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Chapter 12 Wireless Sensor Networks


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Chapter 12 Wireless Sensor Networks Outline • • • • • • • • • 12.1 Introduction 12.2 Sensor Network Architecture 12.3 Data Dissemination 12.4 Data Gathering 12.5 MAC Protocols for Sensor Networks 12.6 Location Discovery 12.7 Quality of a Sensor Network 12.8 Evolving Standards 12.9 Other Issues 12.1 Introduction • Sensor networks are highly distributed networks of small, lightweight wireless node, deployed in large numbers to monitor the environment or system. • Each node of the sensor networks consist of three subsystem: – Sensor subsystem: senses the environment – Processing subsystem: performs local computations on the sensed data – Communication subsystem: responsible for message exchange with neighboring sensor nodes • The features of sensor nodes – Limited sensing region, processing power, energy • The advantage of sensor networks – – – – Robust : a large number of sensors Reliable : Accurate : sensor networks covering a wider region Fault-tolerant : many nodes are sensing the same event • Two important operations in a sensor networks – Data dissemination : the propagation of data/queries throughout the network – Data gathering : the collection of observed data from the individual sensor nodes to a sink • The different types of sensors – Seismic, thermal, visual, infrared 12.1.1 Applications of Sensor Networks • Using in military – Battlefield surveillance and monitoring, guidance systems of intelligent missiles, detection of attack by weapons of mass destruction such as chemical, biological, or nuclear • Using in nature – Forest fire, flood detection, habitat exploration of animals • Using in health – Monitor the patient’s heart rate or blood pressure, and sent regularly to alert the concerned doctor, provide patients a greater freedom of movement • Using in home (smart home) – Sensor node can built into appliances at home, such as ovens, refrigerators, and vacuum cleaners, which enable them to interact with each other and be remote-controlled • Using in office building – Airflow and temperature of different parts of the building can be automatically controlled • Using in warehouse – Improve their inventory control system by installing sensors on the products to track their movement 12.1.2 Comparison with Ad Hoc Wireless Networks • Different from Ad Hoc wireless networks – The number of nodes in sensor network can be several orders of magnitude large than the number of nodes in an ad hoc network. – Sensor nodes are more easy to failure and energy drain, and their battery sources are usually not replaceable or rechargeable. – Sensor nodes may not have unique global identifiers (ID), so unique addressing is not always feasible in sensor networks. – Sensor networks are data-centric, the queries in sensor networks are addressed to nodes which have data satisfying some conditions. Ad Hoc networks are address-centric, with queries addressed to particular nodes specified by their unique address. – Data fusion/aggregation: the sensor nodes aggregate the local information before relaying. The goals are reduce bandwidth consumption, media access delay, and power consumption for communication. 12.1.3 Issues and Challenges in Designing a Sensor Network • Issues and Challenges – Sensor nodes are randomly deployed and hence do not fit into any regular topology. Once deployed, they usually do not require any human intervention. Hence, the setup and maintenance of the network should be entirely autonomous. – Sensor networks are infrastructure-less. Therefore, all routing and maintenance algorithms need to be distributed. – Energy problem – Hardware and software should be designed to conserve power – Sensor nodes should be able to synchronize with each other in a completely distributed manner, so that TDMA schedules can be imposed. – A sensor network should also be capable of adapting to changing connectivity due to the failure of nodes, or new nodes powering up. The routing protocols should be able to dynamically include or avoid sensor nodes in their paths. – Real-time communication over sensor networks must be supported through provision of guarantees on maximum delay, minimum bandwidth, or other QoS parameters. – Provision must be made for secure communication over sensor networks, especially for military applications which carry sensitive data. Figure 12.1 Classification of sensor network protocol 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 12.2 Sensor Network Architecture • The two basic kinds of sensor network architecture – Layered Architecture – Clustered Architecture 12.2.1 Layered Architecture • A layered architecture has a single powerful base station, and the layers of sensor nodes around it correspond to the nodes that have the same hop-count to the BS. • In the in-building scenario, the BS acts an access point to a wired network, and small nodes form a wireless backbone to provide wireless connectivity. • The advantage of a layered architecture is that each node is involved only in short-distance, low-power transmissions to nodes of the neighboring layers. Figure 12.2 Layered architecture 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 Unified Network Protocol Framework (UNPF) • UNPF is a set of protocols for complete implementation of a layered architecture for sensor networks • UNPF integrates three operations in its protocol structure: – Network initialization and maintenance – MAC protocol – Routing protocol Network initialization and maintenance • The BS broadcasts its ID using a known CDMA code on the common control channel. • All node which hear this broadcast then record the BS ID. They send a beacon signal with their own IDs at their low default power levels. • Those nodes which the BS can hear form layer one • BS broadcasts a control packet with all layer one node IDs. All nodes send a beacon signal again. • The layer one nodes record the IDs which they hear (form layer two) and inform the BS of the layer two nodes IDs. • Periodic beaconing updates neighbor information and change the layer structure if nodes die out or move out of range. MAC protocol • During the data transmission phase, the distributed TDMA receiver oriented channel (DTROC) assignment MAC protocol is used. • Two steps of DTROC : – Channel allocation : Each node is assigned a reception channel by the BS, and channel reuse is such that collisions are avoided. – Channel scheduling : The node schedules transmission slots for all its neighbors and broadcasts the schedule. This enables collision-free transmission and saves energy, as nodes can turn off when they are not involved on a send/receive operation. Routing protocol • Downlink from the BS is by direct broadcast on the control channel. Uplink from the sensor nodes to BS is by multi-hop data forwarding. • The node to which a packet is to be forwarded is selected considering the remaining energy of the nodes. This achieves a higher network lifetime. UNPF-R • • • Optimize the network performance by make the sensor nodes adaptively vary their transmission range. Because while a very small transmission range cause network partitioning, a very large transmission range reduce the spatial reuse of frequencies. The optimal range (R) is determined by simulated annealing – Objective function : • N : the total number of sensors • n : the number of nodes in layer one • : the energy consumption per packet • d : the average packet delay UNPF-R – If no packet is received by the BS from any sensor node for some interval of time, the transmission range increase by . Otherwise, the transmission range is either decrease by with probability 0.5 x ( n / N ), or increase by with probability [ 1 – 0.5 x ( n / N ) ]. – If , then the transmission range R’ is adopt. Otherwise, R is modified to R’ with probability • T : the temperature parameter – The advantage of the UNPF-R : • Minimize the energy x delay • Maximize the number of nodes which can connect to the BS 12.2.2 Clustered Architecture • A clustered architecture organizes the sensor nodes into clusters, each governed by a cluster-head. The nodes in each cluster are involved in message exchanges with their cluster-heads, and these heads send message to a BS. • Clustered architecture is useful for sensor networks because of its inherent suitability for data fusion. The data gathered by all member of the cluster can be fused at the cluster-head, and only the resulting information needs to be communicated to the BS. • The cluster formation and election of cluster-heads must be an autonomous, distributed process. Figure 12.3 Clustered architecture 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 Low-Energy Adaptive Clustering Hierarchy (LEACH) • LEACH is a clustering-based protocol that minimizes energy dissipation in sensor networks. The operation of LEACH is spilt into two phases : setup and steady. – Setup phase : each sensor node chooses a random number between 0 and 1. If this is lower than the threshold for node n, T(n), the sensor node becomes a cluster-head. The threshold T(n) is calculated as • P : the percentage of nodes which are cluster-heads • r : the current round • G : the set of nodes that has not been cluster-heads in the past 1/P rounds After selection, the cluster-heads advertise their selection to all nodes. All nodes choose their nearest cluster-head by signal strength (RSSI). The cluster-heads then assign a TDMA schedule for their cluster members. – Steady phase : data transmission takes place based on the TDMA schedule, and the cluster-heads perform data aggregation/fusion. After a certain period of time in the steady phase, cluster-heads are selected again through the setup phase. 12.3 Data Dissemination • Data dissemination is the process by which queries or data are routed in the sensor network. The data collected by sensor nodes has to be communicated to the node which interested in the data. • The node that generates data is call source and the information to be reported is called an event. A node which interested in an event is called sink. • Data dissemination consist of a two-step process : interest propagation and data propagation. – Interest propagation : for every event that a sink is interested in, it broadcasts its interest to is neighbor, and across the network. – Data dissemination : When an event is detected, it reported to the interested nodes (sink). 12.3.1 Flooding • Each node which receives a packet (queries/data) broadcasts it if the maximum hop-count of the packet is not reached and the node itself is not the destination of the packet. • Disadvantages : – Implosion : this is the situation when duplicate messages are send to the same node. This occurs when a node receives copies of the same messages from many of its neighbors. – Overlap : the same event may be sensed by more than one node due to overlapping regions of coverage. This results in their neighbors receiving duplicate reports of the same event. – Resource blindness : the flooding protocol does not consider the available energy at the nodes and results in many redundant transmissions. Hence, it reduces the network lifetime. 12.3.2 Gossiping • Modified version of blooding • The nodes do not broadcast a packet, but send it to a randomly selected neighbor. • Avoid the problem of implosion • It takes a long time for message to propagate throughout the network. • It does not guarantee that all nodes of network will receive the message. 12.3.3 Rumor Routing • Agent-based path creation algorithm • Agent is a long-lived packet created at random by nodes, and it will die after visit k hops. • It circulated in the network to establish shortest paths to events that they encounter. • When an agent finds a node whose path to an event is longer than its own, it updates the node’s routing table. Figure 12.4 Rumor routing 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 12.3.4 Sequential Assignment Routing (SAR) • The sequential assignment routing (SAR) algorithm creates multiple trees, where the root of each tree is a one-hop neighbor of the sink. • To avoid nodes with low throughput or high delay. • Each sensor node records two parameters about each path though it : available energy resources on the path and an additive QoS metric such as delay. – Higher priority packets take lower delay paths, and lower priority packets have to use the paths of greater delay, so that the priority x delay QoS metric is maintained. • SAR minimizes the average weighted QoS metric over the lifetime of the network. Figure 12.5 Sequential assignment routing 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 12.3.5 Directed Diffusion • The directed diffusion protocol is useful in scenarios where the sensor nodes themselves generate requests/queries for data sensed by other nodes. • Each sensor node names its data with one or more attributes. • Each sensor node express their interest depending on these attributes. • Each path is associated with a interest gradient, while positive gradient make the data flow along the path, negative gradient inhibit the distribution data along a particular path. – Example : two path formed with gradient 0.4 and 0.8, the source may twice as much data along the higher one – Suppose the sink wants more frequent update from the sensor which have detected an event => send a higher data-rate requirement for increasing the gradient of that path. • Query – Type = vehicle /* detect vehicle location interval = 1 s /* report every 1 second rect = [0,0,600,800] /* query addressed to sensors within the rectangle timestamp = 02:30:00 /* when the interest was originated expiresAt = 03:00:00 /* till when the sink retain interest in this data • Report – Type = vehicle /* type of intrusion seen instance = car /* particular instance of the type location = [200,250] /* location of node confidence = 0.80 /* confidence of match timestamp = 02:45:20 /* time of detection 12.3.6 Sensor Protocols for Information via Negotiation • SPIN use negotiation and resource adaptation to address the disadvantage of flooding. • Reduce overlap and implosion, and prolong network lifetime. • Use meta-data instead of raw data. • SPIN has three types of messages: ADV, REQ, and DATA. • SPIN-2 using an energy threshold to reduce participation. A node may join in the ADV-REQ-DATA handshake only if it has sufficient resource above a threshold. Figure 12.6 SPIN protocol 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 12.3.7 Cost-Field Approach • The cost-field approach considers the problem of setting up paths to a sink. The first phase being to set up the cost field, based on metrics such as delay. The second phase being data dissemination using the costs. • A sink broadcasts an ADV packet with its own cost as 0. • When a node N hears an ADV message from node M, it sets its own path cost to min (LN,LM+CNM), where LN is the total path cost from node N to the sink, LM is the cost of node M to the sink, CNM is the cost from N to M. • If LN updated, the new cost is broadcast though another ADV. • The back-off time make a node defer its ADV instead of immediately broadcast it. The back-off time is r x CMN, where r is a parameter of algorithm. Figure 12.7 Cost-field approach 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 12.3.8 Geographic Hash Table (GHT) • • • • GHT hashes keys into geographic coordinates and stores a (key, value) pair at the sensor node nearest to the hash value. Stored data is replicated to ensure redundancy in case of node failures. The data is distributed among nodes such that it is scalable and the storage load is balanced. The routing protocol used is greedy perimeter stateless routing (GPSR), which again uses geographic information to route the data and queries. 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 12.3.9 Small Minimum Energy Communication Network • • • If the entire sensor network is represented by G, the subgraph G’ is constructed such that the energy usage of the network is minimized. The number of edges in G’ is less than G, and the connectivity between any two nodes is not disrupted by G’. The power required to transmit data between u and v is modeled as – t : constant – n : loss exponent indicating the loss of power with distance from transmitter – d(u,v) : the distance between u and v • It would be more economical to transmit data by smaller hops • Suppose the path between u (i.e. u0) and v (i.e. uk) is represented by r = (u0, u1, … uk), each (ui, ui+1) is edge in G’ – The total power consumed for the transmission is • C : the power needed to receive the data • The path r is the minimum energy path if C(r) ≦ C(r’) for all path’s r’ between u and v in G. • SMECN uses only the ME paths from G’ for data transmission, so that the overall energy consumed is minimized. 12.4 Data Gathering • The objective of the data gathering problem is to transmit the sensed data from each sensor node to a BS. • The goal of algorithm which implement data gathering is – maximize the lifetime of network – Minimum energy should be consumed – The transmission occur with minimum delay • The energy x delay metric is used to compare algorithm 12.4.1 Direct Transmission • All sensor nodes transmit their data directly to the BS. • It cost expensive when the sensor nodes are very far from the BS. • Nodes must take turns while transmitting to the BS to avoid collision, so the media access delay is also large. Hence, this scheme performs poorly with respect to the energy x delay metric. 12.4.2 Power-Efficient Gathering for Sensor Information Systems • PEGASIS based on the assumption that all sensor nodes know the location of every other node. • Any node has the required transmission range to reach the BS in one hop, when it is selected as a leader. • The goal of PEGASIS are as following – – – – Minimize the distance over which each node transmit Minimize the broadcasting overhead Minimize the number of messages that need to be sent to the BS Distribute the energy consumption equally across all nodes • To construct a chain of sensor nodes, starting from the node farthest from the BS. At each step, the nearest neighbor which has not been visited is added to the chain. • It is reconstructed when nodes die out. • At every node, data fusion or aggregation is carried out. • A node which is designated as the leader finally transmits one message to the BS. • Leadership is transferred in sequential order. • The delay involved in messages reaching the BS is O(N) Figure 12.8 Data gathering with PEGASIS 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 12.4.3 Binary Scheme • • • This is a chain-based scheme like PEGASIS, which classifies nodes into different levels. This scheme is possible when nodes communicate using CDMA, so that transmissions of each level can take place simultaneously. The delay is O(logN) 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 12.4.4 Chain-Based Three-Level Scheme • For non-CDMA sensor nodes • The chain is divided into a number of groups to space out simultaneous transmissions in order to minimize interference. • Within a group, nodes transmit data to the group leader, and the leader fusion the data, and become the member to the next level. • In the second level, all nodes are divided into two groups. • In the third level, consists of a message exchange between one node from each group of the second level. • Finally, the leader transmit a single message to the BS. Figure 12.10 Chain-based three-level scheme 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 12.5 MAC Protocols for Sensor Networks • The challenges posed by sensor network MAC protocol – No single controlling authority, so global synchronization is difficult – Power efficiency issue – Frequent topology changes due to mobility and failure • There are three kinds of MAC protocols used in sensor network: – Fixed-allocation – Demand-based – Contention-based • Fixed-allocation MAC protocol – Share the common medium through a predetermined assignment. – It is suitable for sensor network that continuously monitor and generate deterministic data traffic – Provide a bounded delay for each node – However, in the case of bursty traffic, where the channel requirements of each node may vary over time, it may lead to inefficient usage of the channel. • Demand-based MAC protocol – Used in such cases, where the channel is allocated according to the demand of the node – Variable rate traffic can be efficiently transmitted – Require the additional overhead of a reservation process • Contention-based MAC protocol – Random-access-based contention for the channel when packets need to be transmitted – Suitable for bursty traffic – Collisions and no delay guarantees, are not suitable for delay-sensitive or real-time traffic 12.5.1 Self-Organizing MAC for Sensor Networks and Eavesdrop and Register • Self-Organizing MAC for sensor (SMACS) networks and eavesdrop and register (EAR) are two protocols which handle network initialization and mobility support, respectively. • In SMACS – neighbor discovery and channel assignment take place simultaneously in a completely distributed manner. – A communication link between two nodes consists of a pair of time slots, at fixed frequency. – This scheme requires synchronization only between communicating neighbors, in order to define the slots to be used for their communication. – Power is conserved by turning off the transceiver during idle slots. • In EAR protocol – Enable seamless connection of nodes under mobile and stationary conditions. – This protocol make use of certain mobile nodes, besides the existing stationary sensor nodes, to offer service to maintain connections. – Mobile nodes eavesdrop on the control signals and maintain neighbor information. 12.5.2 Hybrid TDMA/FDMA • A pure TDMA scheme minimize the time for which a node has to be kept on, but the associated time synchronization cost are very high. • A pure FDMA scheme allots the minimum required bandwidth for each connection • If the transmitter consumes more power, a TDMA scheme is favored, since it can be switch off in idle slots to save power. • If the receiver consumes greater power, a FDMA scheme is favored, because the receiver need not expend power for time synchronization. 12.5.3 CSMA-Base MAC Protocols • CSMA-based schemes are suitable for point-to-point randomly distributed traffic flows. • The sensing periods of CSMA are constant for energy efficiency, while the back-off is random to avoid repeated collisions. • Binary exponential back-off is used to maintain fairness in the network. • Use an adaptive transmission rate control (ARC) to balance originating traffic and route-through traffic in nodes. This ensures that nodes closer to the BS are not favored over farther nodes. • CSMA-based MAC protocol are contention-based and are designed mainly to increase energy efficiency and maintain fairness. 12.6 Location Discovery • During aggregation of sensed data, the location information of sensors must be considered. • Each nodes couple its location information with the data in the messages it sends. • GPS is not always feasible because it cannot reach nodes in dense foliage or indoor, and it consumes high power • We need a low-power, inexpensive, and reasonably accurate mechanism. 12.6.1 Indoor Localization • Fixed beacon nodes are placed in the field of observation, such as within building. • The randomly distributed sensors receive beacon signals from the beacon nodes and measure the signal strength, angle of arrival, time difference between the arrival of different beacon signals. • The nodes estimate distances by looking up the database instead of performing computations. • Only the BS may carry the database. 12.6.2 Sensor Network Localization • In situations where there is no fixed infrastructure available, some of the sensor nodes themselves act as beacons. • Using GPS, the beacon nodes have their location information, and send periodic beacons signal to other nodes. • In the case of communication using RF signals, the received signal strength indicator (RSSI) can be used to estimate the distance. • The time difference between beacon arrivals from different nodes can be used to estimate location. • Multi-lateration (ML) techniques – Atomic ML – Iterative ML – Collaborative ML Figure 12.11 Atomic multi-lateration 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 Figure 12.12 Iterative multi-lateration 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 Figure 12.13 Collaborative multi-lateration 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 • A mathematical technique called multi-dimensional scaling (MDS), an O(n3) algorithm, is used to assign location to node such that the distance constraints are satisfied. • To obtain the shortest distance between each pair of node. • If the actual positions of any three nodes in the network are known, then the entire network can be normalize. 12.7 Quality of a Sensor Network • The purpose of a sensor network is to monitor and report events take place in a particular area. • Hence, the main parameters which define how well the network observes a given area “coverage” and “exposure”. 12.7.1 Coverage • Coverage is a measure of how well the network can observe or cover an event. • The worst-case coverage defines area of breach, where coverage is the poorest. This can used to improve the deployment of network. • The best-case coverage defines the areas of best coverage. A path along the areas of best coverage is called maximum support path or maximum exposure path. • The coverage problem defined as follows: – – – – – A : a field with a set of sensors S : {s1, s2, …, sn}, where for each sensor si in S (xi, yi) : location coordinate I : initial locations of an intruder traversing F: final locations of an intruder traversing Worst-case • The problem is to identify PB, the maximal breach path from I to F. • PB is defined as the locus of points p in the region A, where p is in PB if the distance from p to the closest sensor is maximized. • Voronoi diagram : partitioning the plane into a set of convex polygon such that all points inside a polygon are closest to the site (sensor) enclosed by the polygon. • The algorithm to find the breach path PB is: – Generate the Voronoi diagram – Create a weighted graph, the weight of each edge in the graph is the minimum distance from all sensors in S. – Determine the maximum cost path from I to F, using BFS. Figure 12.14 Voronoi diagram 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 Best-case • The problem is to identify PS, the maximum support path from I to F. • Delaunay triangulation, which obtain from Voronoi diagram by connecting the sites whose polygons share a common edge. • The algorithm to find the breach path PS is: – Generate the Voronoi diagram – Generate the Delaunay triangulation – Create a weighted graph, the weight of each edge in the graph is the line segment lengths. – Determine the maximum cost path from I to F, using BFS. Figure 12.15 Delaunay triangulation 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 12.7.2 Exposure • • Exposure is defined as the expected ability of observing a target in the sensor field. The sensing power of a node s at point p is modeled as – λand k are constant – d(s,p) is the distance of p from s • All-sensor field intensity : • The closest sensor field intensity : • The exposure during travel of an event along a path p(t) is defined by the exposure function • is the elemental arc length, and t1,t2 are the time instance between which the path is traversed. For conversion from Cartesian coordinates (x(t),y(t)), • • In the simplest case of having one sensor node at (0,0) in a unit field, the breach path or minimum exposure path (MEP) from (-1,-1) to (1,1) . 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 • • • It can also be proved that for a single sensor s in a polygonal field, with vertices v1,v2,..,vn, the MEP between two vertices vi and vj can be determined as follows. The edge (vi,vi+1) is tangent to the inscribed circle at ui. MEP = edge (vi,ui) + arc (ui,uj) + edge (uj,vj) 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 • • • For the generic exposure problem of determining the MEP for randomly placed sensor node in the network, the network is tessellated with grid points To construct an n x n grid of order m, each side of a square is divided into m equal parts, creating (m+1) vertices on the edge. Determined the edge weights, and the MEP is defined as the shortest path by Dijkstra’s Algorithm. 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 12.8 Evolving Standards • The IEEE 802.15.4 low-rate wireless personal area network (LRWPAN) standard research a low data rate solution with multi-year battery life and very low complexity. It intended to operate in an unlicensed, international frequency band. The eighteenth draft of this standard was accepted in MAY 2003. • This standard define the physical and MAC layer specifications for sensor and other WPAN networks. Low power consumption is an important feature targeted by the standard. This requires reduced transmission rate, power efficient modulation techniques, and strict power management techniques such as sleep modes. • Other standard, SensIT project by DARPA which focuses on large distributed military system. 12.9 Other Issues • • • • • 12.9.1 Energy-Efficient Design 12.9.2 Synchronization 12.9.3 Transport Layer Issues 12.9.4 Security 12.9.5 Real-Time Communication 12.9.1 Energy-Efficient Design In node level : • Dynamic power management (DMP) – One of the basic DMP is to shut down several component of the sensor node when no events take place. • Dynamic voltage scaling (DVS) – The processor has a tome-varying computational load, hence the voltage supplied to it can be scaled to meet only the instantaneous processing requirement. – The real-time task scheduler should actively support DVS by predicting the computation and communication loads. • Sensor applications can also be trade-off between energy and accuracy. In network level : • The computation-communication trade-off determines how much local computation is to be performed at each node and what level of aggregated data should be communicated to neighbor node or BSs. • Traffic distribution and topology management algorithms use the redundancy in the number of sensor nodes to use alternate routes so that energy consumption all over the network is nearly uniform. 12.9.2 Synchronization • Two major kinds of synchronization algorithms : – Long-lasting global synchronization , (for entire network lifetime) – Short-lived synchronization, (only for an instant) • Synchronization protocols typically involve delay measurements of control packets. The delay experienced during a packet transmission can be split into four major components : – Send time : sender to construct message – Access time : taken by the MAC layer to access the medium – Propagation time : taken by the bit to be physically transmitted through the medium over the distance separating the sender and receiver – Receive time : receiver receive the message from the channel • The information of time obtained by GPS – Depend on the number of satellites observed by the GPS receiver – Not accuracy, 1µs (worst case) – Not suitable for building, basements, underwater, satellite-unreachable environment post facto • A low-power synchronization scheme • The clocks of the nodes are normally unsynchronized • When event is observed, a synchronization pulse(脈衝) is broadcast by a beacon node • Offer short-lived synchronization, creating only an “instant” of synchronization among the nodes which are within transmission range of the beacon node. • The propagation delay of the synchronization pulse is assumed to be the same for all nodes. Global synchronization protocol • Based on exchange of control signals between neighbor nodes. • A node becomes a leader by election • The leader periodically send synchronization messages to its neighbor, and these message are broadcast in turn to all nodes of network • Fault-tolerance techniques have been added to account for errors on the synchronization message Long-lasting synchronization protocol • Ensure global synchronization of a connect network or within connected partitions of a network • Each node maintain its own local clock (real clock) and a virtual clock to keep track of its leader’s clock • A unique leader is elected for each partition in the network, and virtual clocks are updated to match the leader’s real clock • The leader election : – A small probability (random number) be a leader – Broadcast Leader Announcement (claim) packet, which include the random number, node ID, time of the real clock – A node which receives this packet applies a correction for the propagation delay, and update its virtual clock – If two nodes stake a leadership, compare the random number and node ID, and resynchronizes to the small one • Resynchronization – Dynamic network – Take place in situations such as the merging of two partition due to mobility, where all clock in a partition may need to be updated to match the leader of the other partition. 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 Figure 12.20 shifting of frame on resynchronization • TDMA superframe • Presynch frame – Start and end of superframe • Control frame – Transmit control information • Data frame – TDMA time slots contain data 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 • A positive shift is defined as the transmission of a data packet at an absolute time later than slot in the current frame structure. • A negative shift is defined as advancing the start of a superframe to transmit the data packet earlier than the start of transmission in the current frame structure. – Some data frame will be lost – Buffer • But neighboring links may suffer collision when they follow different clock. Hence, as the resynchronization proceeds radially from the new leader, there is data loss along the head of the resynchronization wave. Out-of-band synchronization • Separate control channel for sending claim and beacon packets • Collision are reduced but the available bandwidth for data transmission is reduced • The cost of the mobile nodes increase because of the need for an additional radio interface In-band synchronization • Figure 12.21 (a) – Control information for synchronization shares the same channel with data packet – A greater number of collision, but avoids an additional channel or bandwidth reservation • Figure 12.21 (b) piggy-backed on data – Control information is piggy-backed onto outgoing data packet – Very low overhead and bandwidth saving. • Figure 12.21 (c) piggy-backed on ack – In data gathering, each sensor send the data to BS, the control information piggy-backed on ack, and move from BS to each node. Figure 12.21 In-band signaling 本圖取自"Ad Hoc Wireless Networks", by C. Siva Ram Murthy and B. S. Manoj, published by Prentice Hall, 2004 12.9.3 Transport Layer Issues • Reliable data delivery – Pump slowly fetch quickly (PSFQ) – Event-to-sink reliable transport (ESRT) Pump slowly fetch quickly (PSFQ) • PSFQ assumes that data loss is due to poor link rather than traffic congestion • The key concept : – Source node distributes data at a slow rate (pump slowly) – Receiver node which experiences data loss retrieve the missing data from immediate neighbors quickly • PSFQ consist of three functions : – Message relaying (pump) – Error recovery (fetch) – Selective status reporting (report) • Pump – Disseminates data to all target nodes, perform flow control, and localizes loss by ensuring caching at intermediate nodes – Hence, the errors on one link are corrected locally without propagating them down the entire path • Fetch – If receiver detect the loss of sequence numbers, it goes into fetch mode – It requests a retransmission from neighbor nodes – Many message losses are batched into a single fetch, which is especially suit for bursty losses. • Report – The farthest target node initiates its report on reverse path of data, and all intermediate nodes add their report – Hence, PSFQ ensure that data segment are delivery to all intended receiver in a scalable and reliable manner Event-to-sink reliable transport (ESRT) • Event-to-sink reliability in place of end-to-end reliability by the transport layer • The sink is required to track reliably only the collective report about the event and not individual reports from each sensor • Observed reliability : – the number of packets that are routed from event to sink • Required reliability : – The desired number of packets for the event to be successfully track • If observed reliability < required reliability ,ESRT increase report freq • Otherwise, decrease the reporting freq for saving energy 12.9.4 Security • The Sybil attack – When a single node presents itself as multiple entities to the network. This can affect the fault tolerance of the network and mislead geographic routing algorithms. • A selective forwarding attack – When certain nodes do not forward any of messages they receive • Sinkhole attack – A node act as BS or a very favorable to the routing – And do not forward any of messages it receive • Wormhole attack – Make the traffic through a very long path by giving false information to the node about the distance between them. – Increase latency • Hello flood attack – Broadcast a Hello packet with very high power, so that a large number of node even far away in the network choose it as the parent. – Increase delay Localized Encryption and Authentication Protocol (LEAP) • LEAP uses different keying mechanisms for different packets depending on their security requirements. • Every sensor node maintains four types of keys: – – – – Individual key : share with BS, preload into the node before deployment Group key : share with all node of the network and the BS Cluster key : share between a node and its neighbor Pairwise share key : share with each neighbor • A common initial key is loaded into each node before deployment. Each node obtain a master key by common key and unique ID. Nodes then exchange hello message, which authenticated by receiver. Compute the neighbor’s master key (by their ID and common key). Compute the shared key based on their master key. Clear the common key in all node after the establishment. • Since no one can get the common key, it is impossible to inject false data or decrypt the earlier exchange message. Also, no node can later forge the master key of any other node. • In this way, pairwise shared key are generated between all immediate neighbors. • The cluster key is established by a node after the pairwise key establishment. • Then group key is established by cluster key. Intrusion Tolerant Routing in Wireless Sensor Networks (INSENS) • The protocol cannot totally rule out attack on nodes, but minimizes the damage caused to network. • It constructs routing tables at each node, bypassing malicious node in the network. • Only BS is allowed to broadcast, no individual node can masquerade as the BS. • Control information about routing must be authenticated by BS, prevent injection of false data. • INSENS has two phase: route discovery and data forwarding • Route discovery phase: – – – – – BS send a request message to all node in the network by multi-hop Any node receiving a request, record the Id of sender. The nodes respond with their local topology by sending feedback The messages is protected using shared key mechanism. BS calculates forwarding table for all node • Data forwarding phase: – Transport data by the routing table. Security Protocol for Sensor Network (SPINS) • • For highly resource-constrained sensor network Two main modules: – Sensor network encryption protocol (SNEP) – Micro-version of time, efficient, streaming, loss-tolerant authentication protocol (mTESLA) • SNEP – Provide data authentication, protection from replay attack – Semantic encrypted, the same message is encrypted differently at different instance in time – Message integrity and confidentiality are maintained using a message authentication code (MAC) • mTESLA – The MAC keys are obtained from a chain of key and one-way function – All nodes have an initial key K0, which is some key in the key-chain – K0=F(K1), K1=F(K2),…, Ki=F(Ki+1) , and given K0…Ki it is impossible to compute Ki+1 12.9.5 Real-Time Communication • Used for surveillance or safety-critical system • Nuclear power plant • Two protocol which support real-time communication in sensor network: – SPEED – RAP SPEED • • • Provide real-time packet transmission Do not require routing table Distributes traffic and load equally across the network • • • SPEED require periodic beacon transmission between neighbor Use two on-demand beacons for delay estimation and congestion detection. Routing of packets is performed by stateless non-deterministic geographic forwarding (SNGF). Using geographic information, packet are forwarded only to the nodes which are closer to the destination. Among the closer nodes, the ones which have least delay have a higher probability of being chosen. If there is no nodes that satisfy the delay constraint, the packet is dropped. And reduce the sending rate to avoid congestion, until the delay is below the average. • • RAP • The application layer program in the BS can specify the kind of event information required, the area to which the query is address, and the deadline within which information is required. • The underlying layers of RAP ensure that the query is sent to all nodes in the specified area, and results are sent back to the BS. • Consist of location address protocol (LAP) , velocity monotonic scheduling (VMS) • LAP use location to address nodes instead of IP. It supports three kind of communication: unicast, area multicast, area anycast. • VMS is based on the concept of packet-requested velocity, which reflect both the timing and the distance constraint. The velocity of a packet is calculated as the ratio of the geographic distance between sender and receiver.