Wireless Sensor Networks

X-MAC: A short preamble MAC protocol for duty-cycled wireless sensor networks

Author:Michael Buettner, Gary V. Yee, Eric Anderson, Richard Han 2006.

Presented by Amora Amir1399284

Delft University of Technology

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Outline

MAC for WSN LPL & Limitition X-MAC Experimental Setup Conclusion

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MAC for WSN:

Medium Access Control (MAC): is to ensure that the channel can be accessed by multiple users.

A duty cycle is the % of awake time . Duty cycled MAC :use awake, sleep intervals to sleep/wakeup Categorized into synchronized as S-MAC , T-MAC and

asynchronous B-MAC, and WiseMAC.

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Low Power Listening(LPL) &limitation

Sender uses longer preamble to allow the receiver to awake periodically.

Simple Asynchronous But suffer from....

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LPL Limitation

● The long preamble in LPL is suboptimal in terms of energy consumption at both the sender and receiver,

● The receiver has to wait the full period until the preamble is finished This wastes energy at both the receiver and transmitter.

● LPL suffers from the overhearing problem, where receivers who are not the target of the sender , This wastes energy .

● The target receiver has to wait for the full preamble before receiving the data packet, the per-hop latency is lower bounded by the preamble length. Over a multi-hop path, this latency can accumulate to become quite substantial..

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X-MAC :

Low power MAC protocol for wireless sensor networks

Aim:-• high throughput• low delay• energy efficiency ....for sensor nodes in duty cycles

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X-MAC :cont’d Short & target in preamble

Minimize overhearing problem.

Reduce latency and reduce energy consumption

Strobed preamble Reduces latency for the

case where destination is awake before preamble completes.

Reduces per-hop latency and energy

Dynamic duty-cycle algorithm

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X-MAC :cont’d

Small pauses between preamble packets permit the target receiver to send an early ack .

Short preamble saves energy at transmitter and receiver and allows for lower latency

.Non-target receivers which overhear the strobed preamble can go back to sleep immediately.

Preamble period must be greater than sleep period.

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Experiments for X-Mac Performance

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X-MAC: Benchmarking parameters

Design goals of the X-MAC protocol for duty-cycled WSNs:Energy-efficiencylow loss % of packets low latency for data high throughput for data duty cycles

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Experimental Setup:

● For our experiments, used testbed TelosB platform. The radio used by the TelosB is the Chipcon CC2420, which is an 802.15.4 compliant device, has a data rate of 250kbps, and operates in the 2.4 Ghz. ISM band. The mote uses an 8 MHz TI MSP430 processor and has 1 MB of external flash.

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X-MAC Performance

● Duty Cycle Under No Contention: Star topology consisting of 1 receiver and up to 9senders preamble length of 500 ms. with the transmissions timed so

as to avoid contention.● Duty Cycle Under Contention

vary the number of nodes in the network all nodes are within range of each other. All senders generate packets at the same average rate.

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Duty Cycle under no contention(star topology)

•Senders duty cycle is 7.0% for X-MAC versus 9.3% for LPL, accounting for 32.5% increase in energy lifetime

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Duty Cycle under contention

X-MAC uses less energy for all sleep periods and generation rates, and is less sensitive to network density.X-MAC still performs significantly better than LPL.

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Transmission succes rates

• X-MAC receives approximately 90%

• LPL loses more packets as density increases.

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Energy Models

The expected energy to receive & send a packet is given by:

Er =(listen cycle energy + sleep cycle energy)* (expected iterations for a preamble to arrive)+(energy to send an ACK)+(energy to receive packet)

Es =(preamble energy + energy per ACK listen)*(expected preamble-listen iterations required)+(energy to send packet)

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Energy Usage

Energy consumption of the LPL protocol increases as network density increases. For X-MAC, remains relatively constant.

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Latency (chain topology of 9 nodes)

X-MAC reduces latency by approximately 50%.

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Conclusions X-MAC, a new approach to low power communication

in WSNs. Evaluation shows savings energy over low power

listening. Gains continue as network density increases.

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Thank You...Questions

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Optimality

Based on derivations:

Energy, latency minimized when preamble and ack periods are set to the lowest values for senders.

Optimal receiver's, sleep and listen times are minimum.