MAC protocols for WSNs

Vinod KulathumaniWest Virginia University

2

Outline

• MAC goals

• MAC protocol assignment categories Random Fixed On-demand

• WLAN MAC

Aloha, CSMA, TDMA, FDMA

• Challenges in WSN

• WSN MAC protocols Basic MAC S-MAC B-MAC O-MAC

3

MAC alphabet soup

• http://www.st.ewi.tudelft.nl/~koen/MACsoup/protocols.php

4

Reading

• A Transmission Control Scheme for Media Access in Sensor Networks, D. Culler, A. Woo

• Versatile Low Power Media Access for Wireless Sensor Networks, J. Polastre et al.

• An Energy-Efficient MAC Protocol for Wireless Sensor Networks, D. Estrin et al.

http://www.isi.edu/~johnh/PAPERS/Ye04b.pdf

• Ultra-Low Duty Cycle MAC with Scheduled Channel Polling, Wei Ye et al.

5

Introduction

• Important attributes of MAC protocols

Collision avoidance

Basic task — medium access control Energy efficiency Scalability and adaptivity

Number of nodes changes overtime Latency Fairness Throughput Bandwidth utilization

6

MAC protocol categories

1. Random assignment

ALOHA, CSMA (Carrier Sense) Predominantly used in wireless networks 802.11, 802.15, etc.

2. Fixed assignment

TDMA (Time Division), CDMA (Code division), FDMA (Frequency division)

Unsuitable for dynamic, bursty traffic in wireless networks

3. On-demand assignment

Token ring Hard to implement: requires static topology or neighbor

discovery

7

Random assignment

• Aloha

Hawaii 1970 Node sends a data when it has data If no ACK received, data is re-send after random backoff No carrier sensing Works for low network contention, peak performance 18%

8

Random assignment

• Contention-based protocols

CSMA — Carrier Sense Multiple Access

Ethernet

Not enough for wireless

Collision detection not possible as in Ethernet

Collision avoidance using carrier sense

A B CHidden terminal: A is hidden from

C’s CS

9

Random assignment

• Contention-based protocols

MACA — Multiple Access w/ Collision Avoidance

RTS/CTS for hidden terminal problem

RTS/CTS/DATA

Node wishing to send data sends a Request to Send frame

Destination node replies with a Clear To Send frame.

Node receiving either the RTS or the CTS frame

refrains from sending data for a given time. MACAW — improved over MACA

RTS/CTS/DATA/ACK

Fast error recovery at link layer IEEE 802.11 Distributed Coordination Function

Largely based on MACAW

10

Fixed assignment

• TDMA (Scheduled protocol)

Each node gets full bandwidth for a pre-allocated time in turns Not suitable for networks whose node density changes

• FDMA (Scheduled protocol)

Each node gets a permanent share of bandwidth Poor bandwidth utilization

• CDMA

Spread spectrum technology Analogous to speaking different languages Multiple coders/decoder needed

11

Energy Efficiency in MAC Design

• Energy is primary concern in sensor networks

• What causes energy waste?

Collisions Control packet overhead Overhearing unnecessary traffic Long idle time

bursty traffic in sensor-net apps

Idle listening consumes 50—100% of the power for receiving

11

12

Energy Efficiency in MAC Design

• TDMA vs. contention-based protocols

TDMA can easily avoid energy waste from all above sources Contention protocols disadvantage in all 4 areas TDMA has limited scalability and adaptivity

Hard to dynamically change frame size or slot assignment when new nodes join

Restrict direct communication within a cluster Contention protocols easily accommodate node changes and

support multi-hop communications

12

13

WSN MAC challenges

• The network tends to operate as a collective structure, rather than supporting many independent point-to-point flows

− Deep multi-hop dynamic topologies, route-through traffic exceeds originating traffic

− Guarantees or fairness over multi-hop communication is challenging due to contention at every hop

• Traffic tends to be variable and highly correlated− Little or no activity/traffic for longer periods and intense traffic

over shorter periods

• Highly constrained resources and functionality− Radio should be turned off most of the time

14

WSN MAC design considerations

• Fairness of the bandwidth allocated to each node for end to end data delivery to sink

Each node acts as a router as well as data originator The traffics compete for the same upstream bandwidth RATE CONTROL!

• Hidden node problem

Solution without RTS/CTS

• Energy efficiency

Transmit, receive, idle consume roughly same amount of energy The cost of dropping a packet varies with place and the packet

15

Contributions of Woo-Culler03

• A Transmission Control Scheme for Media Access in Sensor Networks, D. Culler, A. Woo. ACM Mobicom 2003

• Considers converge-cast,

• bursty traffic model

16

Contributions of Woo-Culler03

• How to avoid sending event synchronization?

Highly synchronized nature of the traffic causes collisions Random listening period?

Listening is costly Use random delay + fixed short listening Turn off radio during the fixed delay period

• Random backoff when channel was busy

This alone not enough Nodes are forwarders and originators If source transmission failed, application needs to randomly

backoff Phase shift to reduce synchrony-livelock This also helps in achieving fairness

17

Contributions of Woo-Culler03

• Implicit acknowledgements

Overhearing forwarding counts as an acknowledgement

• Heuristic for alleviating hidden-node problem

No rts/cts Child reduces potential hidden node problem with its grand parent Does not send packets between “t” and “t+x+packettime” after overhearing packet transmission at t by its parent

• Rate control

Control the rate of originating data of a node

to allow route-through traffic to reach the base station

Linear increase, multiplicative decrease Penalize route-thru traffic less than originating traffic

18

Overall

Advantages:

• Lightweight, control packet overhead is reduced

Disadvantages:

• Assumes periodicity of the originating traffic

• Assumes a converge-cast scenario

19

S-MAC: Introduction

• Sensor-MAC — by Ye, Heidemann and Estrin

• Tradeoffs

Latency / fairness poor Energy efficiency high

• Major components in S-MAC

− Periodic listen and sleep

− Collision avoidance

− Overhearing avoidance

− Message passing

20

Periodic Listen and Sleep

• Problem: Idle listening consumes significant energy

• Solution: Periodic listen and sleep

• Turn off radio when sleeping

• Reduce duty cycle to ~ 10% (150ms on/1.5s off)

sleeplisten listen sleep

21

Periodic Listen and Sleep

• Schedules can differ

• Prefer neighboring nodes have same schedule— easy broadcast & low control overhead

Node 1

Node 2

sleeplisten listen sleep

sleeplisten listen

Schedule 2Schedule 1

Border nodes: two schedules; broadcast twice

22

Periodic Listen and Sleep

• Schedule Synchronization

New node tries to follow an existing schedule Remember neighbors’ schedules

— to know when to send to them Each node broadcasts its schedule once every few periods Re-sync when receiving a schedule update

• Periodic neighbor discovery

Keep awake in a full sync interval over long periods

23

Listen and Sleep - Maintaining Synchronization

• The listen time is divided into two parts:

For sending/receiving SYNC signal. For sending/receiving Data.

24

Collision Avoidance

• Problem: Multiple senders want to talk

• Options: Contention vs. TDMA

• Solution: Similar to IEEE 802.11 ad hoc mode (DCF)

Physical and virtual carrier sense Randomized backoff time RTS/CTS for hidden terminal problem RTS/CTS/DATA/ACK sequence

25

Overhearing Avoidance

• Problem: Receive packets destined to others

• Solution: Sleep when neighbors talk

• Who should sleep?

− All immediate neighbors of sender and receiver

How long to sleep?

− The duration field in each packet informs other nodes the sleep interval

26

Message Passing

• Problem: Sensor net in-network processing requires entire message

• Solution: Don’t interleave different messages

Long message is fragmented & sent in burst RTS/CTS reserve medium for entire message Fragment-level error recovery — ACK

— extend Tx time and re-transmit immediately

• Other nodes sleep for whole message time

• Fairness is lost

Energy efficiency is gained

27

Experiments: two-hop network

• Topology and measured energy consumption on source nodes

Source 1

Source 2

Sink 1

Sink 2

• Each source node sends 1-10 messages /second

• Listen interval 115ms• Duty cycle 50%

28

Experiments: two-hop network

The graph shows the mean energy on radios of source nodes

Idle listening rarely happens

Periodic sleep for Idle listening

• Low load – periodic sleep is key• High load – overhearing

avoidance is key

2929

Adaptive Listen

• Reduces latency due to periodic sleep

• At end of each transmission, nodes wake up for a short time

• Next transmission can start right away

• Example: data flow ABC

A sends RTS, B replies CTS, C overhears CTS C will wake up when AB is done BC can start right away

A B C

30

10-hop Experiments

• Compare S-MAC in 3 different modes

• 10-hop linear network with 11 nodes

0 2 4 6 8 100

5

10

15

20

25

30Energy consumption on radios in the entire network

Message inter-arrival period (S)

En

erg

y co

nsu

mp

tion

(J)

10% duty cycle without adaptive listen10% duty cycle with adaptive listen No periodic sleep

0 2 4 6 8 100

2

4

6

8

10

12Average message latency under the lowest traffic load

Number of hops

La

ten

cy (

S)

10% duty cycle without adaptive listen10% duty cycle with adaptive listen No periodic sleep

31

Summary: SMAC

Advantages

• Well-designed, complete protocol

• addresses deficiencies of 802.11 applied to WSN

• Significantly lower power operation than 802.11

Disadvantages

• SMAC incurs some drawbacks of TDMA schemes

• Topology maintenance, need for synchronization

• Additional complexity at border nodes between two schedules

• Designed for a general set of workloads User sets radio duty cycle; SMAC takes care of rest

• Combines carrier sense, link-layer reliability, RTS/CTS and sleep scheduling into MAC layer.

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Some thoughts on projects

Currently chosen:

Home network – Brian Mcqueen

Emergency Evacuation - Bryan Lemon

DSP programming on Beagleboard - Purnima

Wireless hart for industrial control - Nambu

camera networks for face recognition – Srikanth

Snapshots implementation – SriCharan

Exploiting Cooperative MIMO for sensor networks - Omar

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Some thoughts on projects

Imote Platform evaluation / linux installation / camera testing

Desync

Cyclic automata

Higher layer protocols for cooperative comm physical layer

Smart phone applications – for elder care, home networking, anything that uses an element of communication, not standalone

Consensus protocols, survey and evaluation

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Some thoughts on projects

Camera network calibration, survey and simple implementation

Vehicular networks

intra and inter vehicle

evaluation and testing of existing protocols

Wireless network security for electric grids and cyber infrastructure

Robotics, coordinated actuation

Clustering protocol – evaluation

35

BMAC: versatile low power MAC

• Flexible and tunable

small core and factored functionality bidirectional (set and get) interfaces to MAC functionalities

applications can turn them on and off for adapting to radio environment

RTS/CTS, ACKs may be implemented above BMAC

• Low power operation

Clear Channel Assessment (reducing idle listening) Low Power Listening

36

CCA

• 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 Noise floor is established

• Comparing one signal strength reading with noise floor causes false negatives (noise amplitude fluctuates)

• Instead, detect outliers:

Samples whose energy is significantly below noise floor. This can’t happen if packet is being sent.

37

CCA…

Packet arrives between 22 and 54 ms

38

LPL

• Sleep cycles

Wake up, do carrier sensing Use CCA reduce idle listening If idle go back to sleep Else, synchronize using preamble

• Preamble length matches channel checking period

No explicit synchronization required (unlike S-MAC) Packet checking period and Preamble length - configurable

39

Low Power Listening: Preamble Sampling

Sender

Receiver

Preamble Send data

Preamble samplingActive to receive a message

S

R

|Preamble| ≥ Sampling period|Preamble| ≥ Sampling period

• Preamble is not a packet but a physical layer RF pulse

Minimize overhead

40

LPL effect

• 1-hop 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)

• Better to have large preamble than to check more often

41

Analyzing lifetime

Periodic data gathering

• Periodic sampling

• Periodic transmission

• Tree communication structure

Simplified periodic model

• Each node receives r packets from all its neighbors per second

• Each node broadcasts r packets per second

• Neighborhood size n

42

Effect of Neighborhood Size

0 20 40 60 80 1000

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

Number of neighboring nodes

Eff

ecti

ve d

uty

cyc

le (

%)

200ms check interval100ms check interval50ms check interval25ms check interval10ms check interval

Effect of neighborhood size on node duty cycle• To find optimum check interval

• Lowest point along y-axis for given neighborhood size

43

Node lifetime

44

Effect of Neighborhood Size

• Neighborhood Size affects amount of traffic in a cell

Network protocols typically keep track of neighborhood size

Bigger Neighborhood More traffic

0 20 40 60 80 1000

20

40

60

80

100

120

140

160

180

200

Neighborhood size

Ch

ann

el A

ctiv

ity

Ch

eck

Inte

rval

(m

s)

Expected Lifetime Contour

0.25

0.5

0.75

11.25

1.522.5

45

Implementing RTS/CTS

• RTS-CTS can be implemented over BMAC:

Send RTS using LPL Listen for CTS using LPL Once CTS is heard, disable LPL, CCA Send data as burst Send link layer ACK Re-enable LPL, CCA

• RTS – CTS/ ACK used depending on the situation

46

Throughput

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)

N nodes equidistant from receiver

Send as fast as possible with collision avoidance

47

Throughput vs power consumption

10 nodes within cell

Increase traffic slowly

Latency bound by 10 seconds

48

End-to-end latency

49

Summary - BMAC

Pros

• Flexibility to implement application specific optimization

• Zero protocol overhead for synchronization

Cons

• Each transmission wakes up all nodes in range

• Low duty cycle = high sender preamble

50

Dominant Receiver Power Consumption

Large portion of energy is consumed in receiver radio

One typical surveillance application:

Receiver Radio~2100 J/day

Signal processing ~60 J/day

Everything else ~8 J/day

Receiver Radio

Signal Processing

Other

O-Mac : Receiver centric MAC

51

Receiver Centric vs. Transmitter Centric

Transmitter Centric MAC design

Transmitter implicitly knows receiver will wakeup during transmission

Collision avoidance is transmitter driven (i.e., RTS-CTS, CCA)

Receiver Centric MAC design

Receiver explicitly communicates its wakeup schedule to transmitter

Collision avoidance is receiver driven (i.e., receivers use TDMA)

TransmitterReceiver

TransmitterReceiver

Transmitter

Receiver

52

A problem

• Consider 10 hop linear network from source to destination. For a given 100 byte packet to be sent from source, maximum latency allowed is 2 seconds.

Time to transmit 100 bytes at any node =30mS

Time to poll channel = 3 ms

What is the highest check interval that can be chosen in B-MAC that will achieve given latency?

If this is a repeating pattern [one packet sent every 2 seconds], what is the B-MAC duty cycle?

What is the maximum sleep interval possible in S-MAC (listen window is 115 mS)

What is the S-MAC duty cycle in that case?