Periodic Periodic broadcasting with broadcasting with VBR-encoded videoVBR-encoded video

Despina Saparilla, Keith W. Ross, and Martin Reisslein 1999 IEEE INFOCOM

Hsin-Hua, Lee

Objective

Develop non-uniform segmentation schemes with VBR-encoded video that significantly reduce the initial start-up latency without appreciably degrading image quality.

Introduction(1)

VoD (Video on Demand)True VoD: client-centered

• Arbitrary starting time• Waste of network bandwidth

Near VoD: data-centered• Utilization of network bandwidth and serv

er capacity• Start-up latency

• Batching the same requests before serving• Periodic Broadcasting

Introduction(2)

CBR (Constant Bit Rate) encoding technique Modifying the quantization scale during compression

VBR (Variable Bit Rate) encoding technique Quantization level remains constant

For the same quality level, Ave. Bit-RateCBR is typically 2 times or more the Ave. Bit-RateVBR

with VBR video there is potential for increased system efficiency

Quality degradation

Highly variable bit rate

Introduction(3)

To obtain dramatic reductions in start-up latency with VBR-encoded video, we must allow for some small fraction of packet loss (due to link buffer overflow). Tradeoff between start-up latency and packet-los

s. Proposed Schemes

Bufferless multiplexing Smoothing with bufferless multiplexing Server-buffering Client-prefetching

Near VoD with VBR-Encoded Video(1)

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Notations

Near VoD with VBR-Encoded Video(2)

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Near VoD with VBR-Encoded Video(3)

Each video is divided into K segments according to broadcasting series. General broadcasting series [e1,e2,…,eK-1,eK] ei: the ith segment consists of ei segmentation units,

in general, e1=1. Ni(m): the number of frames in the ith segment of th

e mth video

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Near VoD with VBR-Encoded Video(4)

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Start-up latency vs. Loss Probability(1)

Loss of bits occurs when the aggregate bit rate of the traffic (i.e., from all MK streams) exceeds the link’s capacity, C.

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Start-up latency vs. Loss Probability(2)

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Numerical Example: Geometric Series(1)

Seg. 1Seg. 2

Seg. 3

Seg. 4

Figure 1: Broadcasting strategy for geometric series with ek=2k-1.

• q=K.• receiver storage is unlimited.• M=10, and N(m)=160,000 frames, about 107 mins.• F=25 frames/sec.• C: 85~205 Mbps.

Numerical Example: Geometric Series(2)

Numerical Example: Geometric Series(3)

Two performance measures Start-up latency

Probability of loss

.2 since ,)12(

1-kk

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FNL K

k

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Bufferless Statistical Multiplexing

Latency < 2 mins=> K at least 6

Latency < 0.5 mins=> K at least 9

As K increases, prob. of loss also becomes higher.

K1

K2

K3

K4

GoP Smoothing(1)

Total start-up latency = max. access time for 1st video segment + delay introduced due to smoothing over one GOP period.

Figure 3: Bufferless multiplexing with smoothing over each GOP period

GoP Smoothing(2)

We refer to points that correspond to longer total start-up latencies with no further improvement in Ploss as dominated.

Smoothing over a higher number of GoP periods does not have an adverse effect when low start-up latencies are desirable.

Figure 4: Smoothing over many GOP periods (C=145M bps).

K=7

K=6

K=5

No significant effect !

Buffered Statistical Multiplexing

Add in finite size buffer at the server link.

Total start-up latency = max. access time for 1st video segment + B/C.

To limit loss it is instead preferable to use a smaller K.

K=7

K=6

K=5

Join-the-Shortest Queue Prefetching(1)

prefetched frames

prefetched buffer

server

client

client

clientvirtual buffer

Join-the-Shortest Queue Prefetching(2) The JSQ prefetch policy attempts to b

alance the number of prefetched frames across all virtual buffers.

All the server needs to do is to schedule the broadcast of the frames of the MK video streams as if it were sending them to the MK distinct virtual buffers.

C=145 Mbps

Join-the-Shortest Queue Prefetching(3)

JSQ protocol brings significant improvement over simply multiplexing the video stream onto the bufferless link.

6 x 10-2

3 x 10-4

6 x 10-8

100.7 sec

Join-the-Shortest Queue Prefetching with prefetch delay(1)

Join-the-Shortest Queue Prefetching with prefetch delay(2)

dpre :the prefetch delay in frame periods.

Total start-up latency =(N1+dpre)/F

C=145 MbpsK=7

6.6 x 10-6

100.4 sec

3 x 10-4

9 x 10-5

Join-the-Shortest Queue Prefetching with prefetch delay(3)

VBR and CBR Compared

For the buffered multiplexing, we chose the K value and buffer size combination which gives the lowest delay while having a loss probability less than 10E-7 (essentially a negligible loss probability).