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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

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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 Wireless Networks

Design issues

Design goals

Classification of MAC protocolsContention-based protocolsContention-based protocols with reservation mechanismsContention-based protocols with scheduling mechanisms

MAC protocols for sensor networks

Case study: S-MAC

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Design Issues

Bandwidth efficiencyLimited radio spectrum

efficient bandwidth sharingDefinition bandwidth efficiency: ratio of the bandwidth used for actual data transmissions to the total available bandwidth

optimization problem for MAC protocolsQuality of Service support

Usually complex or no bandwidth reservationresource reservation must consider the nature of wireless channels and

node mobilitySynchronization

Inter-node synchronizationrequired for bandwidth (time slot) reservations

Time synchronization using exchanged control packetscontrol packets must not consume too much of network bandwidth

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Design Issues

Hidden and exposed terminalsUnique 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 arewithin the transmission rangeof the receiverExposed terminal problem – in-ability of a node, which is blockeddue to transmission by a nearbytransmitting node, to transmit toanother node

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Hidden and exposed terminals

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Hidden and exposed terminals

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Design Issues

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Design Issues

Error-prone shared broadcast channelBroadcast nature of radio channelA transmission made by a node is received by all nodes within its direct transmission range

when a node is receiving data, no other node in its neighborhood, apart from the sender, should transmitMany nodes may contend for the channel simultaneously

packet collision probability is quite high in wireless networksA MAC protocol should ensure that all nodes are treated fairly with respect to bandwidth allocation

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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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Design Goals of a MAC Protocol for Ad Hoc Networks

The operation of the protocol should be distributed

The protocol should provide QoS support for real-time traffic

The access delay, which refers to the average delay experienced by any packet to get transmitted, must be kept low

The available bandwidth must be utilized efficiently

The protocol should ensure fail allocation of bandwidth to nodes

Control overhead must be kept as low as possible

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Design Goals of a MAC Protocol for Ad Hoc Networks

The protocol should minimize the effects of hidden and exposed terminal problems

The protocol must be scalable to large networks

It should have power control mechanisms in order to efficiently manage energy consumption of the nodes

The protocol should have mechanisms for adaptive data rate control

It should try to use directed antennas (reduced interference, increased spectrum reuse, reduced power consumption)

The protocol should provide time synchronization among nodes

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Classification of MAC Protocols

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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

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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

Carrier sense multiple access (CSMA)

General behavior1. Sender senses the channel for the carrier signal2. If carrier is present, it times out a random period of time before retrying

CSMA does not overcome the hidden terminal problem as well as the exposed terminal problem

Other approaches / solutionsMultiple access collision avoidanceBusy tone multiple access

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MACA Protocol

Multiple Access Collision AvoidanceUse of additional signaling packets

RTS (ready-to-send)CTS (clear-to-send)

General behaviorIf a packet is to be sent, a RTS packet is transmittedIf the receiver is ready to receive the packet, it answers with a CTS packetOnce the sender successfully receives the CTS without an error, it transmits the data packet

If a packet is lost (collision), the node uses the binary exponential back-off (BEB) algorithm to back off for a random interval of time before retryingEach time a collision is detected, the node doubles its maximum back-off windowRTS and CTS packets carry the expected duration of the data packet transmission to overcome the hidden terminal problem

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MACA 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

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MACAW Protocol

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BTMA Protocol

Busy Tone Multiple AccessThe transmission channel is split intodata and control channelGeneral behavior

When 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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DBTMA Protocol

Dual Busy Tone Multiple AccessBasic idea

RTS/CTS on the control channelTwo busy tones: BTt and BTr

Performance gainBetter network utilization than MACA/MACAW (about twice than most RTS/CTS schemes)

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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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Power Control MAC Protocol

PropertiesRTS/CTS are transmitted with maximum power pmax

RTS-CTS handshake is used to determine the required transmission power pdesired

RTS is received at the receiver with a signal level pr

Calculation of pdesiredRxthresh … minimum necessary received signal strengthc … constant

cRxp

pp threshr

maxdesired *=

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Power Control MAC Protocol

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MAC protocols for sensor networks

Main criteria

(+) Collision avoidance(++) Energy efficiency(++) Scalability and adaptivity(+) Channel utilization(-) Latency(-) Throughput

Goodput(+) Fairness

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Case Study: S-MAC

W. 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.

Primary goalTo retain flexibility of contention-based protocolsWhile improving energy efficiency in multi-hop networks

Approaches to reduce energy consumption from all major sources of energy waste

Idle listeningCollisionOverhearingControl overhead

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S-MAC Design Approaches

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 Design Approaches

Data transmissionRTS-CTS-DATA-ACK exchange

Duration field in each packet toindicate the time needed in thecurrent transmission

Adaptive listening allowsadditional energy savings (nodeswake up immediately after theexchange completes forimmediate contention for thechannel)

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S-MAC Energy Savings

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S-MAC Performance

Energy consumption

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S-MAC Performance

Latency

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S-MAC Performance

Energy vs. Latency and Throughput