1. Energy-Efficient Design Some design issues in each protocol layer Design options for each layer in the protocol stack

2. 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

3. 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.

4. 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

5. 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 ??

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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 Ts Time K wakeups … Measure aggregate rate: _a = K / (t - t0) t0 t Each probing one adjusts: _new = ( _d / _a )

12. 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

13. 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