a 2 -mac: an adaptive, anycast mac protocol for wireless sensor networks hwee-xian tan and mun choon...

A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks

Hwee-Xian TAN and Mun Choon CHAN

Department of Computer Science, School of ComputingNational University of Singapore

Overview

• Introduction• Related Work and Motivation• Protocol Details of A2-MAC• Adaptation in A2-MAC• Performance Evaluation• Conclusion

A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 2

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.


DD

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.


00 11 22 33 44 55 66 77 88 99

active (listening) slots

1 cycle

without duty-cyclingwithout duty-cycling

00 11 22 33 44 55 66 77 88 99

active (listening) slot

inactive (sleep) slotsinactive (sleep) slots

1 cycle

with duty-cyclingwith duty-cycling

Related Work


On-DemandOn-Demand

Requires out-of-band signaling (using low power radio) to wake up nodes for data reception.

E.g. Wake on wireless.

SynchronousSynchronous

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.

AsynchronousAsynchronous

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 A2-MAC

• Based on asynchronous slot model…

• Slots of a node may be unsynchronized with other nodes…


00 11 22 33 44 55 66 77 88 99

active (listening) slot

inactive (sleep) slotsinactive (sleep) slots

2 ms

(10, 1, 2ms)(10, 1, 2ms)

duration of each slot# slots per cycle

# active slots per cycle

Ensuring Communication in A2-MAC

• Using a probing mechanism…


t0 t1 t2 t3 t4 t5 t6 t7 t8

PP PP PP PP

APAP

DATADATA

ADAD

t0 t1 t2 t3 t4t7 t8 t9 t5

S

f1

Packet arrival @ slot t3 of S

f1 wakes up @ slot t2 of f1

Ensuring Communication in A2-MAC

• Probing for active neighbors does not incur additional delays or overheads as compared to existing asynchronous MACs…


PP PP PP DATADATA

A’A’ AA

PP PP DATADATA

APAP AD

AD

tA

tA

strobed preambles

early ACK

preambles as probes

X-MAC

X-MAC

X-MAC

S

f1

f2

A2-MAC

A2-MAC

A2-MAC

S

f1

f2

Anycast + Random Scheduling


Robustness to intermittent link connectivity

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 latencyReduction 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

• A2-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…


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 A2-MAC


Primary Objective(s)Primary Objective(s)

Reduce duty-cycles (and energy consumption) of nodes.

Extend network connectivity and coverage. subject to delay constraint

Key ComponentsKey Components

Forwarder selection.

Duty-cycle selection. SS

v3v3

v4v4

v6v6

DD

v1v1

v2v2 v5

v5

transmission range of S

one-hop neighbors of Scandidate setforwarding set



Forwarding Set and Duty-Cycle SelectionForwarding Set and Duty-Cycle Selection

Lemma 1

Let set of candidate nodes Ni of node i be sorted in descending order of progress, from 1 to |Ni|.

Optimal set of forwarders that minimizes the maximum duty-cycle of the neighbors of i, is the first ni 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.

latency sleepprogress hop per

progressof rate



11

33

44

22 55

66

α14

α15

α24

α13

α25

α26

α3 = α13

α4 = max(α14, α24)

α5 = max(α15, α25)

α6 = α26

Rate of progress constraint Vmin = 2

# slots per cycle v = 1

candidate set N1 = {3, 4, 5}

progresses: d13=1, d14=0.9, d15=0.2

Forwarding set Duty-cycle

{N3} α13=1

{N3, N4} α13=α14=0.5526

{N3, N4, N5} α13=α14=α15=0.619

candidate set N2 = {4, 5, 6}

progresses: d24=1, d25=0.75, d26=0.5

Forwarding set Duty-cycle

{N4} α24=1

{N4, N5} α24=α25=0.6429

{N4, N5, N6} α24=α25=α26=0.5556



The Adaptation AlgorithmThe 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.

SS

v3v3

v4v4

v6v6

DD

v1v1

v2v2 v5

v5




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.

SS

v3v3

v4v4

v6v6

DD

v1v1

v2v2 v5

v5

v7v7




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. 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)


Parameter Value

Transmitting 11.0 mA

Receiving 19.7 mA

Idle 0.426 mA

Sleep 0.001 mA

A2-MAC time slot length 20 ms

A2-MAC cycle length 2 s

Packet size 60 Bytes

Terrain size 250 m × 250 m

Delay tradeoff under varying delay constraints


Percentage connectivity and coverage


Performance with varying network densities


Performance with intermittent link connectivity


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.


based on local topology and given delay constraint

A2-MACA2-MAC

a 2 -mac: an adaptive, anycast mac protocol for wireless sensor networks hwee-xian tan and mun choon...

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