Date post: | 21-Dec-2015 |
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Energy-Efficient Design
• Some design issues in each protocol layer
• Design options for each layer in the protocol stack
Energy Consumption Here are some results on energy consumption
of networking hardware:
Device Sleep power (mW) Idle/Wakeup Power (mW) wakeup time (ms)
WaveLAN 143 1148.6 100
Metricom 93.5 346.9/431 5000
IBM IR - 349.6 100
Newton PDA 164.2 1187.8 N/A
Magic link
PDA 312 700 N/A
Laptop 8000
Energy Consumption
• WaveLAN: transmission power : reception power = 1.4 : 1
• Rockwell sensors: transmission power : reception power = 4:3 (full range); 1:1 (min range)
• Berkeley’s motes: operate at around 10mW.
Mechanisms to reduce power at physical layer
• First-order approximation of the power consumption of CMOS circuitry:– P = C * V^2 * f (C: effective switch capacitance; V is the
supply voltage; f is the clock frequency)
• Ways to reduce power:– Reduce the supply voltage V; (cons: may reduce performance
at the same time, need perf. compensation)– Reduce the switching frequency f (sleep/idle mode); (cons:
no energy reduction due to longer runtime)– Reduce the capacitive load C
• Reduce external access (output, memory, etc.)• Reduce logic state transitions, routing capacitance
Reducing power at MA C and link layer
• Principles:– Only power on when transmitting– Increase the ratio of effective transmissions and avoid
retransmissions, collisions, corruptions– regulate traffic
• Mechanisms to achieve it– Add idle/sleep mode in the transmission state diagram– Regularize transmissions and make them predictable, nodes
can sleep when not transmitting– Avoidance contention loss, congestion loss, retransmissions,
error-prone transmissions, as much as possible• how to achieve ??
Reducing Power at Network Layer• Principles:
– Put routers that do not forward packets into sleeping mode• SPAN, GAF, PEAS
– Reduce signaling packets (e.g. LSP)– On-demand routing– Packet size and fragmentation– Energy-related optimality criteria– System decomposition: do not stress energy-critical nodes, load
balancing– Use intermediate nodes as middleman
• Key issues:– How to guarantee connectivity
PEAS: Probing Environment, Adaptive Sleeping
• Enable long-lived system by exploiting the scale
• How?
– Keep a subset working, turn off the others into sleeping mode
– Sleeping ones replace dead ones
• Two issues
– working nodes should be evenly distributed across the field: Probing Environment
– Replace failed or energy-exhausted working nodes quickly: Adaptive Sleeping
Distribute working nodes evenly: Probing Environment
• Nodes are in sleeping initially– An exponential random time
• When waking up, a node probes within a range R_p– If there is no working node
within R_p, it starts working
– Otherwise, the working one should send back a reply and this one sleeps again
Peas model: why working node are evenly distributed
• Any topology of working nodes is equivalent to placing round peas of radius R_p/2 on a plane
– The centers of two peas are at least R_p apart, when they’re tangent
• Assuming infinite deployment density
– The densest case: each pea is tangent to 6 neighboring peas
– the sparsest case: the space among any 3 adjacent peas is slightly smaller to insert another pea
Detect unpredictable failures: randomized sleeping times
• If wakeups are synchronized to some time points– long “gaps” if a working
one fails unexpectedly
• Randomized sleeping times spread wakeups over time– Any unexpected failure is
detected on time
How to adjust the wakeup rates• A working node
measures the aggregate wakeup rate from its probing neighbors and
• includes the information in the replies to probing neighbors
• Each probing neighbor adjusts its rate accordingly
TsTime
K wakeups
…
Measure aggregate rate: _a = K / (t - t0)
t0 t
Each probing one adjusts: _new = (_d / _a )
Reducing power at the transport layer
• Principles:– Avoid packet loss and retransmissions
– Reduced-power operations during backoffs
– Light-weight connection management
– Design balance at the sender and the receiver
• Mechanisms– State estimation: Do not transmit during channel errors
– Congestion avoidance: Do not overload the network
– Buffering
Reducing Power in the OS (including the FS)
• Principles:– Predictable schedules– Caching– Turn off idle devices (e.g. displays)– CPU scheduling via dynamic voltage scaling
• Mechanisms:– CPU scheduler: idle state– Use application hints & semantics