[SelfOrg], SS 2006 2-3.1

Self-Organization in Autonomous Sensor/Actuator Networks

[SelfOrg]Dr.-Ing. Falko Dressler

Computer Networks and Communication SystemsDepartment of Computer SciencesUniversity of Erlangen-Nürnberg

http://www7.informatik.uni-erlangen.de/~dressler/dressler@informatik.uni-erlangen.de

[SelfOrg], SS 2006 2-3.2

Overview

Self-OrganizationBasic methodologies of self-organization; comparison of central and hierarchical control, distributed systems, and autonomous behavior; examples of self-organization

Mobile Sensor/Actuator NetworksAd hoc routing; reliable communication and congestion control; sensor assistance for mobile robots; applications

Coordination of Autonomous SystemsCoordination and synchronization; communication aspects; clustering

Bio-inspired MechanismsSwarm intelligence; artificial immune system; intra/inter cellular information exchange

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MAC Protocols for Ad Hoc and Sensor Networks

Principles and ClassificationMACA / MACAWS-MACPower Control MAC

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Principal Options and Difficulties

Medium access in wireless networks is difficult mainly because ofImpossible (or very difficult) to send and to receive at the same timeInterference situation at receiver is what counts for transmission success, but can be very different to what sender can observeHigh error rates (for signaling packets) compound the issues

RequirementsAs usual: high throughput, low overhead, low error rates, …Additionally: energy-efficient, handle switched off devices!

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Requirements for Energy-efficient MAC Protocols

RecallTransmissions are costlyReceiving about as expensive as transmittingIdling can be cheaper but is still expensive

Energy problemsCollisions – wasted effort when two packets collideOverhearing – waste effort in receiving a packet destined for another nodeIdle listening – sitting idly and trying to receive when nobody is sending Protocol overhead

Always nice: Low complexity solution

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Design Issues

Distributed nature/lack of central coordinationNodes must be scheduled in a distributed fashionExchange of control information

control packets must not consume too much of network bandwidth

Mobility of nodesVery important factor affecting the performance (throughput) of the protocolBandwidth reservations or control information exchanged may end up being of no use if the node mobility is very highProtocol design must take this mobility factor into consideration

system performance should not significantly affected due to node mobility

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Classification of MAC Protocols

MAC Protocols for Ad Hoc Wireless Networks

Contention-Based ProtocolsContention-Based

Protocols with Reservation Mechanisms

Contention-Based Protocols with

Scheduling MechanismsOther MAC Protocols

Sender-Initiated Protocols

Receiver-Initiated Protocols

Synchronous Protocols

Asynchronous Protocols

Single-Channel Protocols

MultichannelProtocols

• MACAW• FAMA

• BTMA• DBTMA

• RI-BTMA• MACA-BI

• HRMA• FPRP

• MACA/PR• RTMAC

• DPS• DLPS

• MMAC• MCSMA

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Classification of MAC Protocols

Contention-based protocolsNo a priori resource reservationWhenever a packet should be transmitted, the node contends with its neighbors for access to the shared channelCannot provide QoS guarantees

Sender-initiated protocols – packet transmissions are initiated by the sender node

Single-channel sender-initiated protocols – the total bandwidth is used as it is, without being dividedMulti-channel sender-initiated protocols – available bandwidth is divided into multiple channels; this enabled several nodes to simultaneously transmit data

Receiver-initiated protocols – the receiver node initiates the contention resolution protocol

[SelfOrg], SS 2006 2-3.9

Classification of MAC Protocols

Contention-based protocols with reservation mechanismsSupport for real-time traffic using QoS guaranteesUsing mechanisms for reserving bandwidth a priori

Synchronous protocols – require time synchronization among all nodes in the network global time synchronization is generally difficult to achieve

Asynchronous protocols – do not require any global time synchronization, usually rely on relative time information for effecting reservations

Contention-based protocols with scheduling mechanismsFocus on packet scheduling at nodes and also scheduling nodes for access to the channel

requirement for fair treatment and no starvationUsed to enforce priorities among flowsSometimes battery characteristics, such as remaining battery power, are considered while scheduling nodes for access to the channel

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Contention-based Protocols: Main Problems

Hidden and exposed terminals - unique problem in wireless networksHidden terminal problem – collision of packets due to the simultaneous transmission of those nodes that are not within the direct transmission range of the sender but are within the transmission range of the receiverExposed terminal problem – inability of a node, which is blocked due to transmission by a nearby transmitting node, to transmit to another node

S2R1

S1

S3R2

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Main Options to Shut Up Senders

Receiver informs potential interferers while a reception is on-goingBy sending out a signal indicating just thatProblem: Cannot use same channel on which actual reception takesplaceUse separate channel for signalingBusy tone protocol

Receiver informs potential interferers before a reception is on-goingCan use same channelReceiver itself needs to be informed, by sender, about impendingtransmissionPotential interferers need to be aware of such informationMACA protocol

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BTMA – Busy Tone Multiple Access

The transmission channel is split intodata and control channel

General behaviorWhen a node wants to transmit a packet,it senses the channel to check whetherthe busy tone is activeIf not, it turns on the busy tone signal andstarts transmission

Problem: very poor bandwidth utilization

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MACA – Multiple Access Collision Avoidance

Use of additional signaling packetsSender B asks receiver C whether C is able to receive a transmission - Request to Send (RTS)Receiver C agrees, sends out a Clear to Send (CTS)B sends, C acks

Potential interferers overhear RTS/CTSRTS/CTS packets carry the expected duration of the data transmissionStore this information in a Network Allocation Vector (NAV)

Collision handlingIf a packet is lost (collision), the node uses the binary exponential back-off (BEB) algorithm to back off for a random time interval before retryingEach time a collision is detected, the node doubles its maximum back-off window

MACA protocol (used e.g. in IEEE 802.11)

A B C D

RTS

CTS

Data

Ack

NAV indicates busy medium

NAV indicates busy medium

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MACA Problems

RTS/CTS ameliorate, but do not solve hidden/exposed terminal problems

Idle listening: need to sense carrier for RTS or CTS packetsIn some form shared by many CSMA variants; but e.g. not by busy tonesSimple sleeping will break the protocol

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MACAW Protocol

The binary back-off mechanism can lead to starvation of flowsExample

S1 and S2 are generating a high volume of trafficIf one node (S1) starts sending, the packets transmitted by S2 get collided

S2 backs off and increases its back-off windowthe probability of node S2 acquiring the channel keeps decreasing

SolutionEach packet carries the current back-off window of the senderA node receiving this packet copies this value into its back-off counter

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MACAW Protocol

Large variations in the back-off valuesthe back-off window increases very rapidly and is reset after each successful transmission

Solutionmultiplicative increase and linear decrease (MILD) back-off mechanism (increase by factor 1.5)

FairnessMACA: per node fairnessMACAW: per flow fairness (one back-off value per flow)

Error detectionOriginally moved to the transport layerSlow and introducing much overhead

SolutionNew control packet type: acknowledgement (ACK)

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MACAW Protocol

Exposed terminal problemRTS/CTS mechanism does not solves the exposed terminal problem

SolutionNew control packet type: data-sending (DS), a small packet (30 Byte) containing information such as the duration of the forthcoming data transmission

A B C D

RTS

CTS

Data

Ack

DS

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Contention-Based Protocols with Reservation

MACA/PR – MACA with Piggy-Backed ReservationMulti-hop routing protocol based on MACAWMain components

MAC protocolReservation protocolQoS routing protocol

Differentiation of real-time and best-effort packetsGeneral behavior

Slotted mechanismsMaintenance of a reservation table (RT) at each node that records all the reserved transmit and receive slots / windows of all nodes within its transmission rangeNetwork allocation vectors (NAV) for cyclesDestination sequenced distance vector (DSDV) used for routing

TDM-like system for real-time trafficBest-effort traffic using MACAW in free slots

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MACA/PR Protocol

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MAC Protocol Using Directed Antennas

PropertiesOne receiver per node, which can transmit and receive only one packet at any given timeEach transceiver is equipped with Mdirectional antennasEach antenna has a conical radiationpattern spanning an angle of 2π/M radiansBasic RTS/CTS scheme (as used in MACA)

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MAC Protocol Using Directed Antennas

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Sensor-MAC (S-MAC)

Primary goalTo retain flexibility of contention-based protocolsWhile improving energy efficiency in multi-hop networksMACA’s idle listening is particularly unsuitable if average data rate is low (most of the time, nothing happens)

Idea: Switch nodes off, ensure that neighboring nodes turn on simultaneously to allow packet exchange (rendez-vous)

Only in these active periods,packet exchanges happenNeed to also exchange wakeupschedule between neighborsWhen awake, essentiallyperform RTS/CTSUse SYNCH, RTS, CTS phases

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S-MAC Scheduling

Coarse-grained sleep/wakeup cycle

SchedulingLow-duty-cycle operation (1-10%)All nodes choose their own listen/sleep schedulesThese schedules are shared with their neighbors to make communication possible between all nodesEach node periodically broadcasts its schedule in a SYNC packet, which provides simple time synchronizationTo reduce overhead, S-MAC encourages neighboring nodes to adopt identical schedules

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S-MAC Synchronized Islands

Nodes try to pick up schedule synchronization from neighboring nodesIf no neighbor found, nodes pick some schedule to start with If additional nodes join, some node might learn about two different schedules from different nodes

“Synchronized islands”To bridge this gap, it has to follow both schemes

Time

A A A A

C C C C

AB B B B

D D D

A

C

B

D

E E E EE E E

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S-MAC Adaptive Listening

Adaptive listening allows additional energy savings (nodes wake up immediately after the exchange completes for immediate contention for the channel)

A

B

C

D

D

C

Sleep

R

Schedule

R RTS C CTS Data DATA A ACK

Adaptive Listen Period

S

S SYNC Period D

DS DS

Frame n Frame n+1 Frame n+2

Packet transmitted in adaptive listening ALP

Scheduled DATA Period

A

B

C

D

D

C

Data

A

Sleep

R

Schedule

R RTS C CTS Data DATA A ACK

Adaptive Listen Period

S

S SYNC Period D

DS DS

Frame n Frame n+1 Frame n+2

Sleeping

Packet transmitted in adaptive listening ALP

Scheduled DATA Period

Sleeping

A

B

C

D

D

C

Data

A

Sleep

R

C

R

Schedule

R RTS C CTS Data DATA A ACK

Adaptive Listen Period

S

S SYNC Period D

DS DS

Frame n Frame n+1 Frame n+2

Sleeping

Packet transmitted in adaptive listening

ALP

ALP

Scheduled DATA Period

Sleeping

SleepingA

B

C

D

D

C

Data

A

Sleep

R

C

Data

A

R

Schedule

R RTS C CTS Data DATA A ACK

Adaptive Listen Period

S

S SYNC Period D

DS DS

Frame n Frame n+1 Frame n+2

Sleeping

Packet transmitted in adaptive listening

ALP

ALP

Scheduled DATA Period

Sleeping

SleepingA

B

C

D

D

C

Data

A

Sleep

R

C

Data

A

R

Schedule

R RTS C CTS Data DATA A ACK

Adaptive Listen Period

S

S SYNC Period D

DS DS

Frame n Frame n+1 Frame n+2

Sleeping

Packet transmitted in adaptive listening

ALP

ALP

Scheduled DATA Period

Sleeping

Sleeping

Slp

Slp

A

B

C

D

D

C

Data

A

Sleep

R

C

Data

A

R

C

Data

A

R

Schedule

R RTS C CTS Data DATA A ACK

Adaptive Listen Period

S

S SYNC Period D

DS DS

Frame n Frame n+1 Frame n+2

Sleeping

Packet transmitted in adaptive listening

ALP

ALP

Scheduled DATA Period

Sleeping

Sleeping

SleepingSlp

Slp

SleepingA

B

C

D

D

C

Data

A

Sleep

R

C

Data

A

R

C

Data

A

R

Schedule

R RTS C CTS Data DATA A ACK

Adaptive Listen Period

S

S SYNC Period D

DS DS

Frame n Frame n+1 Frame n+2

Sleeping

Packet transmitted in adaptive listening

ALP

ALP

Scheduled DATA Period

Sleeping

Sleeping

SleepingSlp

Slp

ALP

Sleeping

Sleeping

Sleeping

Sleeping

A B C D

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S-MAC Performance – Latency

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S-MAC Performance – Energy Consumption

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Power Control MAC Protocol (PCM)

PropertiesRTS/CTS are transmitted with maximum power pmax

RTS-CTS handshake to determine the required transmission power pdesired

RTS is received at the receiver with a signal level pr

Calculation of pdesiredRxthresh is the minimum necessary received signal strengthc … constant

cRxp

pp threshr

maxdesired *=

measured

known in advance

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Power Control MAC Protocol

A B C D E F G H

transmission range

RTS/CTSrange DATA/ACK

range

carrier sensingrange

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Power Control MAC Protocol

PropertiesAdaptation to changing conditions, e.g. caused by mobilityPeriodical check and re-calculation of the necessary transmission power pdesired

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Summary

Well-established MAC protocols in the ad hoc domainMACA / MACAW / 802.11Similar solutions for hidden/exposed terminal problem

Applicability for wireless sensor networksScalability: 802.11 needs a global sync!Channel utilization: good for 802.11, bad for unsynchronized MACAEnergy efficiency: limited sleeping time in 802.11

Specific developmentsS-MAC: allows multiple syncs, long sleep cycles with adaptive listeningPCM: well-controlled transmission power

Advanced researchCross-layer issues between MAC & network (e.g. neighborshipmanagement)

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References

OverviewH. Karl and A. Willig, Protocols and Architectures for Wireless Sensor Networks, Wiley, 2005.C. S. R. Murthy and B. S. Manoj, Ad Hoc Wireless Networks. Upper Saddle River, NJ, Prentice Hall PTR, 2004.

802.11IEEE, "Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specification," IEEE Std. 802.11-1999 edition, 1999.

S-MACW. Ye, J. Heidemann, and D. Estrin, "An Energy-Efficient MAC Protocol for Wireless Sensor Networks," Proceedings of 21st International Annual Joint Conference of the IEEE Computer and Communications Societies (INFOCOM), vol. 3, New York, NY, USA, June 2002, pp. 1567-1576. W. Ye, J. Heidemann, and D. Estrin, "Medium Access Control with Coordinated Adaptive Sleeping for Wireless Sensor Networks," IEEE/ACM Transactions on Networking (TON), vol. 12 (3), pp. 493-506, June 2004.

PCME.-S. Jung and N. Vaidya, "A Power Control MAC Protocol for Ad Hoc Networks," Proceedings of ACM/IEEE MobiCom, September 2002.

OthersM. Adamou, I. Lee, and I. Shin, "An energy efficient real-time medium access control protocol for wireless ad-hoc networks," Proceedings of WIP session of IEEE Real-time systems symposium, London, UK, 2001. M. Caccamo, L. Y. Zhang, L. Sha, and G. Buttazzo, "An Implicit Prioritized Access Protocol for Wireless Sensor Networks," Proceedings of 23rd IEEE Real-Time Systems Symposium (RTSS'02), Austin, Texas, USA, 2002.