這份投影⽚片是 2007 年製作的，雖然內容有點舊，但是很多觀念是相通的，由於沒時間準備公司的「技術分享會議」只好拿舊東西來說了

說明

2

Medium Access Control (MAC) Protocols for

Wireless Sensor Networks

!!

Presented by Chih-Yu

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Outline● Introduction ● MAC Protocols ● Contention-based ● Schedule-based ● Hybrid

● Conclusion

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Outline● Introduction ● MAC Protocols ● Contention-based ● Schedule-based ● Hybrid

● Conclusion

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Introduction● Wireless Sensor Networks

⽕火災、⼟土⽯石流

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Introduction● Why do we need MAC?

● Wireless channel is a shared medium ● Radios transmitting in the same frequency band

interfere with each other !

● The role of Medium Access Control ● Controls when and how each node can transmit in

the wireless channel ● Solves the contention and collision

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Introduction● MAC Protocols for WSNs ● Primary Concern

● Energy Efficiency ● Reduction of significance

● Contention/Collision (basic task) ● Scalability ● Latency ● Communication Patterns ● Adaptability to changes ● Fairness ● Throughput

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Introduction● Major Sources of Energy Wastes ● Control Packet Overhead

● E.g., RTS/CTS ● Collision

● Retransmission ● Overhearing

● The receiver of a packet is not the intended receiver of that packet

● Idle Listing ● Listening to possible traffic that is not sent

ab

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Outline● Introduction ● MAC Protocols ● Contention-based ● Schedule-based ● Hybrid

● Conclusion

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Classification of MAC Protocols● Contention-based protocols

● Nodes compete in probabilistic coordination ● Examples: ALOHA (pure & slotted), CSMA ● Simple, no time synch, and robust to network

changes ● High idle listening and overhearing overheads

● Solve this by duty cycling

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Classification of MAC Protocols● Schedule-based protocols

● Schedule nodes onto different sub-channels ● Examples: TDMA, FDMA, CDMA ● Collision-Free ● Requires time synch and not robust to changes. ● Low throughput and high latency even during low

contention. ● Low idle listening and overhearing overheads

● Wake up and listen only during its neighbor transmission

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Outline● Introduction ● MAC Protocols ● Contention-based ● Schedule-based ● Hybrid

● Conclusion

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Contention-based MAC Protocols for WSNs● S-MAC ● B-MAC

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S-MAC

W. Ye, J. Heidemann, and D. Estrin, “An Energy-efficient MAC Protocol for Wireless Sensor Networks,” in Proc. of IEEE Infocom, Jun. 2002, pp.1567–1576.

● Basic Idea ● Trades energy efficiency for lower throughput and

higher latency ● Main Components ● Periodic Listen and Sleep ● Collision Avoidance ● Overhearing Avoidance ● Message Passing

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Periodic Listen and Sleep● Nodes periodically sleep

● Turn off radio when sleeping ● Reduce duty cycle to ~10% (150ms on vs. 1.5s off)

● Trades energy efficiency for lower throughput and higher latency

Listen Sleep tSleepListen

Listen Sleep Listen t

Both the synchronization issue and the schedule maintenance issue can be found in the paper.

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Collision Avoidance● Similar to IEEE 802.11 ● Virtual Carrier Sense

● NAV (Network Allocation Vector) ● Physical Carrier Sense ● Four-way handshake

● RTS/CTS/DATA/ACK

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Overhearing Avoidance● Basic Idea ● A node can go to sleep whenever its neighbor is

talking with another node ● Who should sleep? ● The immediate neighbors of sender and receiver

● How to they know when to sleep? ● By overhearing RTS or CTS

● Hog long should they sleep? ● NAV

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Message Passing● How to transmit a long message? ● Transmit it as a single long packet

● Easy to be corrupted ● Transmit as many independent packets

● Higher Control Overhead & Longer Delay ● Divide into fragments, but transmit all in burst !!!

● RTS/CTS reserve medium for the entire sequence

RTSCTS

DataCTS

DataCTS

DataCTS

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B-MAC (Overview)● B-MAC is implemented in TinyOS

!● Major Feature: reconfigurable ● Above B-MAC, one can implement an RTS-CTS

scheme or a TDMS-like scheduling protocol !

● A small core of media access functionality ● arbitration, reliability, low power communication

J. Polastre, J. Hill, and D. Culler, “Versatile low power media access for wireless sensor networks,” in SenSys, Nov. 2004, pp. 95–107.

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B-MAC (Overview)● Channel Arbitration ● Clear Channel Assessment (CCA) & Backoffs !

● Reliability ● Link-layer acknowledgment !

● Low Power Communication ● Low Power Listening (LPL)

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Clear Channel Assessment● This paper proposes a way to estimate noise

floor using signal-processing techniques ● Noise floor can be viewed as a threshold !

● Where a node intend to transmit a packet ● 802.15.4: takes a single sample and compares it to

the noise floor ● B-MAC: outlier detection algorithm

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Clear Channel Assessment

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Clear Channel Assessment● MacControl

● command result_t EnableCCA(); ● command result_t DisableCCA(); !

● By disabling CCA, a scheduling protocol may be implemented above B-MAC !

● If CCA is enabled ● event uint16_t initialBackoff(void* msg); ● event uint16_t congestionBackoff(void* msg);

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Link-layer Acknowledgment● Similar to traditional MAC protocols !

● MacControl ● command result_t EnableAck(); ● command result_t DisableAck(); !

● RTS/CTS/DATA/ACK ?

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Low Power Listening● Low Power Listening

● Sender (Transmit mode) ● Send preamble before transmit the data ● Preamble Length !

● Receiver (Listening mode) ● Check Interval !

● To reliably receive data, the preamble length is matched to the interval that the channel is checked for activity ● If the channel is checked every 100 ms, the preamble must

be at least 100 ms long

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Low Power Listening

Sender

Receiver 1

Receiver 2

Backoff& CCA Preamble

wakeup

Data

wakeupCheck Interval

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Outline● Introduction ● MAC Protocols ● Contention-based ● Schedule-based ● Hybrid

● Conclusion

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Schedule-based MAC Protocols for WSNs● TDMA MAC Protocols ● TRAMA ● Convergecast in Tree-based Wireless Sensor

Networks

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Traditional TDMA MAC Protocols● Divide time into cycles

● A cycle consists of several slots !!!

● Advantages ● Collision-Free, Low Idle Listing and Overhearing Overheads

● Disadvantages ● Synchronization, Low Channel Utilization !

● An important Issue ● Slot Assignment Strategy

1 2 3 4 5 6 7 1 2 3

Cycle

3 4 5 cc 1 2 3 4 5 6 7

Cycle

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Slot Assignment Strategy● Basic Slot Assignment Strategy ● A node should own a slot different from its one-

hop and two-hop neighbors

R

S

S

Collision

One-hop

R

R

S S

Collision

Collision

Two-hop

RS R S

Three-hop

Acknowledgement?

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TRAMA

● TRAMA (TRaffic-Adaptive Medium Access) ● Avoid assigning slots to nodes with no traffic ● Allow nodes to determine when they can switch off

!● Main Components ● Neighbor Protocol ● Schedule Exchange Protocol ● Adaptive Election Algorithm

V. Rajendran, K. Obraczka, and J. Garcia-Luna-Aceves, “Energyefficient, collision-free medium access control for wireless sensor networks,” Wireless Networks, vol. 12, no. 1, pp. 63–78, Feb. 2006.

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Neighbor Protocol

To accommodate topology dynamics: 1. Dynamic: interval between random access periods could be larger 2. Static: interval could be smaller

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Schedule Exchange Protocol

Random Access

SCHEDULE_INTERVAL

SCHEDULE_INTERVAL

Winning Slot(Intended Receiver: a, b, c)

Winning Slot(Intended Receiver : d, e, f)

Winning Slot(Intended Receiver : φ)

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Adaptive Election Algorithm

x

a y

b 10080

60

20

30

50

10 120

90

130

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Convergecast in Tree-based Wireless Sensor Networks

6

4 54

1 2 3 1

0 1 0

3

MAXSLOT=7

2

Level 0

L1

L2

L3

1. Construct a BFS tree T !

2. Traverse the nodes of T in a bottom-up manner

a) if the node v is a leaf node, assign a min slot s(v) which is not interference with its interference neighbors

b) if v is the in-tree node, choose the min slot s(v) which is larger than the children’s slot and do not interference with others

!3. Re-assign the slot in a top-down

mannerY.-C. Tseng and M.-S. Pan, “Quick Convergecast in ZigBee/IEEE 802.15.4 Tree-Based Wireless Sensor Networks”, ACM MobiWac, 2006.

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Outline● Introduction ● MAC Protocols ● Contention-based ● Schedule-based ● Hybrid

● Conclusion

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Hybrid TDMA/CSMA● Z-MAC ● Funneling-MAC

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Z-MAC● Z-MAC (Zebra MAC) ● A hybrid TDMA and CSMA MAC scheme ● The main feature is its adaptability to the level of

contention in the network ● under low contention, it behaves like CSMA, and ● under high contention, like TDMA

I. Rhee, A. Warrier, M. Aia, and J. Min, “Z-MAC: a hybrid MAC for wireless sensor networks,” in SenSys, Nov. 2005, pp. 90–101.

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Z-MAC● Design of Z-MAC ● Neighbor Discovery and Slot Assignment ● Local Framing ● Transmission Control of ZMAC ● Explicit Contention Notification ● Receiving Schedule of ZMAC ● Local Time Synchronization

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Funneling-MAC● Funneling effect in sensor networks

G.-S. Ahn, E. Miluzzo, A. T. Campbell, S. G. Hong, and F. Cuomo “Funneling-MAC: A localized, sink-oriented MAC for boosting fidelity in sensor networks,” in SenSys ’06

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Funneling-MAC● The authors propose a localized, sink-

oriented funneling-MAC !

● Localized ● Using local TDMA scheduling in the intensity

region only ● Sink-oriented ● Manage TDMA scheduling ● Compute and maintain the depth of the intensity

region

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Conclusion● MAC Protocols in WSNs ● Energy Efficiency and Other Concerns ● Contention-based/Schedule-based/Hybrid !

● Future Directions ● Taking the traffic features of WSNs into

consideration ● Communication Patterns: Convergecast, Broadcast,

Local Gossip, Event Reporting, etc.