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August 23, 2001 ITCom2001
Proxy Caching Mechanismswith Video Quality AdjustmentProxy Caching Mechanisms
with Video Quality Adjustment
Masahiro SasabeGraduate School of Engineering Science
Osaka UniversityE-mail : [email protected]
Background
• Growth of computing power• Proliferation of the Internet• Distributed multimedia applications
Increasing of video streaming services over the Internet
High-quality and low-delay video streaming service is required
Technique for low-delay data delivery
• Proxy mechanisms– Widely used in WWW systems– Reduce network load
Applying to the video transferProblems• A current proxy system only handles files whereas a single video stream is very large• A current proxy system only handles files whereas a single video stream is very large
• Client requests on the video quality considerably differ due to heterogeneity in the available bandwidth
• Segmentation of video data
• Video quality adjustment in the proxy
Research Targets
• Proposal of proxy caching mechanismswith video quality adjustment– Low-delay video streaming service
while meeting user’s demand • Data retrieval with consideration
on client’s request• Prefetching data that clients are going to
require in the future
– Reduce required cache buffer size• Segmentation of video data
for retrieval, caching and forwarding• Replacement of cached data
with consideration of size, quality and re-usability of data
Video streaming system
using proxy cache with video quality adjustment
Server
Client 1
Client 2Network
Proxy Request(high quality)
Request(low quality)
Cache Buffer
Forward(high quality)
high qualityCache
high qualityMPEG-2
Video Stream
Request(high quality)
Forward(high quality)Read
low quality
Forward(low quality)
QualityAdjustmen
t
Read
Assumptions on our proposed mechanisms• MPEG-2 video• Unit of data for retrieval, caching and forwarding
– GoP (Group of Pictures)
• A client periodically requests the proxy to send a GoP with user’s demand
• Rate control for sending video data– TFRC (TCP Friendly Rate Control)
• Available Bandwidth Video quality
• Quality adjustment is performed by re-quantization filter
0
5
10
15
20
0 10 20 30 40 50 60
Ave
rage
rat
e [M
bps]
Quantizer scale
Reciprocal of a quantizer scale
TFRC rate
Data retrieval
• Cache hit– The proxy adjusts cached GoP to the request
and transmits it to the client
• Cache miss– Server-Proxy bandwidth is sufficient
• Data retrieval with consideration on current and future demands
– Server-Proxy bandwidth is insufficient• Data retrieval with consideration on trade-off betwe
en quality and delay
Data retrieval- Sufficient bandwidth -
• Considering clients that are going to require the GoP in the future, the proxy retrieves the GoP of maximum quality among their demands
GoP
Beginning End
Video
Client 4
2
Client 3
5
Client 2
3
Client 1
4
User’s request on video quality
5
Cache miss
Data retrieval- Insufficient bandwidth -
• Trade-off between quality and delay
Which does the user give priority to, quality or timeliness?
Introducing a parameter is defined as the ratio of the acceptable quality to the demand
10
timeliness qualityuser’s preference
Data retrieval- Insufficient bandwidth -
• Control based on
Quality of the client’s requestQuality of cached GoP
Quality of the client’s requestQuality of the GoP which the proxy can retrieve from the server
Quality of the client’s requestQuality of the GoP which the proxy can provide
The proxy sends the cached GoP
The proxy retrieves the GoP from the server
The proxy retrieves the GoP of the acceptable qualityfrom the server
<
Yes
Yes
No
No
Prefetching mechanism
Range of examinationfor prefetching(prefetching window)
Position in the video stream
Candidate GoP for prefetching
Requested GoP
Requested quality
Qu
alit
y o
f th
e ca
ched
dat
a
Replacement Algorithm
Beginning End
PriorityHigh Low
7264
6 4 27 5 38
1. Choose a candidate GoP for replacement2. Try the quality adjustment to decrease
the size of the candidate3. Eject the candidate from the cache
Video
No priority
GoP in the prefetching window
Requested GoP by a client
1
Candidate GoP for replacement
65
Simulation model
• Video stream is two hours long• 10 clients watch the same video stream from the
beginning to the end without interactions• The inter-arrival time between two successive client
participations follows the exponential distribution whose average is 1,800 seconds
Server Router Router Router RouterProxy
Client
150 Mbps 50 Mbps
TCP sessions TCP sessions
UDP sessionsUDP sessions
Evaluation criteria
• Required cache buffer size– Amount of cached data
• Playout delay– Maximum jitter
• Degree of satisfaction on delivered video– The average ratio of the delivered video qualit
y to the demand
Simulation result- Amount of cached data -
Time [sec]
0
20
40
60
80
100
120
140
160
0 5000 10000 15000 20000
Traditional method (infinite cache, no quality adjustment)Proposed mechanism (infinite cache, no prefetching, )Proposed mechanism (20 Gbits cache, prefetching, )
Am
ou
nt o
f cac
hed
dat
a [G
bit]
Simulation result- Playout delay -
Client
10
15
20
25
00 1 2 3 4 5 6 7 8 9
5
Pla
yout
de
lay
[se
c]
Traditional method (infinite cache, no quality adjustment)Proposed mechanism (infinite cache, no prefetching, )Proposed mechanism (20 Gbits cache, prefetching, )
Simulation result- Degree of satisfaction on delivered video -
00 1 2 3 4 5 6 7 8 9
1
0.8
0.6
0.4
0.2
Client
De
gre
e o
f sat
isfa
ctio
n o
n d
eliv
ere
d vi
de
o
Traditional method (infinite cache, no quality adjustment)Proposed mechanism (infinite cache, no prefetching, )Proposed mechanism (20 Gbits cache, prefetching, )
Conclusion
• Conclusions– We proposed proxy caching mechanisms wi
th video quality adjustment– Simulation results show that our system can
accomplish a low-delay video streaming service while meeting user’s demand and available bandwidth
• Future works– Reducing playout delay– Considering interactions such as rewinding,
pausing and fast-forwarding
