Chapter 2. Wireless Sensor Networks

Transcription

1 Chapter 2 Wireless Sensor Networks As from the previous chapter we get to know about the potential of wireless sensor networks and how it is useful in application. This chapter gives the detail study about routing challenges and design issues in wireless sensor networks, wireless sensor networks architecture and the routing protocols in WSN. Figure 2.1 Wireless Sensor Networks[20] 12

2 2.1 Routing Challenges and Design Issues in WSNs Node deployment: Node deployment in Wireless sensor networks plays very important role related with the performance of the routing protocols. Based on applications node deployment can be either deterministic or randomized. In deterministic node deployment, the sensors are manually placed and data is routed through pre-determined paths. In random node deployment, the sensor nodes are randomly positioned in an ad hoc manner. So, there are several issues related with random deployment as coverage, optimal clustering etc. which need to be addressed. Energy consumption without losing accuracy: Sensor nodes have limited supply required to performing computations and transmitting information in a wireless environment. Being energy deficient issues of energy consumption in WSN nodes need to be addressed. WSN network lifetime shows a strong dependence on the battery lifetime. In a multihop WSN, each node plays a dual role as data sender and data router. Power failure in some sensor nodes lead to malfunctioning of nodes can cause topological changes and might require rerouting of packets and reorganization of the network. Node/Link Heterogeneity: In case of many applications of sensor networks there might be requirement of deployment of a diverse mixture of sensor nodes with different types and capabilities. Data generation from different sensors nodes in WSN, can be of different rates. Similarly a network can follow different data reporting models and can be subjected to different quality of service constraints. In Such a heterogeneous environment routing techniques become more complex. Fault Tolerance: Fault tolerance is very important issue and deals with sensor nodes which may fail or be blocked due to lack of power, physical damage, or environmental interference. It is desired that the failure of any of the sensor nodes should not affect the overall task of the sensor network. MAC and routing protocols must address the issue in case of many node failures and need formation of new links and routes to the data collection base stations. This includes adjusting transmit powers and signaling rates on the existing links to reduce energy consumption, or rerouting packets in a fault-tolerant sensor network. 13

3 Scalability: In case of Wireless sensor networks in any sensing area deployment of thousands of nodes happens. It is required that every routing scheme operating in the network must be able to work with this huge number of sensor nodes. Addition to this, sensor network routing protocols should be scalable enough to respond to events in the environment. To address energy related issue it is also desired that in routings schemes sensor nodes can remain in the sleep state, till any event happens or initiates. This is also desirable because of data from the few remaining sensors providing a coarse quality. Network Dynamics: In most of the network architectures it is assumed that sensor nodes in a wireless sensor network are stationary. However in many applications mobility of both Base Stations and sensor nodes is sometimes necessary. In such cases routing messages from or to moving nodes becomes more challenging since route stability becomes an important issue, besides energy, bandwidth etc. Talking about applications of wireless sensor network the sensed phenomenon can be either dynamic or static, e.g., it is dynamic in a target detection/tracking application, while it is static in forest monitoring for early fire prevention. Monitoring static events allows the network to work in a reactive mode, simply generating traffic when reporting. Dynamic events in most applications require periodic reporting and consequently generate significant traffic to be routed to the BS. Transmission Media: In a multi-hop sensor network, communicating nodes are linked by a wireless medium. The traditional problems associated with a wireless channel (e.g., fading, high error rate) may also affect the operation of the sensor network. As the transmission energy varies directly with the square of distance therefore a multi-hop network is suitable for conserving energy. Connectivity: The connectivity of WSN depends on the radio coverage. If there continuously exists a multi-hop connection between any two nodes, the network is connected. Connectivity is intermittent if WSN is partitioned occasionally, and sporadic if the nodes are only occasionally in the communication range of other nodes. Coverage: The coverage of a WSN node means either sensing coverage or communication coverage. Typically with radio communications, the communication coverage is significantly larger than sensing coverage. For applications, the sensing coverage defines how to reliably guarantee that an event can be detected. The coverage of 14

4 a network is either sparse, if only parts of the area of interest are covered or dense when the area is almost completely covered. In case of a redundant coverage, multiple sensor nodes are in the same area. Data Aggregation: Sensor nodes usually generate significant redundant data. So, to reduce the number of transmission, similar packets from multiple nodes can be aggregated. Data aggregation is the combination of data from different sources according to a certain aggregation function, e.g., duplicate suppression, minima, maxima and average. It is incorporated in routing protocols to reduce the amount of data coming from various sources and thus to achieve energy efficiency. But it adds to the complexity and makes the incorporation of security techniques in the protocol nearly impossible. Data Reporting Model: Data sensing and reporting in WSNs is dependent on the application and the time criticality of the data reporting. In wireless sensor networks data reporting can be continuous, query-driven or event-driven. The data-delivery model affects the design of network layer, e.g., continuous data reporting generates a huge amount of data therefore, the routing protocol should be aware of data-aggregation Quality of Service: In some applications, data should be delivered within a certain period of time from the moment it is sensed; otherwise the data will be useless. Therefore bounded latency for data delivery is another condition for time-constrained applications. However, in many applications, conservation of energy, which is directly related to network lifetime, is considered relatively more important than the quality of data sent. As the energy gets depleted, the network may be required to reduce the quality of the results in order to reduce the energy dissipation in the nodes and hence lengthen the total network lifetime. Hence, energy-aware routing protocols are required to capture this requirement. 15

5 2.2 Sensor Network Architecture Before the introduction of architecture of WSN the point comes Why Wireless Sensor? if we have wired network and that fulfills the requirement and can be applied to all applications, This is answered with the help of Moore s Law as: 1. Moore s Law is making Sufficient CPU performance available with low power requirements in a small size. 2. Research in Material Science has resulted in novel sensing materials for many chemical, biological, and physical sensing tasks. 3. Transceivers for wireless devices are becoming smaller, less expensive, and less power hungry. 4. Power source improvements in batteries, as well as passive power sources such as solar or vibration energy, are expanding application options. Moore s law presents the main factors by which we get to know that why we need Wireless sensor network. Once we get to know the need of WSN, now the point comes here is the architecture because without knowing it one can not know about the network Architecture of a typical sensor network The two basic kinds of sensor network architecture are classified as layered and clustered architecture Layered Architecture A layered architecture has a single powerful base station (BS), and the layers of sensor nodes around it correspond to nodes that have the same hop-count to the base station (BS) as shown in fig Layered architectures have been used with in-building wireless backbones, and in military sensor-based infrastructure, such as the multi-hop infrastructure network architecture (MINA) [2]. In the in building scenario, BS becomes an access point to a wired network and small nodes form wireless backbone to provide wireless connectivity. The users use hand-held devices like PDAs to communicate to BS via small nodes. Unified Network Protocol Framework (UNPF [2]) is a set of protocols for complete implementation of a layered architecture for sensor networks. 16

6 Three-hop Two-hop One-hop Sensor Coverage BS Figure 2.2. Layered Architecture Clustered Architecture This architecture is based on the concept that higher energy nodes can be used to process and send the information while low energy nodes can be used to perform the sensing in the proximity of the target. Hence this architecture organized the sensor nodes into clusters. Each cluster is governed by a cluster-head[10]. The nodes in each cluster are involved in message exchanges with their respective cluster-heads and these heads send messages to a BS, which is usually an access point connected to a wired network. The creation of clusters and assigning special tasks to cluster heads can greatly contribute to overall system scalability, lifetime, and energy efficiency. Figure 2.3 shows this architecture where any message can reach the BS in at most two hops. Clustering can be extended to greater depths hierarchically. Clustering architecture is especially useful for sensor networks because of its inherent suitability for data fusion. (Data aggregation can be perceived as a set of automated methods of combining the data that comes from many sensor nodes into a set of meaningful information. With this respect, data aggregation is known as data fusion).the data gathered by all members of the cluster can be fused at the cluster-head and only the resulting information needs to be communicated to the BS. Sensor networks should be self-organizing, hence the cluster formation and election of cluster-heads must be an autonomous and a distributed process. This is achieved through network layer protocols which are mainly two layer routing techniques where one layer is used to select cluster heads and the other layer is used for routing. 17

7 Cluster Cluster-head-head Sensor node BS Flow of data Figure 2.3. Clustered Architecture However, most techniques in this category are not about routing, rather on who and when to send or process/aggregate the information. Channel allocation etc., which can be orthogonal to the multihop routing function. 2.3 OSI based Sensor network architecture The sensor network protocol architecture is a stack of following layers: Application layer, Transport layer, Network layer, Data Link layer, and Physical layer (figure 2.4). In addition to these layers wireless sensor network also requires following management planes in order to function efficiently: Power management plane, Mobility plane, and Task Planes. The functionality of Power Management Plane is to minimize power consumption which may include turn off functionality to preserve energy of the network. The function of Mobility Plane is to detect and register movement of nodes to maintain a data route to the sink. The Task plane deals with scheduling of the sensing tasks assigned to the sensor field. It balances among the active and functioning nodes as well as free nodes which are non functional so that free nodes can focus their energy on routing and data aggregation. These management planes play very important role in monitoring the power consumption in the network, movement of nodes, and task distribution among sensor nodes. Further these layers help in reducing power consumption by coordinating sensing tasks and routing. All protocols developed for wireless sensor networks must address all three of these planes. 18

8 Physical Layer Likewise the functioning of basic OSI network model the Physical Layer of wireless sensor networks deals with connectivity related issues if a wireless sensor network. In WSN frequency selection, carrier frequency generation, signal detection, modulation, and encryption is part of physical layer. Energy consumption minimization is major issue at this layer apart from other issues which are the same as those of other wireless networks. The minimum output power required to transmit over a distance d is proportional to d to a power of n, where n varies from 2 to 4 and is closer to four when the antennae are near the ground as is typical in wireless sensor networks[36]. This is due in part to groundreflected rays, which causes partial signal cancellation. This problem is overcome by multi-hop communication and high node density. Task Management Plane Mobility Management plane Power Management Plane Application Layer Transport Layer Network Layer Data Link Layer Physical layer Figure 2.4: OSI based sensor architecture 19

9 Data Link Layer Data link layer deals with the multiplexing of data streams, data frame detection, medium access and error control. At data link layer in a wireless sensor network to address the issue of power consumption and data centric routing there should have a MAC protocol and that MAC protocol must meet two goals: first to create a network infrastructure, (establishing communication links between deployed thousands of sensor nodes) and providing the network self-organizing capabilities. The second goal is to fairly and efficiently share communication resources between all the nodes. At present power consumption being secondary concern most MAC protocols fail to meet these two goals. In wireless sensor networks there is no provision of central controlling agent among a much larger number of nodes than traditional ad-hoc networks. And above this node malfunctioning because of power failures the network topology keep on changing and any MAC protocol for wireless sensor networks must also take this issue into account. The MAC protocols presented here are sensor-mac (SMAC) and Etiquette Protocol for Sensor Networks. SMAC SMAC provides nodes a way to discover their neighbors and establish transmission/reception schedules. The main principles behind SMAC are a periodic listen and sleep schedule[9], collision and overhearing avoidance, and message passing. It works by multi-hop communication and self-configuration of nodes. The focus of SMAC is on system-wide performance and network lifetime. Therefore, applications will have long idle time and must tolerate some latency. The nodes are formed in a flat topology so that neighbors can always communicate with each other and changes are easy to accommodate. All nodes in the network are set to have the same listen and sleep time period. During the listen period, the node waits for communication from its neighbors with its receiver on. During the sleep period, the node turns its receiver off to conserve energy. The nodes are free to choose their own schedule but to reduce overhead and latency, neighboring nodes are synchronized. The nodes then broadcast their schedules and cache the schedules of their neighbors. If two neighbors wish to communicate, one simply waits for the other to wake up. If two nodes wish to communicate to the same node, they send out a request to 20

10 send (RTS) message. The first RTS received is the one honored and is responded to with a clear to send (CTS) message[9]. All nodes will then resume their sleep cycle when the transmission is complete. The downside to this pattern is increased latency due to the periodic sleep schedule and the delay can accumulate on each hop. The latency requirement of a specific application places a fundamental limit on sleep time of nodes. The nodes are free to choose their own listen and sleep schedules using the following rules: On start-up there are three methods to choosing the listen and sleep schedule. The node listens for a specified amount of time and if it does not receive any communication from a neighbor, it arbitrarily chooses its schedule and broadcasts it immediately to its neighbors. This node becomes a synchronizer. If the node receives a neighbor's schedule in that start-up period it will adopt that schedule and wait for a random delay to broadcast to its neighbors. This node is called a follower. The last scenario is that a node creates its own schedule and hears a neighbor's before it is able to broadcast. In this case the node will merge the two schedules and adopt this as its schedule. Some nodes may fail to discover all its neighbors during this start-up period, but may find them later. If a node joins an already established sensor network, it will listen until it discovers an active node. It will then send an introduction packet to this active node, and it will respond by forwarding the new neighbors its schedule table. The new node will treat every node in the table as a potential neighbor and try to contact them during their listen period. This new node will attempt to find a synchronizer among those and follow it. If it is unable to locate one, it will simply choose its own schedule and broadcast it to the network. Synchronization between neighbors must happen periodically to prevent long-term clock drift. The period between updates may be quite long (a magnitude of tens of seconds). Synchronization is done by sending a synchronization (SYNC) packet. The SYNC packet is a very short packet containing only the node ID and the time of its next sleep relative to the moment the sender finishes transmitting the SYNC packet[29]. The receiver then adjusts its timer counters accordingly. Synchronizers also need to periodically synchronize all of its followers. The followers only need to update neighbors not on the 21

11 same schedule. In order to accommodate receiving SYNC messages and DATA, SMAC divides the listen time into two parts, one to listen for SYNC and the second to listen for RTS. Collision avoidance is accomplished through physical and virtual carrier sense. Physical carrier sense is carried out before a node initiates a unicast transmission. The node sends out an RTS message and then waits to receive a CTS message from the intended recipient. If the transmitting node does not receive the CTS, it sleeps until the receiver's next listen period. Each transmission packet contains a duration field indicating how long the remaining transmission will last. If a node intercepts a packet destined for another node, it stores this data in a variable called Network Allocation Vector (NAV). If NAV is not equal to zero, the node knows the transmission medium is busy and it can keep silent until all other transmissions are complete. This is called virtual carrier sense. If both physical and virtual carrier sense give the OK, the node is free to transmit.[29] In traditional MACs every node will continue to listen to all transmissions from all neighbors, but in the Wireless Sensor Network, this is far too expensive and a waste of the nodes' energy resources. SMACS allows interfering nodes to sleep after overhearing an RTS or CTS broadcast. This prevents neighbors from overhearing long DATA packets. In order to avoid all interference, all immediate neighbors of both the sender and receiver should sleep. The RTS and CTS also contain the duration of the transmission, so the overhearing nodes will know how long they should sleep. Message passing is a primary function of networks. A message is defined as a collection of meaningful, interrelated units of data. A message may be divided into a series of short packets or may be transmitted as a single long packet. A long single packet has the disadvantage of a long retransmission time if only a few bits are errors. Smaller packets have the disadvantage of large control overhead and longer delay because the RTS/CTS ritual must be performed for every packet. The SMAC approach is to fragment a long message into small fragments and transmit them in bursts. The sender will use a single RTS for the burst and reserve the medium for enough time to transmit all the fragments. The sender then waits for an acknowledgement (ACK) after each fragment is transmitted. If it fails to receive the ACK, it extends the reservation time using the duration field in the next transmission and resends the lost packet. The other reason for using the ACK 22

12 approach is that if a neighbor of the receiver wakes up or a new node joins the network in mid-transmission, it will not hear the DATA. If the receiver did not send ACK frequently, the new node may mistakenly infer from its carrier sense that the medium is clear. If it starts transmitting, it will interfere with the receiver of our original transmission. The ACK also carries the duration so the receiver's neighbors can sleep appropriately.ac has been shown to effectively reduce energy consumption, but the system must be able to tolerate a high level of latency. Etiquette Protocol Etiquette Protocol is a power-conserving protocol designed to rest on top of an existing MAC protocol [9]. The MAC still handles the micro-details of communications, such as channel contention. The main ideas behind Etiquette Protocol are: communication is scheduled in a completely distributed manner nodes can dynamically adapt their schedule in response to packet load communication between neighbors is done by appointments the onus of communication lies with the sender Nodes start by scheduling 'office hours' at regular intervals. Neighbor nodes request 'appointments' for sending data during these office hours. The appointment packet specifies the duration of the appointment and the receiver either grants or denies the appointment. The grant packet will specify the time and duration of the appointment. Both nodes will sleep until the time of the appointment because all the nodes know the precise interval of communication. Nodes are free to choose their own office hours independently of other nodes with the condition that the office hour period cannot exceed a specified time, namely Pmax. Pmax is set to create an upper bound on the latency of the network. A lower Pmax will create a more responsive network, but since holding office hours depletes the energy of a node, the cost will be higher power consumption. A Sensor Network in which latency is not a primary concern will have a higher Pmax, thus conserving more energy. This forcers a node to hold office hours at least once in Pmax units of time[29]. At the start of its office hours, the node broadcasts an announcement containing the duration and period. 23

13 When a new node enters a network and is powered on, its first concern is to establish office hours so it may communicate with its neighbors. To do this, it scans the channel for a set amount of time and gathers the office hours of all its immediate neighbors. It then chooses its own office hours in such a way as to minimize overlap with the office hours of its neighbors. To determine a neighbor's office hours, a node will turn its radio on and is guaranteed to hear an announcement within Pmax time. It caches all overheard office hour announcements for future reference. Once the announcement is heard, the node is then able to request an appointment. The appointment request contains four parameters: the appointment length, appointment type, desired time interval for appointment, and number of appointments needed. The appointment length specifies the size of the payload to be communicated. The type can be one-time or periodic. Periodic appointments are especially useful in data gathering applications where data must be sent on a regular basis. The use of periodic appointments avoids the need to request an appointment for each packet to be sent. The desired time interval may be requested by the sender to avoid conflicts with other appointments it may have already scheduled. The node may also need to request a number of appointments if the data is greater than the maximum packet size of the network and must be split into several smaller packets. The receiver then looks for an appropriate slot when the channel is expected to be free. However, it only has locally available information: its own set of appointments, office hour schedule, and usually incomplete knowledge of appointments of its neighbors. It must select a time-slot that does not interfere or overlap any of these other obligations and meets all the constraints set forth in the request. If no suitable time-slot is available, the node may overlap the appointment with its office hours, effectively reducing the duration of its office hours. This may be more energy efficient for the node, but may prevent some neighbors from setting up appointments. If a suitable slot is found, the node sends a grant-appointment message containing the start time of the appointment. If not, it sends a deny-appointment message. If the node is granted an appointment, it records the appointment time, enters sleep mode, and waits for the appointment time. If the requesting node does not receive a response, it waits for a request-timeout interval and resends the request. There is a retry-threshold and if the number of requests exceeds 24

14 this threshold, it assumes the receiving node is no longer in the network and discards all messages intended for that destination. When the appointment time is reached, both nodes turn on their radios. Because the appointment time is communicated as an offset of the time the grant-appointment message was sent, the appointment time recorded by the sender may be different than that of the receiver. In order to compensate for this difference, the nodes use a 'guard band' around the appointment slot and actually power their radios on at the beginning of the guard band. This prevents the loss of data due to one node preemptively transmitting. Etiquette Protocol offers several options to help optimize performance. The first allows nodes to send small packets during the receiver's office hours without having to make an appointment. This is done when the packet being sent is smaller than the combined size of the appointment-request and grant-appointment messages. Etiquette Protocol also allows nodes to adapt their office hours according to the load on the node. If the node is idle for longer than a given threshold during their office hours, the node reduces the length[18]. On the other hand, if the fraction of idle time drops below another threshold, the office hours have become too crowded and the node will increase the duration of its office hours. The choice of thresholds is dependent on the requirements of the application. When an event monitoring system causes bursts of communication data, a highly aggressive strategy should be used to change the office hours. Conversely, a more conservative approach will work for habitat monitoring systems that report consistently and periodically. If an inappropriate adaptive strategy chooses office hours that are too short, latency will increase. If the duration of office hours is too long, the node will be idle for a significant period of time, which is a waste of energy. Network Layer The network layer in a WSN deals with routing in the network. As battery consumption plays very important role in WSN, network layer must be designed with the following criteria s in priority: power efficiency, data-centric networking and WSNs have attributebased addressing and location awareness. As we discussed earlier the Data Link layer deals how two nodes talk to each other and the network layer is tresponsible to routing for communication i.e; deciding with which intermediate nodes communication is 25

15 possible. Simplest example of routing is flooding[29]. According to this routing technique, each node receiving data repeats it by broadcasting the data to every neighbor unless the max hop lifetime of the data has been reached or the receiving node is the destination. The major advantage of flooding is the simple technique of routing. It requires no complexity related with maintenance and route discovery. On the other hand the shortcomings are also there. The first issue is that of implosion. Implosion occurs when two nodes (A and B) share multiple neighbors. Node A will broadcast data to all n of these neighbors. Node B will then receive a copy of the data from each of them. The second issue is that of overlap, when two nodes share the same sensing region. If a stimulus occurs within this overlap, both nodes will report it. Power consumption is major issue in WSN and the last and most crucial problem related with this is resource blindness. In Flooding data is passed to every node by not taking energy resources into account. Another issue is from Flooding is gossiping. In gossiping, when a node receives data, it randomly chooses a neighbor and sends the data to it. Gossiping avoids the problem of implosion, but does not address the other two concerns and contributes to the latency of the network. Ideal Dissemination Ideal dissemination is one step up from flooding. According to this technique, a shortest path route is used to send data from the originating node. This approach guarantees every node will receive every piece of information exactly once. As no redundancy is there so no energy is wasted in sending or receiving redundant data. However, the overhead is substantial which is involved in keeping track of the shortest paths. Apart from this, ideal dissemination does not have technique to identify the nodes not interested in a particular piece of information. SPIN(Sensor Protocols for Information via Negotiation) SPIN[36] is one of major protocols which are part of a little more sophisticated family of protocols. With negotiation and resource adaption SPIN addresses the deficiencies of classic flooding. The problems of implosion are resolved by negotiation and it also makes sure that useful and desired information is disseminated. Meta data is being sent with the data by every node in negotiation. And the size of meta data must be shorter than the data to be sent to improve the efficiency, with condition that meta-data 26

16 describing two distinguishable pieces of data must be different. Likewise, if two pieces of data are indistinguishable, they will share the same meta-data. The format of the metadata is application specific and not specified by SPIN. There are three types of messages used by SPIN: 1.ADV 2. REQ 3. DATA. A node first broadcasts an ADV message whenever a node has data to share. It also contains metadata. A neighbor then responds with REQ if it is interested in the advertised data, and in sent DATA. This method of negotiation ensures that only interested nodes will receive data. Also ADV and REQ contain only meta-data,they are cheaper to send and receive. Nodes poll their resources before data transfer to find out how much energy is available. The resource manager is available at each node to keep track of resource consumption and to calculate cost of performing computations and sending and receiving data. Using this information, nodes are able to use their resources effectively. One implementation of SPIN[36], SPIN-2[36], employs a Low-Energy Threshold[60]. When energy is abundant, the node functions as normal. However, when the resource manager detects the nodes power supply is reaching the low-energy threshold, the node with decline to participate in any stage of the protocol if it believes it will not have enough power to complete the rest of the stages without going below the low-energy threshold. This prolongs the life of the node and allows it to perform only high priority functions. SPIN is a more sophisticated and energy aware schema for data dissemination. It reduces the amount of energy expended, solves the problems of implosion, overlap, and resource blindness, and ensures that only interested nodes will expend energy to receive data. Transport Layer The transport layer is responsible for communication of a system with the outside world. This layer is above physical,data link and network layer. It makes end to end kind of communication between two communicating nodes of a network. Due to unavailability of global addressing Communication in WSN, linking from the sink to the user is a problem. In place of that attribute-based naming is used to indicate the destinations of DATA packets. 27

17 Application Layer The functionality of this layer is to make the hardware and software of lower layers transparent to the Sensor Network Management Applications. SMP(sensor management protocol) is used to address this issue in WSN. The system administrators and programmers interact with the Sensor Network using SMP. Here also absence of global identification and infrastructure less nature of sensor networks comes into consideration. There are following rules, SMP provides to enable interaction between applications and sensor networks: Data aggregation, attribute-based naming, and clustering Exchange data related to the location finding algorithms Time synchronization Moving sensor nodes Turning nodes on or off Querying WSN configuration status, reconfiguring the WSN Authentication, key distribution, and security 2.4 Sensor Node Deployment Strategies Potential-field-based approach for node deployment is deeply discussed in [3], in which nodes are treated as virtual particles, subject to virtual forces. These forces repel the nodes from each other and from obstacles, and ensure that an initial, compact configuration of nodes will quickly spread out to maximize the coverage area of the network. In addition to these repulsive forces, nodes are also subject to a viscous friction force. This force is used to ensure that the network will eventually reach the state of static equilibrium, i.e. all nodes will ultimately come to Fig.2.4: Protocol stack representation of the Architecture a complete stop. The viscous force does not, however, prevent the network from reacting to changes in the environment; if something is moved, the network will automatically reconfigure itself for the modified environment before return once again to a static equilibrium. Thus, nodes move only when it is necessary to do so, saving a great deal of energy. A hybrid approach based on clustering in [39] is used for load balancing, where the 2-D mesh is partitioned into 1-D arrays by row and by column. Two scans are used in sequence: one for all rows, followed by the other for all columns. 28

18 Within each row and columns, the scan operation is used to calculate the average load and then to determine the amount of overload and under load in clusters. Load is shifted from overloaded clusters to under load clusters in an optimal way to achieve a balanced state. Each cluster covers a small square area and is controlled by cluster head, knows the information about cluster s position in the 2-D mesh and the number of sensors in the cluster. Limited motilities based approach is discussed in, where sensor can flip (or hop) only once to a new location and the flip distance is bounded. In this framework, the problem is to determine the optimal way for flip based sensors to maximize the coverage in the network. After detecting the coverage holes, the sensors move to new position to prevent coverage hole. Such movement can be realized in practice by propellers that are powered by fuel, coiled springs that unwinds for flipping. In this model, sensors can flip only once to a new location. The total force on a node is the sum of all the forces given by other sensors together with obstacles and preferential coverage in the area. In [50], three protocols are evaluated for sensor network to maximize the sensor coverage with less time, movement distance and message complexity. These protocols first discover the existence of coverage holes in the target area based on the sensing service required by the application. After discovering a coverage hole, the protocols calculate the target positions of these sensors, where they should move. These three protocols are VEC (VECtor-based), VOR (VORonoibased) and Minimax based on the principle of moving sensors from densely deployed areas to sparsely deployed areas. For static environment, deterministic deployment is used since the location of each sensor can be predetermined properly. The stochastic deployment is used when the information of sensing area is not known in advance or is varied with time, that is the position for sensor deployment cannot be determined [9, 29]. In [4], a centralized deterministic sensor deployment method, DT-score is the basis. Given a fixed number of deployable sensors, DT-score aims to maximize the area coverage of sensing area with obstacles. In the first phase of DT-score, a contour-based deployment is used to eliminate the coverage holes near the boundary of sensing area and obstacles. In the second phase, a deployment method based on the Delaunay Triangulation is applied for uncovered regions. Before 29

19 deploying a sensor, each candidate position generated from the current sensor configuration is scored by a probabilistic sensor detection model. 2.5 Cross Layer Architecture Introduction In wireless sensor network (WSN) one essential feature is a low power consumption of sensor nodes, that is, small devices equipped with a short range wireless transceiver, a small processor, and advanced sensing functionalities. Another key requirement for WSN is a self-configuring capability, the importance of this increase with the size of the network. In any bigger network at least some of the nodes must also be capable of multihop data transmission despite low memory and computational capacity. There exists many routing protocols that may function well in ad-hoc networks, but these protocols cannot be adapted directly to wireless sensor networks. The memory and other requirements of these protocols are usually too demanding for tiny devices. Most of these algorithms and networking methods must be taken care in more than one layer. It is obvious, therefore, that the interaction between layers cannot be ignored. One solution is that necessary data could be transmitted through service access points (SAP) and processes the tasks in each OSI layer's Layer Management Entity (LME) section. This would, however, increase the computational requirements in a protocol stack. For this reason, it is suggested that some parts of the system responsible for the power saving characteristics and network management could be implemented in a cross-layer module working in parallel with the traditional protocol stack. It is a software architecture where cross-layer management entity and low protocol stack has been combined. The architecture is aimed for wireless sensor network nodes with reduced resources. This cross-layer architecture is versatile and an adaptive solution for WSN nodes with limited resources. This architecture combines a low protocol stack and a cross-layer management entity with shared data structures and some special functions. Figure 2.5 shows the principle of cross-layer architecture in a WSN node. Above the data link layer, the architecture branches into two parallel areas. The application and the 30

20 protocol stack are responsible for the application-specific data transmission and the crosslayer management entity takes care of network management. The cross-layer management entity is further divided into two parts - Management Entity and Shared Data Structures. The reason why the cross-layer management entity sits on the data link layer is that in practice it uses the services of the data link layer like multiplexing and error-free data transmission offered by the link Description of Cross-layer Architecture The messages are divided so that protocol stack handles all data transfer between applications and the cross-layer entity handles control messages (figure 2.5).The application uses the services provided by the protocol stack and cross-layer management entity. The interface between the cross-layer management entity and the protocol stack/application employs the client/service principle. Application Protocol stack Data Cross-layer-management entity M1 M2 Mn Data Link Layer Layer Management Entity Physical Layer Functions Layer Management Entity Primitives M1,M2 Mn Modules Fig.2.5: Sensor network's cross-layer architecture 31

21 2.6 Classification of Routing Protocols for Wireless Sensor Networks In general, routing in WSNs can be divided into flat-based routing, hierarchical-based routing, and location-based routing depending on the network structure. In flat-based routing, all nodes are typically assigned equal roles or functionality. In hierarchical-based routing, however, nodes will play different roles in the network. In location-based routing, sensor node positions are exploited to route data in the network. A routing protocol is considered adaptive if certain system parameters can be controlled in order to adapt to the current network conditions and available energy levels. Furthermore, these protocols can be classified into multipath-based, query-based, negotiation-based, or QoSbased routing techniques depending on the protocol operation. In addition to the above, routing protocols can be classified into three categories, namely, proactive, reactive, and hybrid protocols depending on how the source finds a route to the destination. In proactive protocols, all routes are computed before they are really needed, while in reactive protocols, routes are computed on demand. Hybrid protocols use a combination of these two ideas. When sensor nodes are static, it is preferable to have table driven routing protocols rather than using reactive protocols. A significant amount of energy is used in route discovery and setup of reactive protocols. Another class of routing protocols is called the cooperative routing protocols. In cooperative routing, nodes send data to a central node where data can be aggregated and may be subject to further processing, hence reducing route cost in terms of energy use. Many other protocols rely on timing and position information. We use a classification according to the network structure and protocol operation (routing criteria). Fig.2.6 shows the classification of routing protocols in WSN. 32

22 Routing Protocols in WSN Network Structure Protocols Operation Flat Hierarchical Location Negotiation Multi-path Query QoS Network Network Based Based Based Based Based Routing Routing Routing Routing Routing Routing Routing SPIN LEACH GEAR SPIN Directed Directed Directed Protocol GMR Diffusion Diffusion Diffusion PEGASIS PBM Rumor Rumor TEEN & MFR,DIR Routing Routing APTEEN & GEDIR MCFA MECN Gradient SOP SPAN Based VGA Routing Figure 2.6 Routing protocols in WSN Based on the figure 2.6 here are some of the routing protocols of WSN which are frequently used is discuss in detail: Sensor Protocols for Information via Negotiation (SPIN) SPIN is a flat-based network routing protocol. A family of adaptive protocols called SPIN is designed to address the deficiencies of classic flooding by negotiation and resource adaptation. The SPIN family of protocols is designed based on two basic ideas: sensor nodes operate more efficiently and conserve energy by sending data that describe the sensor data instead of sending the whole data, e.g., image, and sensor nodes must monitor the changes in their energy resources. 33

23 ADV REQ DATA Step 1 Step 2 Step 3 ADV REQ DATA Step 4 Step 5 Step 6 Figure 2.7: The SPIN Protocol SPIN has three types of messages, i.e., ADV, REQ, and DATA. Before sending a DATA message, the sensor broadcasts an ADV (Advertise) message containing a descriptor, i.e., meta-data, of the DATA as shown in Step 1 of Fig.2.7. If a neighbor is interested in the data, it sends a REQ message for the DATA and DATA is sent to this neighbor sensor node as shown in Steps 2 and 3 of Fig. 2.7 respectively. The neighbor sensor node then repeats this process as illustrated in Steps 4, 5, and 6 of Fig As a result, the sensor nodes in the entire sensor network, which are interested in the data, will get a copy. SPIN is based on data-centric routing where the sensor nodes broadcast an advertisement for the available data and wait for a request from interested sinks Low-Energy adaptive clustering hierarchy (LEACH) It is a clustering based protocol that minimizes energy dissipation in sensor networks. Nodes are randomly selected as cluster-heads[46]. The protocol performs periodic reelection, so that the high energy dissipation experienced by the cluster-heads in communicating with the BS is spread across all nodes of the network. Each iteration of selection of cluster heads is called a round. The operation of LEACH is divided into setup and steady phases. 34

24 In 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 T(n)= P/{1-P[r mod (1/P)]} where P is the desired percentage of nodes which are cluster heads, r is the current round, and G is the set of nodes that has not been cluster-heads in the past 1/P rounds. This ensures that all sensor nodes eventually spend equal energy. After selection, the clusterheads advertise their selection to all nodes. All nodes choose their nearest cluster-head when they receive advertisements based on the received signal strength. The clusterheads then assign a TDMA schedule for their cluster members[46]. The steady phase is of longer duration in order to minimize the overhead of cluster formation. During the steady phase, data transmission takes place based on the TDMA schedule, and cluster heads perform data aggregation and /fusion through local computation. The BS receives only aggregated data from cluster-heads, leading to energy conservation. After a certain period of time in the steady phase, cluster-heads are selected gain through the set-up phase Directed Diffusion The directed diffusion data dissemination paradigm is proposed in where the sink sends out interest, which is a task description, to all sensors as shown in Fig. 2.8(a). The task descriptors are named by assigning attribute value pairs that describe the task. Each sensor node then stores the interest entry in its cache[34]. The interest entry contains a timestamp field and several gradient fields. As the interest is propagated throughout the sensor network, the gradients from the source back to the sink are set up as shown in Fig. 2.8(b). When the source has data for the interest, the source sends the data along the interest s gradient path as shown in Fig. 2.8(c). The interest and data propagation and aggregation are determined locally. Also, the sink must refresh and reinforce the interest when it starts to receive data from the source. The directed diffusion is based on datacentric routing where the sink broadcasts the interest. 35

25 Figure 2.8[34] Directed Diffusion (a) propagate interest,(b) set up gradient and (c) send data Rumor Routing Rumor routing is a variation of directed diffusion and is mainly intended for applications where geographic routing is not feasible. In general, directed diffusion uses flooding to inject the query to the entire network when there is no geographic criterion to diffuse tasks. However, in some cases there is only a little amount of data requested from the nodes and thus the use of flooding is unnecessary[42]. An alternative approach is to flood the events if the number of events is small and the number of queries is large. The key idea is to route the queries to the nodes that have observed a particular event rather than flooding the entire network to retrieve information about the occurring events. In order to flood events through the network, the rumor routing algorithm employs long-lived packets, called agents. When a node detects an event, it adds such event to its local table, called events table, and generates an agent. Agents travel the network in order to propagate information about local events to distant nodes[58]. When a node generates a query for an event, the nodes that know the route, may respond to the query by inspecting its event table. Hence, there is no need to flood the whole network, which reduces the communication cost. On the other hand, rumor routing maintains only one path between source and destination as opposed to directed diffusion where data can be routed through multiple paths at low rates. Simulation results showed that rumor routing can achieve 36

26 significant energy savings when compared to event flooding and can also handle node's failure. However, rumor routing performs well only when the number of events is small. For a large number of events, the cost of maintaining agents and event-tables in each node becomes infeasible if there is not enough interest in these events from the BS. Moreover, the overhead associated with rumor routing is controlled by different parameters used in the algorithm such as time-to-live (TTL) pertaining to queries and agents. Since the nodes become aware of events through the event agents, the heuristic for defining the route of an event agent highly affects the performance of next hop selection in rumor routing Geographic and Energy Aware Routing (GEAR) The use of geographic information while disseminating queries to appropriate regions since data queries often include geographic attributes. The protocol, called Geographic and Energy Aware Routing (GEAR), uses energy aware and geographically-informed neighbor selection heuristics to route a packet towards the destination region[60]. The key idea is to restrict the number of interests in directed diffusion by only considering a certain region rather than sending the interests to the whole network. By doing this, GEAR can conserve more energy than directed diffusion. GEAR was compared to a similar non-energy-aware routing protocol GPSR, which is one of the earlier works in geographic routing that uses planar graphs to solve the problem of holes. In case of GPSR, the packets follow the perimeter of the planar graph to find their route. Although the GPSR approach reduces the number of states a node should keep, it has been designed for general mobile ad hoc networks and requires a location service to map locations and node identifiers. GEAR not only reduces energy consumption for the route setup, but also performs better than GPSR in terms of packet delivery. 2.7 Position Based Routing All routing protocols can be classified based on different standards. According to the network structure, there are flit-based, hierarchical-based and position-based. Considering protocol operation,these routing algorithms can be classified into query- 37