Sensor Networks Overview Sensor networks have recently emerged as an important computing platform • Sensor Nodes are usually: • Typically less mobile • More densely deployed than mobile ad-hocnetworks(MANETs). • Limited memory andcommunication capabilities • Limited battery lifetime • Left unattendede.g., in hostile environments, which makes it difficult or impossible to re-charge or replace theirbatteries.

Reasons Of Energy Consumption Energy consumption in a sensor node can be due to either “useful” or “wasteful” sources. • Useful energy consumption • Transmitting/ receiving data • processing query requests • forwarding queries/data to neighboring nodes. • Wasteful energy consumption • Idle listening to the media • retransmitting due to packet collisions • overhearing, and generating/handling control packets.

Reduce Energy Consumption For Wasteful Sources: Several MAC protocols attempt to reduce. For Useful Sources : A number of protocols have also been proposed to reduce useful energy consumption. These protocols can be classified into three classes : • First classcontrols the transmission power level at each node to increase network capacity while keeping the network connected. • Second class makes routing decisions based on power optimization goals. • Third class decides which nodes should participate in the network operation (be awake) and which should not (remain asleep) . (nodes require of locations knowledge via GPS-capable antennae / message exchange).

Reduce Energy Consumption (2) Hierarchical clustering techniques can aid in reducing useful energy consumption. Clustering is particularly useful for : • Applications that require scalability to hundreds or thousands of nodes. (need for load balancing and efficient resource utilization.) • Applications requiring efficient data aggregation , Load Balancing • Routing protocols • One-to-many, many-to-one or one-to-all (broadcast) communication.

Clustering And Clusterheads • The essential operation in sensor node clustering is to : • Select a set of cluster heads among the nodes in the network. • Cluster the rest of the nodes with these heads. • Cluster heads are responsible for : • Coordination among the nodes within their clusters (intra-cluster coordination). • Communication with each other and/or with external observers on behalf of their clusters (inter-cluster communication).

Network Lifetime Network Lifetime: • Time until the first node / the last node in the network depletes its energy (dies). For example, in a military field where sensors are monitoring chemical activity, the lifetime of a sensor is critical for maximum field coverage. How To Prolong Network Lifetime ? • Reducing the number of nodes contending for channel access, • Summarizing information and updates at the cluster heads (through intra-cluster coordination), • Routing among cluster heads, which has a relatively small network diameter.

Clustering Can Reduce The Communication Overhead • Clustering can reduce the • communication overhead for both • single-hop and multi-hop networks. • Sensors periodically transmit information to a remote observer (base station). • With clustering, nodes transmit their information to their cluster heads. • A cluster head aggregates the received information and forwards it over to the observer.

Primary Goals Of HEED • Prolonging network lifetime by distributing energy consumption • Terminating the clustering process within a constant number of iterations • Minimizing control overhead (to be linear in the number of Nodes) • Producing well-distributed cluster heads.

Problem Statement A set of n sensor nodes are dispersed uniformly and independently in a rectangular field. Sensor nodes are • quasi-stationary. • Links are symmetric(same transmission power level) • The network serves multiple mobile/stationary observers, which implies that energy consumption is not uniform for all nodes. • location-unaware, i.e. not equipped with GPS capable antenna. • equally significant (have similar capabilities (processing / communication)). • left unattended after deployment. • Each node has a fixed number of transmission power levels.

No Assumptions Are Made About : • Homogeneity of node dispersion in the field • Network density or diameter • Distribution of energy consumption among sensor nodes • Proximity of querying observers.

HEED Requirements • Each node is mapped to exactly one cluster. • The node can directly communicate with its cluster head (via a single hop). • Clustering is completely distributed. Each node independently makes its decisions based on local information. • Clustering terminates within a fixed number of iterations. • At the end of each TCP, each node is either a cluster head, or an ordinary node that belongs to exactly one cluster. • Clustering should be efficient in terms of processing complexity and message exchange. • Cluster heads are well-distributed over the sensor field and have high average residual energy compared to regular nodes

The HEED Protocol Mainly has three phases • Define Clustering Parameters • Residual Energy • Communication cost • Protocol operation • HEED Analysis

Defining Clustering Parameters Parameters For Electing Cluster heads: • Primary parameter: residual energy (Er) • Goal : Prolong network lifetime • Used to select initial clusterheads • Secondary parameter: communication cost • Goal : Increase energy efficiency and further prolong network lifetime • Used to break tiesi.e., maximize energy and minimize cost • A tie means :A node falls within the “range” of more than one cluster head or,Two tentative cluster heads fall within the same range. • Is a function of cluster properties, such as • Cluster size • Whether or not variable power levels are permissible for intra-cluster communication

Communication cost Definitions • if all intra-cluster communication must use the same power level The cost is proportional to • Nodedegree—if requirement on LOAD DISTRIBUTION. • 1/(nodedegree)– if requirement on DENSE CLUSTERS. • If each node is allowed to use the minimum power level to reach its cluster head • AMRP (Average Minimum Reachability Power) Goal : For minimum intra - cluster communication energy • If a node has to select its cluster head among nodes not including itself, the closest neighbor within its cluster range (the neighbor reached using the smallest power level) can be selected as its cluster head.

Protocol Operation Probability Of Becoming A Cluster Head: • Before a node starts executing HEED, it sets its probability of becoming a cluster head, CHprob, as follows : • Cprob : Initial percentage of cluster heads among all N nodes (say 5%) • Eresidual : Estimated current residual energy in the Node • Emax : A reference maximum energy (corresponding to a fully charged battery), which is typically identical for all nodes. • The CHprob value of a node is not allowed to fall below a certain threshold pmin(e.g., 10−4)

HEED – Algorithm at node v • Discover neighbors within cluster range (Snbr) • Compute and broadcast cost to Snbr • Compute the initial cluster head probability • CHprob = max(Cprob * (Er/Emax) , pmin) I. Initialization II. Main Processing (Repeat) • If v received some cluster head messages, choose one head with min cost • If v does not have a cluster head, elect to become • a cluster head with CHprob . • CHprob = min(CHprob * 2, 1) • Repeat until CHprob reaches 1 III. Finalization • If cluster head is found, join its cluster • Otherwise, elect to be cluster head

Pseudocode

HEED – Analysis (1) HEED has the following lemmas : • Lemma 1 : HEED terminates in Niter = O(1) iterations • Proof: • The worst case : low Eresidual. Then CHprob= pmin. However, CHprob doubles in every step, and phase II of the protocol terminates one step (iteration) after CHprobreaches 1 • Lemma 2 : At the end of phase III of the HEED protocol, a node is either a cluster head or a regular node that belongs to a cluster. • Lemma 3 : HEED has a worst case processing time complexity of O(n) per node, where n is the number of nodes in the network .

HEED – Analysis (2) • Lemma 4 : HEED has a worst case message exchange complexity of O(1) per node, i.e., O(N) in the network. • An ordinary node is silent until it sends one join message to a cluster head. • The number of these join messages in the network is less than N, since at least one node will decide to be a cluster head during the clustering process. • Hence, the number of messages exchanged in the network is upper-bound by Niter × N, i.e., O(N) since Niter is constant.

HEED – Analysis (3) • Lemma 5 : The probability that two nodes within each other’s cluster range are both cluster heads is small, i.e., cluster heads are well-distributed. Consider the following worst case scenario : • Assume that v1 and v2 are two isolated neighboring nodes, each one does not have any other neighbor in close proximity. • We compute the probability, pnbr, that at the end of phase III, both of them are cluster heads (assume that they are fully synchronized). • Assume that neither of the two nodes decides to be a cluster head before its CHprobreaches 1. Otherwise, one of them will concede to the other.

Inter-Cluster Communication (1) • Rt : inter-cluster transmission range • Rc : the cluster transmission range • Lemma 6 :(Blough and Santi’02) • Assumen nodes are dispersed uniformly and independently in an area R=[0,L]2 • Assume that the area is divided into square cells of size (Rc / √2)× (Rc / √2). • If Rc2n=aL2lnL, for some a>0, Rc << L, and n>>1, then • limn,N→∞E(Minimum number of nodes in a cell) = 1, so eachcell contains at least one node L Rc / √2 Rc Rc / √2

2.7Rc v2 2.7Rc Rt v1 Inter-Cluster Communication (2) • Lemma 7 :There exists at least one cluster head in any (2 + 1/√2) Rc × • (2 + 1/√2) Rcarea. • Lemma 8 :For any two cluster heads v1 and v2 in two neighboring areas A • and B of size (2+ 1√2) Rc × (2+ 1√2) Rc, v1 and v2 can communicate if • Rt ≥ 6Rc. B A

Inter-Cluster Communication (3) • Theorem 1 :HEED produces a connected multi-hop cluster head graph (overlay) • Proof (by contradiction) :Assume previous 3 lemmas hold. • Assume that HEED produces two connected components (graphs) of cluster heads. • G1 = (V1,E1) and G2 = (V2,E2), such that any v1∈ V1 can not communicate with any v2∈ V2. • Assume that V2 lies on the right of V1, and that a cluster head v1∈ V1 lies on the rightmost border of V1. • v1 is able to communicate with a cluster head v2 on its right side, since the condition in Lemma 8 holds. • v2 must reside inside V2, which contradicts with the initial assumption that a Cluster head in one component cannot communicate with one in the other component. Therefore, V1 and V2 are connected.

Inter-Cluster Communication (4) Assume that transmission proceeds from all nodes in the top left cell to an observer in the bottom right cell. Eo- energy consumed for transmitting 1 bit of data by all the nodes in the top left cell without clustering Ec - energy consumed for transmitting 1 bit of data by all the nodes in the top left cell using clustering and data aggregation Energy gain Eg = Eo − Ec . Now assume that: • Nodes are dispersed uniformly at random in a ﬁeld, • One unit of energy is consumed for transmitting 1 bit of data per one unit of distance • Every cell (as deﬁned above) has one cluster head. then Eg > 0, if n > 2√ 2(L/Rc )2 .

Performance Evaluation (1) • Simulation Environment • 1000 nodes uniformly spread across 2000 x 2000 • Minimum probability for becoming a cluster head (pmin) - 0.0005 • Initial CHprob= Cprob = 5% and Cluster radius – 25 to 400 m. • Residual Energy levels – 20 and # of Experiments – 100 • Comparison with generic weight–based clustering protocols (GC) WCA, DCA, • Clustering is distributed only based on local information • Selected cluster heads with the highest weights (residual energy) • A node has only one cluster head • No assumptions about node dispersion field • Number of iterations is function of network diameter • Time and message complexities are O(N) and O(1) • Guaranteed that no two cluster heads are neighbor

Performance Evaluation (2) • Number of iterations to terminate • Number of iterations in HEED can be deterministically computed using Lemma 1, which is independent of the cluster radius. • For GC, the number of iterations grows quickly as the cluster radius increases. (radius implies neigbors for each node ) • GC takes only 3 iterations to terminate for a cluster radius of 25. The number of iterations, however, grows to 85 for a cluster radius of 400. • HEED takes 6 iterations to terminate for all cluster ranges. (Cprob = 5%, Eresidual is close to E max)

Clusterhead Characteristics (1) • HEED cluster heads are comparable to those selected by GC in terms of number, distribution, and energy availability. • HEED cannot guarantee optimal head selection in terms of energy, since it uses the secondary parameter to resolve conflicts. • GC, a weight-based approach, does guarantee that the highest energy node will be the cluster head within its cluster range.

Clusterhead Characteristics (2) • If it is required to balance load on • cluster heads, then it is important to • have clusters with small variance in • the number of nodes they cover. • The maximum degree cost type and • GC show similar results. • For minimum degree cost, the • standard deviation is the lowest, • because ties are broken by joining the • smaller degree node, thus balancing • the cluster sizes. • AMRP results lie between the two • extremes. Therefore, AMRP provides a • compromise between load balancing • and cluster density.

Clusterhead Characteristics & Node Syncronization • HEED produces a higher percentage of non-single node clusters than GC for all cost types. • The maximum number of nodes in a cluster in HEED is on the average smaller than that of GC for all cost types • Node synchronization is not critical for the operation of HEED. selected cluster heads in both cases have comparable residual energy.

Clustering Applications • Use of HEED for • Energy efficient routing protocols • Efficient Data Aggregation • Because prolonging network lifetime is especially important for unattended networks used in environmental monitoring. • HEED vs. gen-LEACH • HEED clustering improves network lifetime over gen-LEACH clustering for all cost types. • HEED expends less energy in clustering than gen-LEACH. • HEED prolongs network lifetime, compared to gen-LEACH and to direct communication.

Conclusion And Future Work • HEED is a new energy-efficient approach for clustering nodes in sensor networks. • Periodically selects cluster heads according to their residual energy and a secondary parameter, such as node proximity to its neighbors or node degree. • The clustering process terminates rapidly. • The protocol incurs low overhead in terms of processing cycles and messages exchanged. • Achieves fairly uniform cluster head distribution across the network. • Considers cluster quality, e.g., load-balanced clusters or dense clusters. • Future Work • Extend the protocol to multi-level hierarchies. • Cluster size constraints in HEED • Incorporate multiple external mobile observers into HEED.

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