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.
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 3
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.
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 4
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
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 5
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…
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 6
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…
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 7
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…
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 8
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
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 9
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…
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 10
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
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 11
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
Adaptation in A2-MAC
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 12
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
Adaptation in A2-MAC
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 13
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
Adaptation in A2-MAC
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 14
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
Adaptation in A2-MAC
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 15
The Adaptation AlgorithmThe 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.
SS
v3v3
v4v4
v6v6
DD
v1v1
v2v2 v5
v5
v7v7
Adaptation in A2-MAC
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 16
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.
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)
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 17
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
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 18
Percentage connectivity and coverage
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 19
Performance with varying network densities
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 20
Performance with intermittent link connectivity
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 21
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.
A2-MAC: An Adaptive, Anycast MAC Protocol for Wireless Sensor Networks 22
based on local topology and given delay constraint
A2-MACA2-MAC