Lesson 11: Applications (Video) Flashcards
What can you tell about bit rate for video and it’s comparison to the bit rate of audio or browsing through an online photo gallery?
Video browsing requires more bandwidth than photo browsing which is about twice that of music.
(video 2Mbps > photo 320 bps > music 128 bps)
What are the characteristics of streaming stored video?
video is interactive continuous playback (no freezing) normally stored on a CDN
What are the characteristics of streaming live audio and video?
many simultaneous users in different locations
delay-sensitive (10 sec max)
What are the characteristics of conversational voice and video over IP?
3+ users
high delay-sensitive (0.4 sec max)
loss-tolerant
How does the encoding of analog audio work (in simple terms)?
Audio is encoded by taking many (thousands) of samples per second, and then rounding each sample’s value to a discrete number within a particular range
Pulse Code Modulation (PCM) may record speech at 8,000 Hz at 8 bits and audio CD at 44,100 Hz @ 16bits
What are the three major categories of VoIP encoding schemes?
narrowband, broadband, and multimode
With regards to telephony and VoIP, what are the functions that signaling protocols are responsible for?
1) User location
2) Session establishment - handling the callee accepting, rejecting, or redirecting a call.
3) Session negotiation - the endpoints synchronizing with each other on a set of properties for the session.
4) Call participation management - handling endpoints joining or leaving an existing session.
What are 3 metrics used to measure the quality of VoIP?
- end-to-end delay
- jitter
- packet loss
One of the QoS VoIP metrics is “delay jitter”. How does this phenomenon occur?
data packets experience different delays and are received at different times. The result can be more delayed packets and gaps in audio recieved.
How does FEC (Forward Error Correction) deal with the packet loss in VoIP? What are the tradeoffs of FEC?
transmits redundant data alongside the main transmission.
tradeoffs:
redundant data may be of a lower quality
transmission requires more bandwidth
receiver may need to receive more before playing
How does interleaving deal with the packet loss in VoIP/streaming stored audio? What are the tradeoffs of interleaving?
packets mix chunks of audio together. If a packet is lost, small gaps are preferred to one large one.
tradeoffs:
wait longer before starting
How does error concealment technique deal with the packet loss in VoIP?
when a packet is missing, it repeats last packet or interpolates between the surrounding packets to guess at the sound.
Provide a high-level overview of adaptive video streaming.
video is:
1) recorded
2) compressed
3) secured using DRM
4) replicated amongst geographic servers within CDN
5) downloaded by user
6) decoded and rendered on the screen
An efficient video compression can be achieved in two-ways. What are these ways?
1) spacial redundancy - pixels within an image can be similar.
2) temporal redundancy - consecutive frames can be similar
What are the 4 steps of JPEG compression?
- Transform RGB image into color (Cr, Cb) and brightness ( Y ) components
- Obtain frequency domain matrix by applying the Discrete Cosine Transformation to 8x8 blocks.
- Compress the resulting matrix using a pre-defined Quantization table.
- Perform a lossless encoding.
Explain video compression.
In video compression the initial or i-frame is encoded as a jpeg. Next frames in the same scene are encoded and called predicted frames (or p-frames).
Additionally, a Bi-frame (or b-frame may be encoded between the i & p-frames to improve coding efficiency.
What is the difference between constant bitrate encoding and variable bitrate encoding (CBR vs VBR)?
CBR - output size of the video is fixed
VBR - output size remains the same on an average, but varies based on the underlying scene complexity.
Which protocol is preferred for video content delivery - UDP or TCP? Why?
TCP
- if an i or p-frame is lost decoding is impossible (TCP ensures delivery)
- TCP provides congestion control.
What was the original vision of the application-level protocol for video content delivery and why was HTTP chosen eventually?
Original vision was to have specialized video servers that remembered the state of the clients. Intelligence is on the server and clients do minimal work.
HTTP was in existence, providers can use existing CDNs, bypassing middleware with HTTP was possible.
When streaming video, what is a “byte-range request” and why is it useful?
A request for a small sample of the video file.
This is useful to avoid downloading the entire video file such as when browsing.
What is bitrate adaptation
The ability of a server to adapt to a bitrate specified by the client during a video stream.
For example, A server will host video in short segments at various bitrates (250kbps, 500 kbps, 3Mbps, etc.). Depending on the bandwidth available, the client will request the segment and quality. Throughout a video bitrate may vary or adapt.
How does the bitrate adaptation work in DASH?
Video in DASH is divided into chunks and each chunk is encoded in multiple bitrates. The bitrate adaptation algorithm at the client adapts the video bitrate to request based on network conditions.
What are the goals of the bitrate adaptation algorithm?
optimize the user’s viewing quality of experience
this is done by:
- low or zero re-buffering
- high quality video
- low quality variations
- low start latency
What are the different signals that can serve as an input to a bitrate adaptation algorithm?
- network throughput
- video buffer: full buffer can allow time to download high quality vid, low buffer can be filled quickly with low quality.
