qos - kiv.zcu.czledvina/vyuka/pds/pds-tut/cs757-cn/757-13-qosdet.pdf · multimedia net w o rks...
TRANSCRIPT
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Deterministic QOS
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Quality-of-Service
� Video and audio need Quality-of-Service (QoS)
guarantees:
{ delay
{ jitter
{ throughput
{ loss rate
� A deterministic service gives worst-case guarantees.
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Quality-of-Service
� A deterministic service gives worst-case delay guarantees:
Delay � X
.� A statistical service makes probabilistic service
guarantees:
Pr[Delay > X] < " or Pr[Loss] < " :
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Multimedia Networks
Sender
TrafficPolicer
Receiver
AdmissionControl
� Multimedia connections have QoS and tra�c
parameters.
� Multimedia networks need resource reservation.
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Why is Resource Reservation Di�cult?
� Compressed digital video has a variable bit rate.
0 100 200 300 400 5000
50
100
150
200
Num
ber
of C
ells
Peak Rate
Average Rate
Frame Number
� Problem: How do we provide deterministic QoS without
peak-rate reservation?
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Design Space for QoS
On a fundamental level, the design space for QoS is
3-dimensional.
TrafficCharacterization
Scheduling
Admission Control
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Peak Rate Allocation
TrafficCharacterization
Scheduling
Admission Control
PeakPeak
Peak
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Deterministic QoS
TrafficCharacterization
Scheduling
Admission Control
EmpiricalEnvelope
EDF
Deterministic
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Assumptions
� There are P ow classes, Cq is the set of ows in class q.
� We �rst consider a single node.
� Flow j 2 Cq has a delay bound dq.
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Design Space for Deterministic QoS
TrafficCharacterization
Scheduling
Admission Control
Empirical E
nvelope
FIFO
Static Priority
WFQ
EDF
Leaky Bucke
t
Peak Rate
Deterministic(sufficient)
Deterministic(necessary+sufficient)
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TraÆc Characterization
� Aj(t1; t2) are arrivals from ow j in [t1; t2).
� Denote: AC(:) =P
j2C Aj(:).
� Arrivals from ow j are characterized by a subadditive
deterministic envelope A�j as follows
Aj(t; t+ �) � A�j(�) 8t;8� :
� TraÆc is conditioned (shaped, policed) to comply to A�j .
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Practical Deterministic Envelopes
Time
Tra
ffic
σ
ρ
Time
Tra
ffic
σ
ρ
P
Most deterministic envelopes used in practice are Leaky
Buckets (or Token Buckets):
� Simple Leaky Bucket: A�(t) = � + � t.
� Peak-Rate Constrained LB: A�(t) = minfP t; � + � tg.
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Best Possible Deterministic Envelopes
8000
6000
4000
2000
004080
120160200
0 50 100 150 200 0 50 100 150 200
A
E*
Cu
mu
lati
veTimeTime
Tra
ffic
� Best deterministic envelope for a given trace is the
empirical envelope E�j :
E�j (�) := supt�0Aj(t; t+ �); for all � � 0 :
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Packet Scheduling
Input Links Output Links
Fabric
Scheduler
� A connection j has a delay bound dj .
� Packet scheduling discipline determines delay .
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Admission Control
Schedulability Condition:
Given a packet scheduler and a set of connections. The
connections are said to be schedulable if a violation of the
delay bounds will never occur.
Schedulability Condition
=
Delay Bound Test for Admission
Control
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Scheduling and Network Utilization
� First-Come-First-Served (FCFS)
{ Simplest, o�ers only one delay bound.
� Earliest-Deadline-First (EDF)
{ Sophisticated, optimal in terms of schedulability.
� Static Priority (SP)
{ Compromise, o�ers �xed number of delay bounds.
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First-Come-First-Served (FCFS)
3 2 1
� Exact Admission Control Test:
d �
Xj2N
A�j(t)� t t � 0
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Earliest-Deadline-First (EDF)
32d = 20
d = 30
1d = 10
t=302
23
3 1
45 29
1
� Exact Admission Control Test (Liebeherr/Wrege/Ferrari):
t �
Xj2N
A�j(t� dj) + max
k;dk>tsk t � 0
where maxk;dk>t sk � 0 for t > maxk2N dk
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Static Priority (SP)
2
1
33
11
3
2 2
� Exact Admission Control Test (Liebeherr/Wrege/Ferrari):
(9� � dp)
t+ � �
Xj2CpA�j(t) +
p�1Xq=1X
j2CqA�j(t+ �) + maxr>p
sr
for all p; t � 0
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Experimental Setup
� Single 155 Mbps switch.
� Three connection groups Low, Medium, High Delay .
Delay Burst
Index Bound Size Rate
j dj Bj rj
Low 1 12 ms 4,000 cells 10-155 Mbps
Medium 2 24 ms 2,000 cells 10-155 Mbps
High 3 36 ms 4,000 cells 10-155 Mbps
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Evaluation20
1010
2020
40
4080
80
155
155
10
20
40
80
155
EDF
2010
10
2020
40
4080
80
155
155
10
20
40
80
155
SP
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VBR Video Networks with
Deterministic Quality-of-Service
Constraints
J�org Liebeherr
Department of Computer Science
University of Virginia
1
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MPEG Video Compression
MPEG compression uses three variable-size frame types:
� Intra-coded (I) frames
� Predictive-coded (P) frames
� Bidirectionally predictive-coded (B) frames
PP I
1 2 3 4 5 6 7 8 9 10
I B B B B B B
=) MPEG encoders generate variable-bit rate (VBR) video.
4
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Tra�c Characterization
For QoS networks with deterministic service we need a
worst-case tra�c characterization.
� A[t; t+ � ] is actual tra�c in interval [t; t+ � ].
� Worst-case characterization of tra�c is a function A� with:
A[t; t+ � ] � A�(�) 8t;8�
A� is a time-invariant bound of the actual tra�c.
� A� must be sub-additive,
A�(t1+ t2) � A�(t1) +A�(t2):
10
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What is the best A� ?
� De�ne the \Empirical Envelope" E:
E(�) := maxt
A[t; t+ � ]
� E is a time-invariant bound.
� E is best possible time-invariant bound.
E(�) � A�(�) 8A�
11
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Constructing the Empirical Envelope
Trace of VBR Video:
1501005000
20
40
200
200
180
160
140
120
100
80
60
Frame Number
Nu
mb
er
of
Cell
s
`Integral' of the Trace:
200150100500
1000
0
2000
3000
4000
5000
6000
7000
8000
9000
Frame NumberC
um
ula
tive N
um
ber
of
Cell
s
Empirical Envelope E
12
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Searching for a "more practical" A� ?
1. Use few parameters.
2. Accurately describe actual tra�c.
3. Be sub-additive.
4. Enforceable by simple policing mechanisms.
Here:
`Leaky bucket' and `Multi-level leaky bucket' traf-
�c models.
14
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Approximations of the Empirical
Envelope
� Construct an upper bound for the Empirical Envelope with
leaky buckets.
� Result is an approximation of the empirical envelope.
17
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Input: E and a time � .
Output: A set of parameter (�i; �i).
Procedure Find Parameters (E, � )
n = 0
While � > 0 Do
n = n+ 1
�n = max
0�t<��1
� � t(�E(t)� tE(� ))
�
�n =E(� )� �n
�
Output (�n; �n)
� = minf t j �n + �n t = E(t)g
End While
End Procedure
Example
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Empirical Evaluation
Determine the maximum utilization on a link : : :
� : : : with empirical envelope.
� : : : with leaky bucket tra�c policing.
� Single link with 45 Mbps
� Workload on link is obtained from MPEG traces.
18
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Workload for Empirical Evaluations
� Lecture:
{ 10-minute MPEG-1 trace showing a videotaped lecture.
{ 160 � 120 pixels per frame; 30 frames per second.
{ Frame pattern is IBBPBB.
� Movie
{ 30-minute MPEG-1 trace of \Jurassic Park".
{ 384 � 288 pixels per frame; 24 frames per second.
{ Frame pattern is IBBPBBPBBPBB.
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Maximum Achievable Utilization
1
0.75
0.5
0.2520
40
60
80
100
120
Average Rate
00 100 200 300 400 500
Peak Rate
Envelope
Delay Bound (ms)
Average U
tilizatio
n# o
f C
on
necti
on
s
Lecture
140
120
100
80
60
40
20
0
1
0.75
0.5
0.25
0 100 200 300 400 500
Average Rate
Envelope
Peak Rate
Av
erag
e Utiliza
tion#
of
Co
nn
ecti
on
s
Delay Bound (ms)
Movie
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Max. Utilization with Multi-Level Leaky Buckets
4 LBs
5 LBs
2 LBs
1 LB
0 100 200 300 400 500Delay Bound (ms)
# o
f C
on
nec
tio
ns
100
80
60
40
20
0.4
0.3
0.2
0.1
Av
erag
e Utiliza
tion
3 LBs
6 LBs/Envelope
0
Lecture
6 LBs
1 LB
2 LBs 3-5 LBs
0.1
0.2
Av
era
ge
Uti
liza
tio
n
0.4
0.3
60
50
40
30
20
10
00 100 200 300 400 500
Delay Bound (ms)
# o
f C
on
nec
tio
ns 8 LBs/Envelope
Movie
(LB = Leaky Bucket)
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Achievable Utilization Using Di�erent Schedulers
0 10 20 30 40 50 60 70 80 90
80ms30ms10 ms
20 ms200ms
1000ms80 ms
Movie
Peak Rate
200 msLecture
Deadline
# M
ovie
Con
nec
tion
s
0
10
20
# Lecture Connections
30
50
60
40
0
EDF
Peak Rate
80ms30ms10 ms
20 ms200ms1000ms200 ms
80 ms
MovieLectureDeadline
9080706050403020100
60
50
40
30
0
10
20
# M
ovie
Con
necti
on
s# Lecture Connections
FCFS
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Achievable Utilization Using Di�erent Schedulers
0 10 20 30 40 50 60 70 80 90
80ms30ms10 ms
20 ms200ms
1000ms80 ms
Movie
Peak Rate
200 msLecture
Deadline
# M
ovie
Con
nec
tion
s
0
10
20
# Lecture Connections
30
50
60
40
0
EDF
9080706050403020100
Peak Rate
80ms30ms10 ms
20 ms200ms
1000ms200 ms80 ms
MovieLectureDeadline
# M
ovie
Con
nec
tion
s# Lecture Connections
60
50
40
30
0
10
20
Static Priorities