A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks Hwee-Xian TAN and Mun Choon CHAN Department of Computer Science, School of Computing National University of Singapore

Overview Introduction Related Work and Motivation Protocol Details of A 2 -MAC Adaptation in A 2 -MAC Performance Evaluation Conclusion A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks2

Wireless Sensor Networks Perform collaborative tasks such as tactical surveillance and environmental monitoring. Face challenges in deployment, such as intermittent connectivity and energy constraints. Usually duty-cycled to reduce energy consumption and prolong network lifetime. A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks3 D D Intermittent connectivity Node failure resulting from energy constraints

Wakeup Scheduling Key component in design of duty-cycled MAC to reduce energy consumption. – Each node remains in low-power sleep mode most of the time. – Wakes up periodically to sense for channel activities. Effective in reducing energy consumption due to sporadic characteristics of sensor traffic. Incurs high sleep latency. A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks active (listening) slots 1 cycle without duty-cycling active (listening) slot inactive (sleep) slots 1 cycle with duty-cycling

Related Work A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks5 On-Demand  Requires out-of-band signaling (using low power radio) to wake up nodes for data reception.  E.g. Wake on wireless. Synchronous  Nodes wake up during same designated time slots to communicate.  Reduces idle listening.  Requires tight time synchronization and pre- negotiation of schedules.  E.g. S-MAC, T-MAC, D-MAC, R-MAC. Asynchronous  Schedules of senders and receivers are decoupled.  Does not require synchronization.  Nodes wake up periodically to check for channel activity.  Low Power Listening (LPL)  Node remains awake if channel activity is detected; resumes sleeping otherwise.  E.g. B-MAC, X-MAC, C-MAC.

Wakeup Schedule of A 2 -MAC Based on asynchronous slot model… Slots of a node may be unsynchronized with other nodes… A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks active (listening) slot inactive (sleep) slots 2 ms (10, 1, 2ms) duration of each slot# slots per cycle # active slots per cycle

Ensuring Communication in A 2 -MAC Using a probing mechanism… A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks7 t0t0 t1t1 t2t2 t3t3 t4t4 t5t5 t6t6 t7t7 t8t8 P P P P P P P P APAP APAP DATA ADAD ADAD t0t0 t1t1 t2t2 t3t3 t4t4 t7t7 t8t8 t9t9 t5t5 S f1f1 Packet slot t 3 of S f 1 wakes slot t 2 of f 1

Ensuring Communication in A 2 -MAC Probing for active neighbors does not incur additional delays or overheads as compared to existing asynchronous MACs… A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks8 P P P P P P DATA A’ A A P P P P DATA APAP APAP ADAD ADAD tAtA tAtA strobed preambles early ACK preambles as probes X-MAC S f1f1 f2f2 A 2 -MAC S f1f1 f2f2

Anycast + Random Scheduling A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks9 Robustness to intermittent link connectivity  Transient characteristics of PHY leads to intermittent link connectivity.  Typical MAC protocols attempt multiple retransmissions across same link  inefficient.  Using anycast, node can dynamically select forwarder based on prevailing link conditions. Reduction in latency  Transmitter can send packets to any node in its forwarding set as soon as one of them is awake.

Interoperability with Routing Protocol A 2 -MAC is interoperable with any routing protocol that provides: – Set of candidate forwarding nodes; and – Metric that indicates progress made by each forwarder. E.g. hopcount to destination, geographical distance, ETX… A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks10 In this paper, we use Maximum Forward Progress (MFP), which forwards packets based on geographical locations. In this paper, we use Maximum Forward Progress (MFP), which forwards packets based on geographical locations.

Adaptation in A 2 -MAC A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks11 Primary Objective(s)  Reduce duty-cycles (and energy consumption) of nodes.  Extend network connectivity and coverage. subject to delay constraint Key Components  Forwarder selection.  Duty-cycle selection. S S v3v3 v3v3 v4v4 v4v4 v6v6 v6v6 D D v1v1 v1v1 v2v2 v2v2 v5v5 v5v5 transmission range of S one-hop neighbors of S candidate set forwarding set

Adaptation in A 2 -MAC A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks12 Forwarding Set and Duty-Cycle Selection Lemma 1  Let set of candidate nodes N i of node i be sorted in descending order of progress, from 1 to |N i |.  Optimal set of forwarders that minimizes the maximum duty-cycle of the neighbors of i, is the first n i forwarders with largest progress. Lemma 2  To meet the rate of progress constraint, the maximum duty-cycles of the (selected) forwarders of i is minimized iff their associated duty-cycles are the same.

Adaptation in A 2 -MAC A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks α 14 α 15 α 24 α 13 α 25 α 26 α 3 = α 13 α 4 = max(α 14, α 24 ) α 5 = max(α 15, α 25 ) α 6 = α 26 Rate of progress constraint V min = 2 # slots per cycle v = 1 candidate set N 1 = {3, 4, 5} progresses: d 13 =1, d 14 =0.9, d 15 =0.2 Forwarding setDuty-cycle {N 3 }α 13 =1 {N 3, N 4 }α 13 =α 14 = {N 3, N 4, N 5 }α 13 =α 14 =α 15 =0.619 candidate set N 2 = {4, 5, 6} progresses: d 24 =1, d 25 =0.75, d 26 =0.5 Forwarding setDuty-cycle {N 4 }α 24 =1 {N 4, N 5 }α 24 =α 25 = {N 4, N 5, N 6 }α 24 =α 25 =α 26 =0.5556

Adaptation in A 2 -MAC A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks14 The Adaptation Algorithm Phase I  Node with undetermined candidate nodes computes forwarding set; and duty-cycle requirements of each forwarder.  Underdetermined candidate node with largest progress is (iteratively) added to (current) forwarding set, and new duty-cycle is computed. Iteration stops when local constraints are met.  Final forwarding set and duty-cycle required from neighbors in current round is the configuration that provides the minimum duty-cycle requirements. S S v3v3 v3v3 v4v4 v4v4 v6v6 v6v6 D D v1v1 v1v1 v2v2 v2v2 v5v5 v5v5

Adaptation in A 2 -MAC A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks15 The Adaptation Algorithm Phase II  Each undetermined node computes interim duty-cycle based on duty-cycle requirements from neighbors.  Undetermined node with largest interim duty- cycle fixes its duty-cycle, and becomes a determined node. S S v3v3 v3v3 v4v4 v4v4 v6v6 v6v6 D D v1v1 v1v1 v2v2 v2v2 v5v5 v5v5 v7v7 v7v7

Adaptation in A 2 -MAC A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks16 The Adaptation Algorithm Phase I  Node with undetermined candidate nodes computes forwarding set; and duty-cycle requirements of each forwarder.  Underdetermined candidate node with largest progress is (iteratively) added to (current) forwarding set, and new duty-cycle is computed. Iteration stops when local constraints are met.  Final forwarding set and duty-cycle required from neighbors in current round is the configuration that provides the minimum duty-cycle requirements. Phase II  Each undetermined node computes interim duty-cycle based on duty-cycle requirements from neighbors.  Undetermined node with largest interim duty- cycle fixes its duty-cycle, and becomes a determined node. Adaptation algorithm proceeds in bi-phase rounds, and is guaranteed to terminate.

Performance Evaluation Simulator: GloMoSim Benchmarks: – X-MAC – opt-MAC (optimal among approaches using same duty- cycle for all nodes) A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks17 ParameterValue Transmitting11.0 mA Receiving19.7 mA Idle0.426 mA Sleep0.001 mA A 2 -MAC time slot length20 ms A 2 -MAC cycle length2 s Packet size60 Bytes Terrain size250 m × 250 m

Delay tradeoff under varying delay constraints A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks18

Percentage connectivity and coverage A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks19

Performance with varying network densities A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks20

Performance with intermittent link connectivity A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks21

Concluding Remarks An adaptive, anycast based MAC protocol that utilizes: – Asynchronous random wakeup schedule – Anycast mechanism – Adaptive forwarding set selection – Adaptive duty-cycle selection Can achieve better connectivity and coverage, and outperforms existing asynchronous MAC protocols. A 2 -MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks22 based on local topology and given delay constraint A 2 -MAC