Power Saving and Clock Sync Ten H. Lai

Problem, Problem, Problem! ???

Energy Efficiency Done at every level from physical to application. Energy-efficient routing. Energy-efficient MAC. Energy-efficient everything.

Power Saving at MAC Layer Beacon interval awake sleep Beacon window ATIM window

Time Sync Is Necessary/Important Really ? What if it is difficult or impossible to synchronize clocks?

To sync or not to sync? Yes global synchronization No no synchronization Partially local synchronization

No Synchronization (0) “Power-Saving Protocols for IEEE 802.11-Based Multi-Hop Ad Hoc Networks” INFOCOM 2002 Y.C.Tseng, C.S. Hsu, T.Y. Hsieh NCTU

No Synchronization (1) Basic idea: nodes be awake more frequently. Extreme case: awake all the time. Beacon interval awake sleep

No Synchronization (1) Dominating-Awake-Interval Awake ≥ BI/2 + BW Beacon interval awake sleep

No Synchronization (1) Dominating-Awake-Interval Awake > BI/2 + BW Beacon interval awake sleep

No Synchronization (2) Periodical-Fully-Awake-Interval

No Synchronization (2) Quorum-based 1 2 3 4 2 3 4 6 7 8 5 6 7 8 1 4 16 1 4 16 1 2 3 4 2 3 4 6 7 8 10 11 12 13 14 15 16 5 6 7 8 10 11 12 13 14 15 16

Local Synchronization (0) “An Energy-Efficient MAC Protocol for Wireless Sensor Networks” INFOCOM 2002 W. Ye, J. Heidemann, D. Estrin UCLA

Local Synchronization (1) Offset 10:10 10:04 -0:01 0:05 0:01 10:09 - 0:05

Local Synchronization (2) Nodes of same color -- synchronize with each other. Nodes of different colors – know each other’s timing

Local Synchronization (3) B C

Problem, Problem, Problem! Power saving ??? Physical MAC Routing Awake-sleep Global no partial sync Analysis & Comparison Clock Sync

Which one? To sync or not to sync? Yes (global sync) No (no sync) Partially (local sync) Which one?

Analysis of energy saving (1) No data traffic Parameters Parameter Value Beacon Interval length 100ms Beacon window length 3ms ATIM window length 7ms PFAI T value 4 Quorum-based n value 6 Avg. num. of sch. in border nodes for Local Synch. 2.7 Total nodes in Local Synch. 100

Analysis of energy saving (2) Clock synchronization method Awake time ratio No Synchronization DAI 53% PFAI 32.5% Quorum-based 35.4% Global Synchronization 10% Local Synchronization 2 schedules 19% 3 schedules 28.4% 4 schedules 37.87%

Global Synchronization: pros and cons Best performance in energy saving Needs a good synchronization algorithm

No Synchronization – pros and cons Simple -- no need for clock sync Less efficient in power saving 1 4 16 1 4 16

No Synchronization: Analysis A has a packet for B in interval 4. Q: When should A send it? In every yellow interval Or when yellow meets red. Q: When will yellow meet red  ? 1 4 16 1 4 16

The Wisdom of Diamond Sutra No synchronization, is not really no synchronization, it is just called no synchronization.

No Synchronization – pros and cons Less efficient in power saving Simple -- no need for clock sync Simpler – clock sync is simpler and more scalable ? 1 4 16 1 4 16

Time Sync in the “No Sync” Scheme Why is it simpler, more scalable? Beacon window ATIM window

A major drawback with no sync Broadcast/multicast is inefficient

Local Synchronization: pros and cons More scalable Inefficient with multiple schedules Protocols incomplete Broadcast/multicast is inefficient?

All of them To sync or not to sync? Which one? Yes (global sync) No (no sync) Partially (local sync) Which one? Normal situation Neighbor discovery Transient situation All of them

Possible Protocol ? Normally, use the global sync scheme. Switch to the no sync scheme when necessary (for neighbor discovery). Use the partial sync scheme while merging. ?

Problem, Problem, Problem! Power saving ??? Physical MAC Routing Awake-sleep Global no partial sync Analysis & Comparison Clock sync

Follow-ups on no-sync “Asynchronous Wakeup for Ad Hoc Networks,” Mobihoc’03 “Quorum-Based Asynchronous Power-Saving Protocols for IEEE 802.11Ad Hoc Networks,” ICPP’03 (Best paper award)

No Synchronization Quorum-based 1 2 3 4 2 3 4 6 7 8 5 6 7 8 10 11 12 1 4 16 1 4 16 1 2 3 4 2 3 4 6 7 8 10 11 12 13 14 15 16 5 6 7 8 10 11 12 13 14 15 16

View T as a matrix and pick a row and a column as the quorum T = {0, 1, …, n-1} Quorum: a subset of T View T as a matrix and pick a row and a column as the quorum Property A: No matter how asynchronous, every two nodes have at least one overlap in every T intervals. 0 3 15 0 3 15

Desired Property for the Power Saving Problem Property PSP: No matter how asynchronous, every node’s beacon window is covered by every other node’s active period at least once per T intervals. 0 3 15 0 3 15

Questions Feasible quorum systems: quorum systems with Property PSP. How to characterize all feasible quorum systems? Any optimal feasible quorum system? What if we want to have m overlaps?

Feasible Quorum System A sufficient condition (rotation closure property): For any two quorums A, B in the system, A ∩ rotate (B, i) ≠ Φ 0 3 15 0 3 15 0 3 15

Quorum Size T = {0, 1, …, n-1} Quorum: the smaller, the better (energy efficient) Closure property |quorum| ≥ √n

Specific Feasible Quorum Systems Grid Quorum System (≈2√n) Torus Quorum System (≈√2n ) Cyclic Quorum System (≈√n) Finite Projective Plane Quorum System (≈√n)

Quorum Systems with a Single Quorum T = {0, 1, …, n-1}. H is a subset of T. {H} is a quorum system iff … H is a difference set of T. H is a difference set of T iff for every i in T, i = x-y mod n for some x, y in H. {0, 1, 2, 4} is a difference set of {0,1, …, 7}.

Quorum Systems with multiple overlaps E-Torus Quorum System e-torus(k1) and e-torus(k2) have (k1+k2)/2 overlaps. Can be used to dynamically adjust the number of overlaps.

K=4

Problem, Problem, Problem! Power saving ??? Physical MAC Routing Awake-sleep S-MAC Global no partial sync Analysis & Comparison Clock sync

S-MAC: an energy-efficient MAC In IEEE INFOCOM 2002, By Ye, Heidemann, Estrin IEEE 802.11-like CSMA/CA

802.11 MAC S-MAC RTS(t1) DATA(t3) A B C CTS(t2) ACK Back off Turn

??? What’s next? Power saving Awake-sleep(802.11) S-MAC Physical MAC Routing Awake-sleep(802.11) S-MAC Global no partial sync Analysis & Comparison Clock sync