delays are included in "end-to-end delay"?
• How does "delay jitter" occur?
• What are the mitigation techniques for delay jitter?
• Compare the three major methods for dealing with packet loss in VoIP protocols.
• How does FEC (Forward Error Correction) deal with the packet loss in VoIP? What are the tradeoffs of FEC?
• How does interleaving deal with the packet loss in VoIP/streaming stored audio? What are the tradeoffs of interleaving?
• How does the error concealment technique deal with the packet loss in VoIP?
• What developments lead to the popularity of consuming media content over the Internet?
• Provide a high-level overview of adaptive video streaming.
• Which protocol is preferred for video content delivery - UDP or TCP? Why?
• What was the original vision of the application-level protocol for video content delivery, and why was HTTP chosen eventually?
• Summarize how progressive download works.
• How to handle network and user device diversity?
• How does the bitrate adaptation work in DASH?
• What are the goals of bitrate adaptation?
• What are the different signals that can serve as an input to a bitrate adaptation algorithm?
• Explain buffer-filling rate and buffer-depletion rate calculation.
• What steps does a simple rate-based adaptation algorithm perform?
• Explain the problem of bandwidth over-estimation with rate-based adaptation.
• Explain the problem of bandwidth under-estimation with rate-based adaptation.Compare the bit rate for video, photos, and audio.
Videos have the highest bit rate, with photos being the second and audio being last.
What are the characteristics of streaming stored video?
Stored video is streamed, interactive, and has continuous playout.
What are the characteristics of streaming live audio and video?
Similar to streaming stored, however, many simultaneous users are requesting the video. Live audio or video is also delay-sensitive.
What are the characteristics of conversational voice and video over IP?
Real-time and delay-sensitive, also loss-tolerant
How does the encoding of analog audio work (in simple terms)?
Quantization, the rough conversion of continuous waves to discrete waves
What are the three major categories of VoIP encoding schemes?
- Narrowband
- Broadband
- Multimode
What are the functions that signaling protocols are responsible for?
- User location — 呼叫者所在的位置。
- Session establishment — 處理被叫方接受、拒絕或重定向呼叫。
- Session negotiation — 端點在會話的一組屬性上相互同步。
- Call participation management — 處理加入或離開現有會話的端點。
What are three QoS VoIP metrics?
- end-to-end delay
- jitter
- packet loss
What kind of delays are included in “end-to-end delay”?
Total delay from mouth to ear. This includes:
- The time it takes to encode the audio
- The time it takes to put it in packets
- All the normal sources of network delay that network traffic encounters such as queueing delays
- Playback delay which comes from the receiver’s playback buffer
- Decoding delay which is the time it takes to reconstruct the signal
How does “delay jitter” occur?
Buffer sizes, queuing delays, network congestion, etc.
What are the mitigation techniques for delay jitter?
Maintaining a buffer called the “jitter buffer” or the “play-out-buffer”. This mechanism smooths out or hides the variation in delay between different received packets, buffering them and playing them out for decoding at a steady rate.
Compare the three major methods for dealing with packet loss in VoIP protocols.
- Forward Error Concealment (FEC) — transmitting redundant data alongside the main transmission, allows the receiver to replace lost data with the redundant data.
- Interleaving — Mixing chunks of audio together so that if one set of chunks is lost, the lost chunks aren’t consecutive.
- Error concealment — basically guessing what the lost audio packet might be.
How does FEC (Forward Error Correction) deal with the packet loss in VoIP? What are the tradeoffs of FEC?
The more redundant data transmitted, the more bandwidth is consumed.
How does interleaving deal with the packet loss in VoIP/streaming stored audio? What are the tradeoffs of interleaving?
The receiving side has to wait longer to receive consecutive chunks of audio , increasing latency.
How does the error concealment technique deal with the packet loss in VoIP?
Basically guessing what the lost audio packet might be.
What developments lead to the popularity of consuming media content over the Internet?
- Bandwidth for the core network and last-mile access links have increased over the years.
- Compression technologies have become more efficient.
Provide a high-level overview of adaptive video streaming.
Which protocol is preferred for video content delivery — UDP or TCP? Why?
- TCP.
- TCP provides reliability, congestion control.
What was the original vision of the application-level protocol for video content delivery, and why was HTTP chosen eventually?
Allows the server to be stateless and the intelligence to download the video is left to the client. Allows content provides to use the already existing CDN. Bypasses middleboxes and firewalls because HTTP is well-known.
Summarize how progressive download works.
The client sends byte-range requests for part of the video, instead of requesting the entire video. The client pre-fetches some video and stores it in the playout buffer.
How to handle network and user device diversity?
Using bitrate adaptation — avoids using one static bitrate. Allows clients to determine the bitrate and then increase / decrease based upon network conditions.
How does the bitrate adaptation work in DASH?
Dynamic Streaming over HTTP (DASH) just uses dynamic bitrate adaptation. Videos are divided into chunks and encoded at multiple bit rates. The client adapts the quality video / bitrate it’s requesting from the server based upon network conditions.
What are the goals of bitrate adaptation?
- Low or zero re-buffering
- High video quality
- Low video quality variations
- Low startup latency
What are the different signals that can serve as an input to a bitrate adaptation algorithm?
- Network throughput
- Video buffer
Explain buffer-filling rate and buffer-depletion rate calculation.
- The network bandwidth divided by the chunk bitrate is the buffer-filling rate.
- The buffer-depletion rate or the output rate is simply 1 (1 second). How fast can we watch video.
What steps does a simple rate-based adaptation algorithm perform?
- Estimation of future bandwidth
- Quantization is the continuous throughput mapped to a discrete bitrate
Explain the problem of bandwidth over-estimation with rate-based adaptation.
Client requests high quality video, however, the bandwidth has dropped tremendously — causes the video buffer to deplete. The player takes time to converge to the correct bandwidth.
Explain the problem of bandwidth under-estimation with rate-based adaptation.
Under-estimation can cause the network bandwidth to be monopolized by a greedier client.
