Wireless Sensor Networks

Wireless Sensor Networks Presenter: Carlos Pomalaza-Ráez carlos@ee.oulu.fi International Workshop on Wireless Ad Hoc NetworksMay 31 – June 3, 2004 University of Oulu, Finland http://www.ee.oulu.fi/~carlos/IWWAN_04_WSN_Tutorial.ppt

Outline • Introduction • Examples of sensor networks and sensor nodes • WIRO – A sensor node developed at CWC • Typical features of WSN • Design considerations • Sensor Network Protocol Stack • Energy consumption model – Physical layer • MAC power saving mechanisms • Data aggregation and Data centrality • Transport and Applications layers

Outline • Networking Issues • MAC • Routing • Transport layer • Summary • Energy Efficiency Issues • Node energy model for multihop WSN • Energy efficient error control mechanisms • Cooperative communications • Distributed source coding

Introduction What is a sensor? A device that produces a measurable response to a change in a physical or chemical condition, e.g. temperature, ground composition. • Sensor Networks • A large grouping of low-cost, low-power, multifunctional, and small-sized sensor nodes • They benefit from advances in 3 technologies: • digital circuitry • wireless communication • silicon micro-machining

Sensing Networking Computation Wireless Sensor Networks (WSN) New technologies have reduced the cost,size, and power of micro-sensors and wireless interfaces Circulatory Net EnvironmentalMonitoring Structural

Some Applications of WSN • Battlefield Detection, classification and trackingExamples: AWAIRS (UCLA & Rockwell Science Center) • Habitat Monitoring Micro-climate and wildlife monitoring Examples: • ZebraNet (Princeton) • Seabird monitoring in Maine’s Great Duck Island (Berkeley & Intel)

Some Applications of WSN • Structural, seismic Bridges, highways, buildings Examples: Coronado Bridge San Diego (UCSD), Factory Building (UCLA) • Smart roads Traffic monitoring, accident detection, recovery assistance Examples: ATON project (UCSD) highway camera microphone • Contaminants detectionExamples: Multipurpose Sensor Program (Boise State University)

WSN Communications Architecture Sensing node Sensor nodes can be data originators and data routers Internet Sink Manager Node Sensor nodes Sensor field

Examples of Sensor Nodes

Sensor Node Evolution

CPU board - Controls all other WIRO boards and is needed in all WIRO stacks. It has an AVR Mega128 microcontroller running at 7.37 MHz and a 4 Mb serial flash memory. The CPU has a 128 kB flash memory for programs, 4 kB of SRAM, and 8 ADCs • RF board – It has an RFM model TR3100 radio transceiver chip capable of up to 576 kbps speeds. The radio interface on this board is configured for 230.4 kbps. Data encoding and decoding can use the onboard CPLD (Complex Programmable Logic Device) or the microcontroller on the CPU board. The transceiver uses ASK modulation • Power supply board – It has electronics to charge a battery pack from the USB bus and to provide the other boards with 5V, +3.3V and +1.8V voltages • Sensor board – It has a 2-axis accelerometer, a 2-axis magnetometer, as well as pressure, temperature and humidity sensors • Prototype board and Test-Pad board WIRO Platform WIRO (WIreless Research Object ) is a modular embedded system developed by the Centre for Wireless Communications, Oulu, Finland. The system consists of a set of boards 35 mm x 35 mm in size. They are: CPU Board 2 Euro coin & RF Board WIRO Box

Tx Rx Sleep WIRO – Power Consumption RF Board Total Power Consumption

Active Sleep WIRO – Power Consumption Sensor Board Total Power Consumption

WIRO – Power Consumption

Typical Features of WSN • A very large number of nodes, often in the order of thousands • Asymmetric flow of information, from the observers or sensor nodes to a command node • Communications are triggered by queries or events • At each node there is a limited amount of energy which in many applications is impossible to replace or recharge • Almost static topology • Low cost, size, and weight per node • Prone to failures • More use of broadcast communications instead of point-to-point • Nodes do not have a global ID such as an IP number • The security, both physical and at the communication level, is more limited than conventional wireless networks

Design Considerations • Fault tolerance – The failure of nodes should not severely degrade the overall performance of the network • Scalability – The mechanism employed should be able to adapt to a wide range of network sizes (number of nodes) • Cost – The cost of a single node should be kept very low • Power consumption – Should be kept to a minimum to extend the useful life of network • Hardware and software constraints – Sensors, location finding system, antenna, power amplifier, modulation, coding, CPU, RAM, operating system • Topology maintenance – In particular to cope with the expected high rate of node failure • Deployment – Pre-deployment mechanisms and plans for node replacement and/or maintenance • Environment – At home, in space, in the wild, on the roads, etc. • Transmission media – ISM bands, infrared, etc.

Sensor Network Protocol Stack Power Management – How the sensor uses its power, e.g. turns off its circuitry after receiving a message. Application Mobility Management – Detects and registers the movements of the sensor nodes Task Management Mobility Management Transport Power Management Network Task Management – Balances and schedules the sensing tasks given to a specific region Data Link Physical

Physical Layer • Frequency selection – The use of the industrial, scientific, and medical (ISM) bands has often been proposed • Carrier frequency generation and Signal detection – Depend on the transceiver and hardware design constraints which aim for simplicity, low power consumption, and low cost per unit • Modulation • Binary and M-ary modulation schemes can transmit multiple bits per symbol at the expense of complex circuitry • Binary modulation schemes are simpler to implement and thus deemed to be more energy-efficient for WSN applications • Low transmission power and simple transceiver circuitry make Ultra Wideband (UWB) an attractive candidate • Baseband transmission, i.e. no intermediate or carrier frequencies • Generally uses pulse position modulation • Resilient to multipath • Low transmission power and simple transceiver circuitry Application Transport Network Data Link Physical

Physical Layer Energy consumption minimization is of paramount importance when designing the physical layer for WSN in addition to the usual effects such as scattering, shadowing, reflection, diffraction, multipath, and fading. Radio Model – Energy Consumption ETC = energy used by the transmitter circuitry ETA = energy required by the transmitter amplifier to achieve an acceptable signal to noise ratio at the receiver

Physical Layer Assuming a linear relationship for the energy spent per bit by the transmitter and receiver circuitry eTC, eTA, and eRC are hardware dependent parameters An explicit expression for eTA can be derived as,

Physical Layer (S/N)r = minimum required signal to noise ratio at the receiver’s demodulator for an acceptable Eb/N0 NFRx =receiver noise figure N0 = thermal noise floor in a 1 Hertz bandwidth (Watts/Hz) BW = channel noise bandwidth λ = wavelength in meters α = path loss exponent whose value varies from 2 (for free space) to 4 (for multipath channel models) Gant = antenna gain ηamp = transmitter power efficiency Rbit = raw bit rate in bits per second

Code Time Frequency Data Link Layer The data link layer is responsible for the multiplexing of the data stream, data frame detection, medium access and error control. Ensures reliable point-to-point and point-to-multipoint connections in a communication network Application Transport Network Data Link Physical • Medium Access Control (MAC) • Let multiple radios share the same communication media • Functions: • Local Topology Discovery and Management • Media Partition By Allocation or Contention • Provide Logical Channels to Upper Layers MAC protocols for sensor networks must have built-in power conservation mechanisms, and strategies for the proper management of node mobility or failure

Wireless MAC Protocols Wireless MAC protocols can be classified into two categories, distributed and centralized, according to the type of network architecture for which they have been designed. Protocols can be further classified, based on the mode of operation, into random access protocols, guaranteed access protocols, and hybrid access protocols Wireless MAC protocols DistributedMAC protocols CentralizedMAC protocols Randomaccess Randomaccess Guaranteedaccess Hybridaccess Since it is desirable to turn off the radio as much as possible in order to conserve energy some type of TDMA mechanism is often suggested for WSN applications. Constant listening times and adaptive rate control schemes have also been proposed.

Power Saving Mechanisms • The amount of time and power needed to wake-up (start-up) a radio is not negligible and thus just turning off the radio whenever it is not being used is not necessarily efficient • The energy characteristics of the start-up time should also be taken into account when designing the size of the data link packets. The values shown in the figure below clearly indicate that when the start-up energy consumption is taken into account the energy per bit requirements can be significantly higher for the transmission of short packets than for longer ones

Error Control Error control is an important issue in any radio link. In general terms there are two modes of error control: • Forward Error Correction (FEC) – There is a direct tradeoff between the overhead added to the code and the number of errors that can be corrected. The number of bits in the code word impacts the complexity of the receiver and transmitter. If the associated processing power is greater than the coding gain, then the whole process in energy inefficiency. • Automatic Repeat Request (ARQ) – Based on the retransmission of packets that have been detected to be in error. Packets carry a checksum which is used by the receiver to detect errors. Requires a feedback channel. With FEC one pays an a priori battery power consumption overhead and packet delay by computing the FEC code and transmitting the extra code bits. In return one gets a reduced probability of packet loss. With ARQ one gambles that the packet will get through and if it does not one has to pay battery energy and delay due to the retransmission process. Whether FEC or ARQ or a hybrid error control system is energy efficient will depend on the channel conditions and the network requirements such as throughput and delay.

Network Layer Basic issues to take into account when designing the network layer for WSNs are: Application Transport Network • Power efficiency • Data centric – The nature of the data (interest requests and advertisement of sensed data) determines the traffic flow • Data aggregation is useful to manage the potential implosion of traffic because of the data centric routing • Rather than conventional node addresses an ideal sensor network uses attribute-based addressing, e.g. “region where humidity is below 5%” • Locationing systems, i.e. ability for the nodes to establish position information • Internetworking with external networks via gateway or proxy nodes Data Link Physical

Routing Phenomenonbeing sensed Data aggregation takes place here Sink Multihop routing is common due to limited transmission range • Low node mobility • Power aware • Irregular topology • MAC aware • Limited buffer space Some routing issues in WSNs

Data Aggregation It is a technique used to solve the problem of implosion in WSNs. This problem arises when packets carrying the same information arrive at a node. This situation can happen when more than one node senses the same phenomenon. This is different than the problem of “duplicate packets” in conventional ad hoc networks. Here it is the high level interpretation of the data in the packets is that determines if the packets are the “same.” Even for the case when the packets are deemed to be different they could still be aggregated into a single packet before the relaying process continues. In this regard data aggregation can be considered as data fusion. Data coming from multiple sensor nodes are aggregated, if they have about the same attributes of the phenomenon being sensed, when they reach a common routing or relaying node on their way to the sink. In this view the routing mechanism in a sensor network can be considered as a form of reverse multicast tree. Phenomenon being sensed

Data Centrality In data-centric routing, an “interest ” dissemination is performed in order to assign the sensing tasks to the sensor nodes. This dissemination can take different forms such as: • The sink or controlling nodes broadcast the nature of the interest, e.g. “four legged animals of at least 50 Kg in weight” Four-legged animal of at least 50 Kg Sink Flow of the request

Data Centrality • Sensor nodes broadcast an advertisement of available sensed data and wait for a request from the interested sinks Tiger, tiger, burning bright,In the forest of the night,What immortal hand or eyeCould frame thy fearful symmetry? Flow of the advertisement Sink

Flooding & Gossiping Flooding is a well known technique used to disseminate information across a network. It is a simple, easy to implement reactive mechanism that could be used for routing in WSNs but it has severe drawbacks such as, • Implosion – When duplicated messages are sent to the same node • Overlap – When two or more nodes share the same observing region, they may sense the same stimuli at the same time. As a result, neighbor nodes receive duplicated messages • Resource blindness – Does not take into account the available energy resources. Control of energy consumption is of paramount importance in WSNs, a promiscuous routing technique such as flooding wastes energy unnecessarily Gossiping is a variation of flooding attempting to correct some of its drawbacks. Nodes do not indiscriminately broadcast but instead send a packet to a randomly selected neighbor who upon receiving the packet, repeats the process. It is not as simple to implement as the flooding mechanism and it takes longer for the propagation of messages across the network.

Proposed Routing Techniques SPIN – Sensor Protocols for Information via Negotiation(†) – Attempts to correct the major deficiencies of classical flooding, in particular the indiscriminate flow of packets with the related energy waste. The sensor nodes minimize the amount of traffic and transmissions by first sending an advertisement of the nature of the sensed data in a concise manner followed by the transmission of the actual data to only those nodes that are interested in receiving it. ADV • SPIN messages • ADV- advertise data • REQ- request specific data • DATA- requested data • Resource management • Nodes decide their capability of participation in data transmissions A B REQ A B DATA A B (†)W. Heinzelman, J. Kulik, and H. Balakrishnan, “Adaptive Protocols for Information Dissemination in Wireless Sensor Networks,” Proc. 5th ACM/IEEE Mobicom Conference (MobiCom '99), Seattle, WA, August, 1999.

Proposed Routing Techniques Data Funneling(†) – Attempts to minimize the amount of communication from the sensors to the information consumer node (sink). It facilitates data aggregation and tries to concentrate, e.g. funnel, the packet flow into a single stream from the group of sensors to the sink. It also attempts to reduce (compress) the data by taking advantage that the destination is not that interested in a particular order of how the data packets arrive. Setup phase: • Controller divides the sensing area into regions • Controller performs a directional flood towards each region • When the packet reaches the region the first receiving node becomes a border node and modifies the packet (add fields) for route cost estimations within the region • The border node floods the region with modified packet • Sensor nodes in the region use cost information to schedule which border nodes to use (†) D. Petrovic, R. C. Shah, K. Ramchandran, and J. Rabaey, “Data Funneling: Routing with Aggregation and Compression for Wireless Sensor Networks,” SNPA 2003, pp. 1-7.

Proposed Routing Techniques Data Funneling Data Communication Phase: • When a sensor has data it uses the schedule to choose the border node that is to be used • It then waits for time inversely proportional to the number of hops from the border • Along the way to the border node, the data packets join together until they reach the border node • The border node collects all packets and then sends one packet with all the data back to the controller

Transport Layer TCP variants developed for the traditional wireless networks are not suitable for WSNs where the notion of end-to-end reliability has to be reinterpreted due to the “sensor” nature of the network which comes with features such as: Application Transport Network Data Link • Multiple senders, the sensors, and one destination, the sink, which creates a reverse multicast type of data flow Physical • For the same event there is high level of redundancy or correlation in the data collected by the sensors and thus there is no need for end-to-end reliability between individual sensors and the sink but instead between the event and the sink • On the other hand there is need of end-to-end reliability between the sink and individual nodes for situations such as re-tasking or reprogramming • The protocols developed should be energy aware and simple enough to be implemented in the low-end type of hardware and software of many WSN applications

Proposed Transport Layer Techniques Pump Slowly, Fetch Quickly (PSFQ)(†) – Designed to distribute data from a source node by pacing the injection of packets into the network at relatively low speed (pump slowly) which allows nodes that experience data loss to aggressively recover missing data from their neighbors (fetch quickly). Goals of this protocol are: • Ensure that all data segments are delivered to the intended destinations with minimum special requirements on the nature of the lower layers • Minimize number of transmissions to recover lost information • Operate correctly even in situations where the quality of the wireless links is very poor • Provide loose delay bounds for data delivery to all intended receivers PFSQ has been designed to guarantee sensor-to-sensor delivery and to provide end-to-end reliability for control management distribution from the control node (sink) to the sensors. It does not address congestion control (†) C-Y Wan, A. T. Campbell, and L. Krishnamurthy, “PSFQ: A Reliable Transport Protocol For Wireless Sensor Networks,” First ACM International Workshop on Wireless Sensor Networks and Applications (WSNA 2002), Atlanta, September 28, 2002, pp. 1-11.

Proposed Transport Layer Techniques Event-to-Sink Reliable Transport (ESRT) (†) – Designed to achieve reliable event detection (at the sink node) with a protocol that is energy aware and has congestion control mechanisms. Salient features are: • Self-configuration – even in the case of a dynamic topology • Energy awareness – sensor nodes are notified to decrease their frequency of reporting if the reliability level at the sink node is above the minimum • Congestion control – takes advantage of the high level of correlation between the data flows corresponding to the same event • Collective identification – sink only interested in the collective information from a group of sensors, not in their individual reports (†) Y. Sankarasubramaniam, O. B. Akan, and I. F. Akyildiz, “ESRT: Event-to-Sink Reliable Transport in Wireless Sensor Networks” Proceedings of ACM MobiHoc`03, Annapolis, Maryland, USA, June 2003, pp. 177-188.

Application Layer There has not been as much development for this layer as for the other layers. Several general potential areas have been suggested as listed below but little work of substance has been reported in any particular area Application Transport Network Data Link • Sensor Management Protocol (SMP) – Carries out tasks such as: • Turning sensors on and off • Exchanging data related to the location finding algorithms • Authentication, key distribution, and other security tasks • Sensor movement management Physical • Interest Dissemination – Interest is sent to a sensor or a group of sensors. The interest is expressed in terms of an attribute or a triggering event. • Advertisement of Sensed Data – Sensor nodes advertise sensed data in a concise and descriptive way and users reply with requests of data they are interested in receiving

Distributed Source Coding (DSC) Aims to take advantage of the high level of correlation of the data collected by spatially close sensor nodes in response to an event. Application Layer The goal is to remove this redundancy in a distributed manner. There is the need to be able to make reliable decisions from the contribution of a large number of individual unreliable components with a considerable amount of system redundancy. Any method that can strip this redundancy in a distributed manner, e.g. minimizing inter-node communications, will make efficient use of the bandwidth and also save energy. One way to remove the redundancy is by joint processing based on exchange of information between the sensors(†). Proposed DSC methods make use of the Slepian-Wolf coding theorem that states that if the joint distribution quantifying the sensor correlation structure is known then there is no theoretical loss in performance using DSC under certain conditions. (†) S. Pradhan and K. Ramchandran, “Distributed Source Coding Using Syndromes (DISCUS): Design and Construction,” IEEE Trans. Information Theory, vol. 49, no. 3, March 2003, pp. 626-643

Distributed Source Coding (DSC) X Encoder 1 Joint Decoder Y Encoder 2 The encoders collaborate and a rate of H(X,Y) is sufficient X Encoder 1 Joint Decoder Y Encoder 2 The encoders do not collaborate. The Slepian-Wolf theorem says that a rate H(X,Y) is also sufficient provided decoding of X and Y is done jointly. It puts more burden on the decoding side

Some Words About Cross-Layer Design Motivations: • Avoid Conflicting Behavior – For example a routing protocol that favors smaller hops to save transmission energy consumption does require a proper MAC protocol to coordinate the transmissions along the data flow that minimizes contention and keeps the transceivers off as much as possible • Remove Unnecessary Layers – Some applications do not require all layers • New Paradigm – WSNs do not have many of the features of the conventional networks for which the OSI protocol layer stack model has proven to be successful. Therefore it is quite possible that a different mix of layers might prove to be more efficient for many WSN applications

Networking Issues • Unlike conventional wireless networks, the protocols designed for the efficient networking of nodes in a WSN have to allow for a closer collaboration or awareness among the layers of the protocol stack, in particular the first three layers Application Transport Network Data Link • For example, the MAC protocols must try to have the radio transceivers in a sleeping mode as much as possible in order to save energy, however if the MAC protocol is not jointly designed with the routing algorithms (network layer) the overall performance of the network could be severely degraded, e.g. excessive packet delay Physical • Conversely, WSN routing algorithms designed with the concepts of data centric and data aggregation create special requirements on the underlying MAC protocols that should be met for the routing mechanisms to work as intended • These observations can be extended to the design of other layers as well since WSNs call for new networking paradigms

Example of a MAC Protocol for WSN Sensor-MAC (S-MAC)(†) – Is an energy-aware protocol that illustrates design considerations that MAC protocols for WSNs should address. Assumptions made in the design of S-MAC are: Data Link • Most communications will be between neighboring sensor nodes rather than between a node and a base station • There are many nodes that are deployed in a casual, e.g. not precise, manner and as such the nodes must be able to self-configure • The sensor nodes are dedicated to a particular application and thus per-node fairness (channel access) is not as important as the application level performance • Since the network is dedicated to a particular application the application data processing can be distributed through the network. This implies that data will be processed as whole messages at a time in store-and-forward fashion allowing for the application of data aggregation techniques which can reduce the traffic • The application can tolerate latency and has long idle periods (†)W. Ye, J. Heidemann and D. Estrin, “An Energy-Efficient MAC Protocol for Wireless Sensor Networks,” In Proceedings of the 21st International Annual Joint Conference of the IEEE Computer and Communications Societies (INFOCOM 2002), New York, NY, USA, June, 2002, pp. 1-10.

Sensor-MAC (S-MAC) The main features of S-MAC are: • Periodic listen and sleep • Collision and Overhearing avoidance • Message passing The basic scheme for each node is: • Each node goes into periodic sleep mode during which it switches the radio off and sets a timer to awake later • When the timer expires it wakes up and listens to see if any other node wants to talk to it • The duration of the sleep and awake cycles are application dependent and they are set the same for all nodes • Requires a periodic synchronization among nodes to take care of any type of clock drift

Sensor-MAC (S-MAC) • The listen and awake periods are much longer than typical clock drift rates • The duration of the sleep and awake cycles are application dependent and they are set the same for all nodes • Unlike conventional TDMA schemes S-MAC tolerates a much looser synchronization among neighboring nodes • Requires a periodic synchronization among nodes to take care of any type of clock drift • Nodes are free to choose their own listen/sleep schedules but to reduce control overhead the protocol prefers that neighboring nodes are synchronized • Because of the multihop scenario not all neighbors can be synchronized, e.g. Nodes A and B are neighbors but they are synchronized to their “other” neighbors, C and D respectively. Nodes broadcast their schedules from time to time to ensure that neighboring nodes can talk to each other even if they have different schedules. If multiple neighbors want to talk to a node, they need to contend for the medium.

Sensor-MAC (S-MAC) Choosing and Maintaining Schedules • Each node maintains a schedule table that stores schedules of all its known neighbors • To establish the initial schedule the following steps are followed: • A node first listens for a certain amount of time • If it does not hear a schedule from another node, it randomly chooses a schedule and broadcasts its schedule immediately • This node is called a Synchronizer • If a node receives a schedule from a neighbor before choosing its own schedule, it just follows this neighbor’s schedule, i.e. becomes a Follower and it waits for a random delay and broadcasts its schedule • If a node receives a neighbor’s schedule after it selects its own schedule, it adopts both schedules and broadcasts its own schedule before going to sleep • It is expected that very rarely a node adopts multiple schedules since every node tries to follow existing schedules before choosing an independent one

Sensor-MAC (S-MAC) Maintaining Synchronization • Timer synchronization among neighbors is needed to prevent clock drift. The updating period can be relatively long (tens of seconds) • Done by periodically sending a SYNC packet that only includes the address of the sender and the time of its next sleeping period • Time of next sleep is relative to the moment that the sender finishes transmitting the SYNC packet • A node will go to sleep when the timer fires • Receivers will adjust their timer counters immediately after they receive the SYNC packet • A node periodically broadcasts a SYNC packet to its neighbors even if it has no followers

Sensor-MAC (S-MAC) Maintaining Synchronization(cont.) • Listen interval is divided into two parts: one for receiving SYNC packets and the other for receiving RTS (Request To Send)

Sensor-MAC (S-MAC) Collision and Overhearing Avoidance • Similar to IEEE 802.11, i.e. use RTS/CTS mechanism to address the hidden terminal problem • Perform carrier sense before initiating a transmission • If a node fails to get the medium, it goes to sleep and wakes up when the receiver is free and listening again • Broadcast packets are sent without RTS/CTS • Unicast packets follow the sequence of RTS/CTS/DATA/ACK between the sender and receiver • Duration field in each transmitted packet indicates how long the remaining transmission will be, so if a node receives a packet destined for another node, it knows how long it has to keep silent • The node records this value in network allocation vector (NAV) and sets a timer for it • When a node has data to send, it first looks at NAV. If this value is not zero, then the medium is busy (virtual carrier sense) • The medium is determined as free if both virtual and physical carrier sense indicate the medium is free • All immediate neighbors of both the sender and receiver should sleep after they hear the RTS or CTS packet until the current transmission is over

Sensor-MAC (S-MAC) • Message Passing • A message is a collection of meaningful, interrelated units of data • Transmitting a long message as a packet is disadvantageous as the re-transmission cost is high if the packet is corrupted • Fragmentation into small packets will lead to high control overhead as each packet should contend using RTS/CTS • S-MAC fragments message into small packets and transmits them as a burst • Only one RTS and one CTS packets are used • Every time a data fragment is transmitted the sender waits for an ACK from the receiver, if it does not arrive the fragment is retransmitted and the reservation is extended for the duration of the fragment • Advantages: • Reduces latency of the message • Reduces control overhead • Disadvantage: • Node-to-node fairness is reduced, as nodes with small packets to send will have to wait until the message burst is transmitted

Wireless Sensor Networks