mcmac: a power efficient, short preamble multi-channel mac

McMAC: a power efficient, short preambleMulti-Channel MAC protocol for wireless

sensor networks

Riccardo Bonetto, Michele Rossi, Nicola Bui, Michele Zorzi

Department of Information Engineering, University of PaduaVia Gradenigo 6/b, 35131, Padua, Italy

March 23, 2012

{bonettor, bui, rossi, zorzi}@dei.unipd.it WNS3 2012 March 23, 2012 1 / 21

OUTLINE

Outline

1 MAC protocols for WSNs

2 Preamble sampling based protocols

3 McMAC: basic idea

4 McMAC: CSMA-CA

5 Transmission algorithm

6 Synchronization

7 Buffering

8 NS-3 implementation

9 Results

10 Conclusions and future work


MAC protocols for WSNs


Wireless nodes’ transceiver is the major responsible for energyconsumption.

MAC protocols for WSNs must be designed for an efficient use of thetransceiver in order to mitigate the three major causes of energy wastage:

Idle listening

Collisions

Overhearing






Idle listening

Collisions

Overhearing

Phenomenon arising when one node keepsits transceiver in receiving mode while notreceiving useful data.






Idle listening

Collisions

Overhearing

Tx1 Tx2Rx

radio wavesnode Rx experi-ences a collisioncaused by Tx1 andTx2 transmittingat the same time






Idle listening

Collisions

OverhearingTx

Rx

I

radio waves

I receives an unde-sired transmissionfor Rx


Preamble sampling based protocols


CSMA-CA based;

No synchronization required;

Can support mobility;

Scalable.

DutyCycling

Channelsensing

Preamble

BackoffTx data

Tx request

Channel busy

backoff expired

Backoff numberexceeded

Channel idle

End preamble

End tx



BMAC vs XMAC

One of the first preamble sampling based protocols is BMAC [9].

Duty Cycling

Receiving preamble Rx Data

sleeping

active

Preamble Data

Tx Data

Rx side

Tx side

XMAC [2] breaks the long BMAC preamble into a sequence of shortstrobed preambles. Sensor nodes equipped with XMAC save energy andtime:

Strobed preambles

Data

Tx data

Rx Side

Tx Side

sleeping

active

Ack


McMAC: basic idea

McMAC

McMAC is a novel protocol combining the flexibility of preamble samplingbased protocols with the efficiency of the strobed preamble technique andthe exploitation of the whole IEEE 802.15.4 spectrum.

Basic idea:

Assign to each node i a “base channel” chi , to do that:

Let I be the set of the nodes’ identifiers;

Let {11, 12, · · · , 26} be the set of IEEE 802.15.4 channels’ identifiers.

Then define the hash function h as:

h : I → {11, 12, · · · , 26}

and let:chi = h(i) ∀i ∈ I


McMAC: basic idea

When node i wants to send a packet to node j , according to the mappinggiven by h, it has to perform the following actions:

Set the transceiver to channel h(j) = chj ;

Evaluate the state of the channel;

Start sending the preambles or abort transmission.

TxRequest for j

Select chj = h(j)

CSMA-CAmodule start transmissionreturn FAILURE

IDLEBUSY


Transmission algorithm

Transmitting a packet

Starting transmission

Send Strobed preamble

Set TRX to RX mode

Ack received

Set TRX to TX mode

TX DATA

Ack required

Set TRX to RX mode

Ack received Return SUCCESS to upper layer

inter preambletime elapsed

preamble sendingtime elapsed

FAILURE

Ack time elapsedFAILURE

Yes

Yes

YesNoYes

No

No

Yes

No

Yes

No

No


Synchronization

Synchronization

To reduce the number of preambles sent, a zero overhead synchronizationheuristic has been implemented.

Sleeping

Awake

Node i

Node j

Drift D Cycle period Cp

Tx Req

x

Then:

x = D + SleepingTimeLeft − CyclePeriod if i was sleeping when itreceived the transmission request;

x = D + AwakeTimeLeft − CyclePeriod if i was awake when itreceived the transmission request.


Buffering

Buffering

A packets queue has also been implemented in order to handle hightransmission rates.

End Tx To node n

Buffer empty

Otherpackets for n

Take the firstpacket in queue,regardless of its

destination

Send the firstpacket in queue for

n immediately

Start a newtransmission

Start duty cycling

No

No Yes

Yes


NS-3 implementation

Spectrum

Spectrum ([1]) is an NS-3 framework aimed at dynamically representingthe electromagnetic spectrum.

Frequency bands are represented as vectors;

Each cell of the vector represents a sub-band;

The number of cells forming the vector determines the precision ofthe representation;

The power level in each sub-band is assumed to be constant and isrepresented using a double;

The precision of the representation is user defined.


NS-3 implementation

Transmission medium representation

The IEEE 802.15.4 spectrum has been represented using the Spectrumframework with a granularity of 1 MHz.

The PSD of the transmitted signals is shaped according to the standardIEEE 802.15.4 PSD mask:

IEEE 802.15.4PSD mask

logarithmic scale

[WHz

]

MHz

txPSDtxPSD · 0.5

txPSD · 0.1

txPSD · 0.0.1

Central frequency

Current channel1MHz resolution


NS-3 implementation

McMAC implementation

McMAC is based on the IEEE 802.15.4 physical layer implementationprovided in [8].


NS-3 implementation

To represent simultaneous transmissions over different channels:

Each node stores a private SpectrumValue object representing thespectrum in its sensing range.Each time a transmission is going on, for each node in the network,the local spectrum representation is updated according to the receivedpower.

Start Tx with Ptx on channel chtx

for each node n onchannel chi : 11 ≤

chi ≤ 26

compute:Prx = f (Ptx)

chi = chtxTrigger reception

to n

Update localspectrum

representation withinterference Prx

These steps areperformed by theSingleModelSpectrumChannel

Update per-formed callingLrWpanPhy:AddNoisePsd(

Ptr<SpectrumValue>)

YesNo


Results

Simulations set up

To obtain statistically valid results, several simulations have been run.

Number of nodes forming the network: N = 64.The tunable parameters were:

The number of nodes allowed to transmit packets:

τi =

{2i 0 ≤ i ≤ 4

4 · i 5 ≤ i ≤ 16

The number of available channels: {1, 8, 16};The arrival time of transmission requests: Poisson process with rateλ ∈ {1, 1

2 ,14}.

It’s worth to point out that McMAC coincides to XMAC when restrictingthe number of available channels to 1.


Results

Results

Percentage of successfully sent packets for XMAC (left) and McMAC with 16 channels (right).

20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96

100

4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64

Se

nt

Pa

cke

ts/T

x r

eq

ue

sts

[%

]

Number of transmitters

McMacsynchronized McMacMcMac with memory

20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96

100

4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64S

en

t P

acke

ts/T

x r

eq

ue

sts

[%

]Number of transmitters



Results

Results

Fraction of time during which the transceiver was active for XMAC (left) and McMAC with16 channels (right).

0

0.2

0.4

0.6

0.8

1

4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64

Active

tim

e/t

ota

l o

pe

ratio

na

l tim

e



0

0.2

0.4

0.6

0.8

1

4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64

Active

tim

e/t

ota

l o

pe

ratio

na

l tim

e




Results

Results

Average number of preambles sent per packet for XMAC (left) and McMAC with 16 channels(right).

200

400

600

800

1000

1200

1400

1600

4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64

Se

nt

pre

am

ble

s/S

en

t p

acke

ts



200

400

600

800

1000

1200

1400

1600

4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64

Se

nt

pre

am

ble

s/S

en

t p

acke

ts




Conclusions and future work

The presented results show that:

McMAC exhibits better performance in terms of throughput withrespect to the widely accepted protocol XMAC;

McMAC exhibits good energy saving skills;

The number of transmitted preambles is quite high when using 16channels .

Results motivate further work to:

Reduce the number of preambles sent;

Add a lightweight and efficient neighbor discovery procedure;

Provide a full integration of McMAC in ns-3;

Implement McMAC in a real testbed.



Questions?

QUESTIONS?



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mcmac: a power efficient, short preamble multi-channel mac

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