Versatile Low Power Media Access for Wireless Sensor

Networks

Sarat Chandra Subramaniam

Goals Low Power operation Effective collision avoidance Simple and predictable Small code size and RAM usage Tolerable to changing RF/networking

conditions Scalable to large numbers of nodes

In a nutshell (1) Low power operation achieved by:

Clear Channel Assessment (reducing idle listening)

Low Power Listening Adaptive preamble sampling

Effective collision avoidance Factoring of MAC functionalities MAC reconfigurability

In a nutshell (2) Tolerant to changing RF conditions Scalable to large number of nodes

Significant Contributions More flexible and more tunable as small

core and factored functionality RTS/CTS, ACKs, etc are considered higher layer

functionality (services) Has bidirectional (set and get) interfaces to

MAC functionalities Applications can turn them on and off –

therefore adaptable to radio environment Clear channel assessment with outlier

detection

Core MAC functionalitiesFUNCTION METHOD

Channel Arbitration CCA (sense if channel is busy) and packet backoffs (if busy, then backoff – congestion backoff).Start by backing off – initial backoff

Reliability Link Layer Acks

Low power comms Low Power Listening (LPL)

Reconfigurability All core functionalities can be configured

(either modifiable or modifiable and removable)

Use? Adaptability to traffic conditions Scalability to include larger/smaller number of

nodes Adaptability to radio environment

CCA (1) BMAC solution: ‘software automatic gain

control’ Signal strength samples taken when channel is

assumed to be free Samples go in a FIFO queue (sliding window) Median added to an EWMA filter Once noise floor is established, a TX requests

starts monitoring RSSI from the radio

CCA (2) Comparing signal strength with noise floor

causes false negatives (noise amplitude fluctuates).

Detect outliers: Samples whose energy is significantly below

noise floor. This can’t happen if packet is being sent.

CCA Results 0=busy, 1=clear Packet arrives between 22 and 54 ms

LPL (1) Familiar Wake-up – Active –Sleep

Mechanism Has CCA – potentially reducing idle

listening Preamble length matches channel

checking period No explicit synchronization required (unlike S-MAC)

Packet checking period and Preamble length - configurable

LPL (2) Single-hop application

doing periodic data sampling

Sampling rate (traffic pattern) defines optimal check interval

Check interval Too small: energy wasted

on idle listening Too large: energy wasted

on transmissions (long preambles)

In general, it’s better to have larger preambles than to check more often!

Lifetime Modeling (1) Lifetime of node determined by energy

consumption Various components are:

Energy for receiving Energy for transmitting Energy for listening Energy for sensing Sleep energy

Key: Energy depends on time taken to achieve all of the above

Lifetime Modeling (2) All the times are known – eg for listening,

time depends on preamble length and channel check interval

Lifetime estimated at compile-time or run-time

Provides feedback to network services to configure MAC

Beauty of reconfigurability Example of achieving RTC-CTS channel

acquisition (all this is implemented by services above the MAC): Send RTS using LPL cycle Listen for CTS using LPL cycle Once CTS is heard, disable LPL, CCA at both ends Send data as burst Send link layer ACK Re-enable LPL, CCA

RTS – CTS/ ACK etc used depending on the situation.

Adaptive Preamble Sampling Mentioned, but not explained. WiseMAC implements adaptive preamble

sampling. Preamble sampling = process of listening

for activity on the radio. It is done during LPL. Adaptive preamble sampling indicates the

adaptability of LPL?

Experimental results: throughput

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Throughput of a congested channel

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B-MACB-MAC w/ ACKB-MAC w/ RTS-CTSS-MAC unicastS-MAC broadcastChannel Capacity

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Throughput vs power consumption

Energy vs Latency

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B-MACS-MACAlways On

S-MAC Default Configuration

B-MAC Default Configuration

11 10 9 3 2 111 10 9 3 2 1

Summary B-MAC is small, extensible and flexible. CCA increases channel utilization. LPL results in decreased power listening. B-MAC may be better or equal S-MAC

performance in almost all scenarios.