High-Performance Prefetching Protocols for VBR Prerecorded Video 윤 지 숙 Martin Reisslein, Keith Ross

VoD Architecture

Observation Prerecorded Video –The traffic in each video is known in advance –Video can be fetched, while video is being played Frequent periods of time during which the networks bandwidth is under utilized

Improvement Better Bottleneck Bandwidth Utilization Improve the quality of video playout –Constant quality playout without client buffer satvation Via: Prefetching Protocol – Provide to whom need more Transmission schedule of a connection on-line as function of the buffer contents at all the clients

Protocols for two architecture Centralized Decentralized

Architecture for Centralized Scheme

Notation

When a Client Requests Server makes Admission Control Once the server grants the request, –Server starts to transmit packets(in predeterminedorder) –Client starts to display as a few frames arrive Every 1/F second, client decode and display a frame For every 1/F seconds, Server keeps track of P l (j) – P l+1 (j) = [P l (j) + A l (j) –1] +

Join-the-Shortest-Queue Find j * with the smallest p(j) and Repeat ‘til –Transmit one frame from connection j * –Increment p(j * ) Stopping rule –Z + X σ(j*) (j * ) <= R/F –b(j * ) + X σ(j*) (j * ) <= B(j)

Decentralized Prefetching

A Protocol inspired by TCP Network accepts a new connection –If link util = F * Σ j= 1 x avg (j)/R <= 95% Senders have send window(w l ) –New connection starts with w 0 = 1 –When all acknowledgements are received in time w l+1 = w l + Δw 0 <= a l <= w l, P l+1 (j) = [P l (j) + A l (j) –1] + –If positive acknowledgement is not received, Set the w l back to 1 If p l = 0, error concealment technique If p l > 0, retransmit the lost frame

Refined Protocol Prevent client buffer overflow –b l + x k <= B Dynamic Send Window –Δw l = Δw max (1 - b l /B) e Randomized Transmission – δ l drawn from Uniform over [-1/2F, 1/2F] –Subsequent transmission = start + (l-1)/F + δ l

For Wireless Environments Where there is location-dependent, time- varying, and bursty errors of wireless links –“A prefetching for Continuous Media Streaming in Wireless Environments” Join-the-Shortest-Queue Channel Probing –Bad Channel Sends at most one packet per forward slot

Protocol Base station keeps track of p(j) and b(j) –b(j) : # of packets in the client j’s buffer –p(j) : length of prefetched video segment –b(j) +(-) Xn(j) –P(j) +(-) Tn(j) More transmission capacity to clients with small reserve –Find J* with smallest p(j) + z(j)

The thing I like When the server schedules transmission, it takes into account of client buffer level Also reacts to link(channel) state Consider the characteristics of prerecorded video(skip the frame when the deadline is passed)

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