BMAC - Versatile Low Power Media Access for Wireless Sensor Networks

Introduction• What is BMAC?

– A configurable MAC protocol for WSNs– Small core

• Factors out higher-level functionality– Energy efficient

• 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

MAC design approaches• Traditional (e.g. SMAC)

– “Black-box” design• Users pre-configure duty cycle• Applications rely on S-MAC to adjust its operation as things change• Designed (and optimized) for a set of workloads that might not always be applicable

• Minimalistic (BMAC)– Small core functionality: media access control– RTS/CTS, ACKs, etc are considered higher layer functionality (services)

• Applications can turn them on and off– More flexible and more tunable

BMAC basics

• Use long preambles (010101010)

• Sleeping nodes will eventually wake up, check the channel and detect that it is active (preamble) and then begin to listen

• It is required that the preamble be long enough so that all nodes will be awake for the data transmission.

BMAC trade-off

• If the time between channel samples is long, then the preamble must be very long.– The preamble must last about as long as the time between checking if the channel is busy

• The amount of energy used transmitting long preambles depends on how often data is sent.

– E.g., if data must be sent only once a day, it might be ok to have a preamble that last an hour, since that would allow a very long time between checking the channel.

– Letting • PT be the power used during transmission of a preamble

• TP be the duration of the preamble

• R the rate of sending data (pkts/sec)

• Echeck the energy required to check the channel

• Then:• Energy for transmitting =

• Energy for receiving =

• Energy for forwarding =

• Optimal value of TP

BMAC trade-off

• If the time between channel samples is long, then the preamble must be very long.– The preamble must last about as long as the time between checking if the channel is busy

• The amount of energy used transmitting long preambles depends on how often data is sent.

– E.g., if data must be sent only once a day, it might be ok to have a preamble that last an hour, since that would allow a very long time between checking the channel.

– Letting • PT be the power used during transmission of a preamble

• TP be the duration of the preamble

• R the rate of sending data (pkts/sec)

• Echeck the energy required to check the channel

• Then:• Energy for transmitting =

• Energy for receiving =

• Energy for forwarding =

• Optimal value of TP =

PT Tpreamble R PTsize

rate

PR Tpreamble

2 R PR

size

rate Echeck 1

Tpreamble

PT Tpreamble R PTsize

rate PR Tpreamble

2 R PR

size

rate Echeck 1

Tpreamble

Tpreamble R PT PR2

2PTsize

rate Echeck 1

Tpreamble

E check

R PTPR2

LPL check interval

• 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!

Sensor sample period – controls the packet rate

Time between listening (checking if the channel is idle)

lifetime 1

total energy 1

PT Tpreamble R E check 1Tpreamble

LPL and neighborhood size

• More neighbors: more transmissions

– More time spent receiving packets– Less time left to go to sleep

• To find the best check interval– Set the reporting rate– Estimate neighborhood size– Best result: check interval that

gives lowest effective duty cycle

Energy vs Latency

• 10-hop network

• Source sends a 100-byte packet every 10 seconds

• SMAC: change the default config

• BMAC: choose optimal check interval

• SMAC again performs worse...– Reason: sync packets, probability

of multiple schedules---less time to sleep

0 2000 4000 6000 8000 100000

50

100

150

200

250

300

350

400

450

500

550

Latency (ms)

En

erg

y (m

J)

Effect of latency on mean energy consumption

B-MACS-MACAlways On

S-MAC Default Configuration

B-MAC Default Configuration

11 10 9 3 2 111 10 9 3 2 1

Throughput vs power consumption

• 10 nodes in a neighborhood• Data must arrive within 10 seconds• Average power consumption per node• Low data rates: SMAC is better

– Very low duty cycle• Power vs throughput

– SMAC: linear– BMAC: sub-linear

• Reason: SMAC duty cycle must increase– More active periods, more SYNC periods

• BMAC: larger preambles at low throughput, progressively becoming smaller

Experimental results: throughput

• “BMAC is about 4.5 faster than SMAC-unicast”

– Not as fast when ACK or RTS/CTS is used

– No combined results...– Differences less pronounced as #

of nodes increases– Another issue: BMAC has CCA,

thus it backs off less frequently (and perhaps the backoff timer is faster)

0 5 10 15 200

2000

4000

6000

8000

10000

12000

14000

16000

0 5 10 15 200

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Throughput of a congested channel

Number of nodes

Per

cen

tag

e o

f C

han

nel

Cap

acit

y

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

Th

rou

gh

pu

t (b

ps)

Clear Channel Assessment

• MAC must accurately determine if channel is clear– Need to tell what is noise and what is a signal

– Ambient noise is prone to environmental changes

• BMAC solution: ‘software automatic gain control’– initialization

• Signal strength samples taken when channel is assumed to be free so that a noise floor is found.

– Check channel• Samples go in a FIFO queue (sliding window)• Median of FIFO is added to an EWMA filter. When the EWMA filter exceeds a threshold, then it is assumed that

the channel is active.– The median filter is known to be robust to impulse noise/outliers– EWMA is not a particularly good way to find a change of state. – A better approach is based on statistical change-point detection (see next slide)

• Once noise floor is established, a TX requests starts monitoring RSSI from the radio

Change point detection

likelihood ratioP observed | change has occured

P observed | no change has occured

If the likelihood ratio is small, then 1. it is unlikely that what we observed could have occured and the change, and 2, it is likely that what was observed occured without the change

typical likelihood ratio PY1Y2 . . .Yn | change has occuredPY1Y2 . . .Yn | no change has occured

PY1 | change has occuredPY2 | change has occured. . .PYn | change has occured

PY1 | no change has occuredPY2 | no change has occured. . .PYn | no change has occured

typical log-likelihood ratio logPY1Y2 . . .Yn | change has occuredPY1Y2 . . .Yn | no change has occured

logPY i | change has occuredPY i | no change has occured

But, if the distributions are Gaussian, then this can be changed into a cummulative summation

S k 1 max0,S k Yk

if Sk >Thresh, then the change has occurred. This is the optimal approach in terms of detecting a change fastest with a given false-alarm rate

CCA: single-sample thresholding vs outlier detection

• Common approach: take single sample, compare to noise floor– Large number of false negatives

• BMAC: search for outliers in RSSI– If a sample has significantly lower energy than the noise floor during the

sampling period, then channel is clear

CCA results

• 0=busy, 1=clear• Packet arrives between 22 and 54 ms• Single-sample thresholding produces several false ‘busy’ signals

Low Power Listening

• Goal: minimize listen cost

• Principles– Node periodically wakes up, turns radio on and checks channel

• Wakeup time fixed

• “Check time” variable

– If energy is detected, node powers up in order to receive the packet– Node goes back to sleep

• If a packet is received

• After a timeout

– Preamble length matches channel checking period• No explicit synchronization required

– Noise floor estimation used to detect channel activity during LPL

Radio powerup sequence of operations

• Goals– Minimize time radio is on– Minimize number of times radio

gets started– Minimize sampling time (stage e)

Conlcusions• BMAC appears to be better than SMAC

– Easier to tune– Has better channel assessment – Doesn’t use explicit sync packets– Doesn’t use RTS/CTS/ACK if it doesn’t have to– Is smaller and less complex

• Are large preambles always good?– Not sure, but:

• They do mention ‘cyclic packet transmissions’• They can be turned off:

– Enable LPL– Send first packet, disable LPL– Send remaining packets in a big burst– Re-enable LPL