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Sense-and-Respond Systems and Play-Back Buffers
Vincenzo LiberatoreDivision of Computer Science
Research supported in part by NSF CCR-0329910, Department of CommerceTOP 39-60-04003, NASA NNC04AA12A, and an OhioICE training grant.
V. Liberatore Control Playback 2
Sense-and-Respond
• Computing in the physical world
• Components– Sensors, actuators– Controllers– Networks
V. Liberatore Control Playback 3
Sense-and-Respond
• Enables– Industrial automation [BL04]– Distributed instrumentation [ACRKNL03]– Unmanned vehicles [LNB03]– Home robotics [NNL02]– Distributed virtual environments [LCCK05]– Power distribution [P05]– Building structure control [SLT05]
• Merge cyber- and physical- worlds– Networked control and tele-epistemology [G01]
• Sensor networks– Not necessarily wireless or energy constrained– One component of sense-actuator networks
V. Liberatore Control Playback 4
Information Flow
• Flow– Sensor data– Remote controller– Control packets
• Timely delivery– Stability– Safety– Performance
V. Liberatore Control Playback 6
Playback Buffers [Infocom 2006]
• Main objective– Smooth out network non-determinism
• Related to– Multimedia buffers– TCP RTO
V. Liberatore Control Playback 7
Multimedia Play-BackSequence number
time
Packet generation
Play-back
Packet arrival
V. Liberatore Control Playback 8
Related Work
• Multimedia buffers– Important source of inspiration– Physics versus multimedia quality– Playback delay computed in advance
• Affects control signal computation
– Round-Trip Times
• TCP RTO– Another source of inspiration
• Upper bound on RTT
– Large time-out cost• Conservative estimate
V. Liberatore Control Playback 11
Main Ideas
• Predictable application time– If control applied early, plant is not in the state
for which the control was meant – If control applied for too long, plant no longer
in desired state
• Keep plant simple– Low space requirements
• Integrate Playback, Sampling, and Control
V. Liberatore Control Playback 12
Algorithm
• Send regular control– Playback time
• Late playback okay
– Expiration
• Piggyback contingency control
V. Liberatore Control Playback 13
Deadwood packets• Old
– Received after the expiration time• Out-of-order
– Later control more appropriate for current plant state• Would get us into a deadlock
– New packet resets the playback timer– Keep resetting until no signal applied– “Quashed” packet
• Discard!
plant
controller
Playback delay XX
V. Liberatore Control Playback 14
Countermand control
• Scenario– Packet i+1 overtakes packet I – i+1 << i
– Likely caused by delay spike
• New signal countermands previous one
plant
controller
Playback delay ii+1
V. Liberatore Control Playback 15
Playback Delays (I)
• Modular component• Compute playback delay and sampling period T• Use short term peak-hopper [EL04]
– Original peak-hopper for TCP RTO• Too conservative for networked control
– Aggressively attempt to decrease
time
V. Liberatore Control Playback 16
Playback Delays (II)
• Aggressively attempt to decrease T• Add upper bound on playback delay
– Avoid dropping deadlock packets– Bound ≤ T+RTT
• Caps and T
• Must estimate lower-bound on RTT– Use symmetric of peak-hopper– Add negative variability estimate to
compensate for short-term memory
17Control PlaybackV. Liberatore
Playback Delays (III)
0
01
r
rr
}1},9375.0,2min{max{ BB
},min{)1(' 01 rrCr
Calculate current RTT variability
':16
'?' min
minminmin rrr
rrrr
},max{)1( 01 rrB
0if then
Positive variability coefficient
Negative variability coefficient
Update min RTT estimate
Age min RTT estimate
Calculate
}2/1,4/4/3max{ CC
18Control PlaybackV. Liberatore
Playback Delays (IV)
min' rT
minrT
16
' minrTTT
if
min' rT
then
else
Attempt to avoid quashed packets
Decrease sampling period
V. Liberatore Control Playback 19
Control Pipes
• Bandwidth and delays– is playback delay– T is sampling period
• 1/T proportional to bandwidth
• Control pipe– T«– Multiple in-flight packets
• Pipe depth– Bound by constraint ≤ T+RTT– Keep pipe predictable
V. Liberatore Control Playback 20
Observer
• Estimate future plant state– Plant sample current state, including local variables– Keep log of outstanding control packets
• Assumption on packet delivery– Future packet delivery is uncertain
• Purge from log– Old packets– Packet that should be overtaken by new control
• Countermands signals generated when delay spike is transient
– Out-of-order packets
V. Liberatore Control Playback 22
Network Model
• Simulated network• Losses: Gilbert model• Delays
– Shifted Gamma distribution
– Heavy tail
– Low probability of out-of-order delivery
– Correlate delays to introduce delay spikes
• Wide-area implementation• Use RT scheduling whenever possible• Use otherwise unloaded machines
– RT made little difference
• Host worldwide, heterogeneous conditions
23Control PlaybackV. Liberatore
Plant
• Scalar linear plant– Plant state x(t)– Input u(t) (control)– Output y(t)– Disturbances v(t), w(t)
• Akin to white noise
• Deadbeat controller– Aggressive
)()()(
)()()()(
twtxty
tvtbutaxtx
1;
aT
aT
e
e
b
akkyu
24Control PlaybackV. Liberatore
Metrics
• Metrics– Root-mean square output– Output: 99-percentile
• Comparison– Open-loop plant u(t)=0– Proportional controller (no buffer)– Proportional controller with constant delays
22 ym
V. Liberatore Control Playback 27
Sampling period
Imperfection of thecontrol pipe
Root-mean-square error
≤T+RTT
V. Liberatore Control Playback 29
Bandwidth Allocation
• Definition– Multiple sense-and-respond
flows– Contention for network
bandwidth
• Desiderata– Stability and performance of
control systems• Must account for physics
– Efficiency and fairness– Fully distributed,
asynchronous, and scalable– Dynamic and self-
reconfigurable
V. Liberatore Control Playback 30
Problem Formulation
• Define a utility fn U(r) that is– Monotonically increasing
– Strictly concave
– Defined for r ≥ rmin
• Optimization formulation
( )
min,
max ( )
s.t. , 1,...,
and
i ii
i li l
i i
U r
r C l L
r r
S
V. Liberatore Control Playback 31
Conclusions (I)• Sense-and-Respond
– Merge cyber-world and physical world– Critically depends on physical time
• Playback buffers integrated with – Sampling (adaptive T)– Control (expiration times, performance
metrics)
• Packet losses– Reverts to open loop plant (contingency
control)