Transcript No Slide Title

On the Interactions
Between
Layered Quality Adaptation
And
Congestion Control for Streaming Video
Mick Feamster, Deepak Bansal, and Hari Balakrishnan
MIT Laboratory of Computer Science
published in
11th International Packet Video Workshop, Kyongju, Korea, April 2001.
This paper presents:
1. mechanisms for smoother playout of layers
with a class of non-AIMD CC algorithms
2. rules for “quality adaptation” for simulcast,
hierarchical encoding and optimistic adaptation
algorithm
3. a mechanism for performing “quality adaptation”
in combination with non-AIMD CC algorithms
Background Knowledge:

Binomial Congestion Control
- generalize TCP’s increase/decrease
rules using the following equations
I: Wt+R
D: Wt+t

Wt +

Wt -

/ Wtk;


> 0
Wtl ; 0 <  < 1
where k, l are the parameters of binomial controls;
Wt is the instantaneous window value, which
governs the transmission rate
Note that,
- for k= 0, l=1, we get AIMD used by TCP
(along with  = 1 and  = 0.5)
- for k=-1, l=1, we get MIMD used by Slow-Start
in TCP
(along with  = 1 and  = 0.5)
- for k=-1, l=0, we get MIAD
- for k= 0, l=0, we get AIAD
- for a binomial congestion control algorithm to
be TCP-friendly, k + l = 1
- one member of the TCP-friendly family is
SQRT (k = l = 0.5)
- SQRT seems to be attractive for streaming
media delivery due to its smaller magnitude of
oscillations
- In SQRT, the reduction in tx rate is
proportional to  W, whereas in AIMD the
reduction is proportional to W.
Wt+R
= Wt + ( ) / Wtk
Wt+R - Wt =
RTT
d W
dt
=

RTT * Wtk

Wtk * RTT

Quality Adaptation
1. Instantaneous Adaptation
- simply send video at the highest possible
layer that can be sent at any given time
- too optimistic
- disadv: the magnitude and frequency of
oscillations in the congestion control algorithm will
govern the perceived quality of video at the receiver
2. Simulcast
- server encodes the bit-stream at various
target bit rates and switches between the previously
encoded layers as the available BW changes
- (for simplicity) assume a constant rate
spacing between successive layers and assume also that
no buffering available to alleviate backoff and the
resulting drop in the tx rate
- the transmission should be switched to a
higher layer only if the higher layer can be sustained
after an immediate backoff
- thus, for a TCP-friendly binomial control,
the video quality that should be sent should be the
highest encoding available such that
BR < R -

Rl
where, BR: the bit rate at which the video stream
is encoded
R : the instantaneous transmission rate
- for TCP-style AIMD where l = 1 and
BR < R/2
- for SQRT where l = 2
BR < R -
 
R

= 0.5,
3. Hierarchical Encoding
- Recall that, with RAP and hierarchical
encoding, figure 2 shows the optimal interlayer buffer
allocation and the total buffering required to survive
one backoff (suppose that smoothing factor = 1)
- with binomial congestion control algorithm, the
same idea can be applied. We will start with the
conditions for adding a new layer. The following two
conditions should hold:
R > (na + 1)C
 i=0 to na-1 bufi >= A
where A is the shaded area in figure 3
t1
t2
- assuming that we have layers available at all encoding
(i.e., continuity assumption), it follows that na+1
corresponds to the encoding of the video data at the
mean rate of transmission. Therefore,
AIMD:
(na+1) C = [ R + ( R/2 ) ] / 2 = 3R/4
SQRT:
(na+1) C = [ R + ( R = R – (


 R) ] / 2
/ 2)  R
- substitute the estimated value of (na+1) C into (3),
we get the following in-equations:
- thus, the buffer required for adding a layer with
SQRT is significantly lower than the one when AIMD
is employed
Results:
- “Binomial Congestion Control” can provide
benefits over AIMD, regardless of the
quality adaptation mechanism used by
reducing the degree of oscillation in the
video server’s sending rate
- “Binomial Congestion Control” can provide
significant benefits when used with buffered
quality adaptation of hierarchically encoded
video by reducing the amount of required
buffering at the receiver in order to playout
at a given layer