case of Wireless Body Area Networks (WBANs) [2] consisting of a set of wearable or implanted sensing devices which can communicate among themselves ...
Tree-Based Routing Protocol for Wireless Sensor Networks Luca Borsani, Sergio Guglielmi, Alessandro Redondi, Matteo Cesana Dipartimento di Elettronica e Informazione, Politecnico di Milano, P.zza Leonardo da Vinci, 32 Milano, Italy {borsani, guglielmi, redondi, cesana}@elet.polimi.it
Abstract—The issue of supporting mobility in Wireless Sensor Networks is recently attracting increasing attention within the research community as new-born application scenarios require the deployment of hybrid network architectures composed of fixed and mobile sensor nodes. In this work, we address the problem of mobility management in WSNs by proposing a tree-based routing protocol able to support mobile sensor nodes. Distinctive features of the proposed routing solution are the provision of bi-directional uplink/dowlink connectivity to mobile sensor nodes, the use of proactive procedures to speed up the association/reassociation phase, a reduced impact of the signalling overhead to manage node mobility by resorting to ”local” handover management procedures. The routing protocol has been implemented on commercial hardware and thoroughly evaluated in terms of reactiveness and overhead through testbed experiments, as well as detailed simulations.
I. I NTRODUCTION Wireless Sensor Networks (WSNs) represent nowadays a powerful transmission commodity to support a vast range of applications and services with heterogeneous goals and deployment scenarios [1]. Classical application arenas of WSNs range form the environmental monitoring for the periodic reporting of remote measures on physical parameters (e.g., humidity, temperature, light, etc. . . ), to safety and security-oriented applications to detect and react to rare events (intrusion detection, natural disaster detection, etc.). Even if WSNs were originally thought to have static network infrastructure, recent applications require sensing nodes to be mounted on mobile entities (human beings, mobile robots, etc. . . ). Think of the case of Wireless Body Area Networks (WBANs) [2] consisting of a set of wearable or implanted sensing devices which can communicate among themselves and/or transmit data from the body to external traffic sinks. WBANs can be indeed useful whenever there is the need to monitor/track nomadic people, e.g., to monitor soldiers in the battle field (military applications), patients in nursing institutes (e-health applications), fire brigades and policemen (security/safety applications). Whatever application environment, the use of WBANs and mobile sensors in general, brings into the world of WSNs the problem of effectively supporting the mobility of single nodes and/or groups of nodes. Namely, the mobile sensors need to be continuously connected to the external network to deliver their sensed data, and viceversa, an external control point may need to seamlessly ”contact” the mobile sensors. Managing and supporting mobility in WSNs bear similar requirements and challenges of other wireless systems (e.g., WLAN mobility). Indeed, mobility support should always be seamless from the mobile node’s perspective, fast enough to track the mobile nodes’ movements, and robust in all its procedures. Yet, the design of effective mobility support strategies within mobile WSNs poses additional challenges with respect to other wireless systems. In fact,
the mobile sensors (and often also the infrastructure ones) are battery operated, thus all the procedure to provide seamless uplink/downlink mobile connectivity to mobile sensors must be highly energy efficient. To this extent, the mobility support must feature a limited overhead in terms of processing and required communication messages to be exchanged among sensor nodes. Moreover, the specific mobility support ”utility” should be fully integrated in a cross-layer fashion with the specific routing solution adopted by the WSN. In this work, we propose a solution to support seamless mobility within a multi-hop WSNs featuring both mobile and statically deployed nodes. The reference network scenario is the one considered by project LAURA (LocAlization and Ubiquitous monitoRing of pAtients for health care support) whose final goal is the design and the implementation of a lightweight system based on Wireless Sensor Networks (WSNs) for the automatic localization and supervision of nomadic patients within a nursing institute [3]. Mobile nodes (sensors) mounted on patients connect to a multi-hop infrastructure statically deployed to finally reach the control center; both uplink and downlink traffic must be supported in the reference scenario. Indeed, sensor nodes mounted on nomadic patients must be able to deliver remotely locally-collected information (uplink), and, dually, receiving configuration information from the control center (downlink). Within this scenario, we take a cross-layer approach and design a mobility-aware tree-based routing protocol which is able to build up and maintain a tree routing topology with mobile leaves (mobile sensor nodes). The proposed solution provides bi-directional connectivity from the root(s) of the tree (traffic gateways) to the mobile nodes and viceversa and features proactive procedures to speed up the association/re-association procedures of the mobile nodes, while limiting the impact of the signalling overhead to manage node mobility. We implemented the proposed scheme on commercial hardware and thoroughly evaluated in terms of reactiveness and overhead through testbed experiments. The paper is organized as follows: Section II overviews the related work in the field of mobile WSNs. we take then a constructive approach by introducing first the procedure to set up a static routing tree (Section III), further showing how to extend it to the case of mobile sensor nodes (Section IV). Section V reports and comments on the numerical results derived from the experimental evaluation of the mobility management procedures, whereas section VI gives our concluding remarks. II. R ELATED WORK Applications for localization, tracking and monitoring of objects and people in indoor environments, usually resort to hybrid sensor
networks composed of fixed and mobile nodes [4]. Hence, mobilityaware routing protocols are required to support seamless communication from/to the mobile nodes and the data sinks. The most common approach in the literature to handle mobility in WSNs ([5], [6], [7], [8], and [9]) consists in modifications of the Low Energy Adaptive Cluster Hierarchy (LEACH) protocol [10]. In LEACH, sensor nodes are organized into local clusters, with one node acting as Cluster-Head (CH). The CH is responsible to deliver all the data coming from non-cluster-head nodes to the PAN coordinator (traffic sink). Since non-cluster-head nodes have a TDMA schedule computed from their CH, they can be switched on only in their time slot, thus reducing energy consumption. On the contrary, since cluster heads must be always active in order to receive data from the cluster and forward it to the PAN coordinator, their lifetime is limited. To avoid the death of a fixed set of sensor nodes, LEACH introduces a randomized rotation of the cluster heads in order to distribute the energy consumption among all nodes in the network. LEACH-Mobile, in short LEACH-M [5], is a modified version of LEACH that introduces support for mobile nodes. In LEACHM the data transmission phase is modified with an explicit requestresponse paradigm. The cluster head broadcasts a call for data to all the nodes in his cluster and waits for responses: mobile nodes that do not answer are marked as possible lost nodes and deleted from the TDMA schedule. Conversely non-cluster-head nodes that do not receive data requests in their TDMA slot because of their mobility, start a procedure to join a new cluster and deliver their data. LEACH-Mobile-Enhanced (LEACH-ME) [6] and Distributed Clustering Algorithm (DCA) [9] introduce modifications in cluster head election to generate steady and balanced clusters that are disturbed minimally by cluster heads movement while other mobility protocols such as CBR-Mobile [8] and M-LEACH [7] try to increase data transfer success rate and energy saving modifying LEACH data transmission phase. In general, LEACH and its modified versions supporting mobile nodes are based on single hop communication, so they work under the assumption that all the nodes in the network can reach directly the PAN coordinator when it’s their cluster-head turn. This assumption is seldom realistic, especially in indoor environments where walls, furniture and people limit the radio range of wireless devices and multi-hop routing is unquestionably necessary. A partially different approach to mobility in WSN that considers multi-hop routing is given in [11]. Here pseudo-cluster are formed, with their cluster heads organized in a multi-hop tree that acts as routing path for mobile nodes. When a moving node needs to transmit its data, it broadcasts an explicit request and analyzes received replies to identify the cluster head which has minimal hop count toward the PAN coordinator. This solution also provides down-link data query from PAN. The distinctive features of the present work compared with the literature can be thus summarized in the two following aspects: (i) the proposed approach addresses mobility in multi-hop WSNs, which, to the best of our knowledge, is still an unresolved issue; (ii) whilst most of the literature in the field resort to simulation-based performance evaluation, the proposed approach has been actually implemented on commercial hardware and experimentally tested in realistic scenarios. III. H IERARCHICAL A DDRESSING T REE (HAT) ROUTING P ROTOCOL In this section we start off by presenting the static HAT routing protocol which builds up a tree-like routing/forwarding topology
among the network nodes. In the next section, we then show how HAT can be modified enhanced to effectively support mobile nodes. The routing tree is rooted at a Personal Area Network (PAN) coordinator which collects the traffic of the entire tree. The hierarchical routing tree is created and maintained through dynamic association (de-association) policies, which allow sensors to retrieve (release) network addresses and join (leave) the routing tree. The assigned addresses feature a hierarchical structure which reflects the tree topology. Referring to Figure 1 which shows an example of address format and management, the network addresses are composed of ordered fields in the form A.B.C.D. Each field is used to address all the nodes at a given depth in the tree (distance in hop from the root). The first field, A, addresses the nodes directly connected to the root of tree (PAN coordinator), the second field the nodes two hops away the root, and so on. The number of fields in the network address (tree depth), and the number of nodes which can be addressed in each field (tree width), obviously depend on the number of bytes available to represent the network address. In this work, unless differently specified, we have considered addresses of 2 bytes, with a maximum number of fields (tree depth) of 5, and each node has a maximum number of sons equal to 8. PAN Coordinator
1.0.0.0.0 2.0.0.0.0 1.2.0.0.0 2.1.0.0.0 1.1.0.0.0 1.2.1.0.0
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Fig. 1.
Example of Address Format and Tree Topology.
A. Association Phase Upon activation, a sensor node starts in the Initialization state. After having initialized all the internal components and variables, as shown in Figure 2(a), the node switches to the Scanning state and starts collecting beacons (BCN) sent by the surrounding nodes. PAN Coordinator
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Fig. 2. Procedure of Network Association. (a) A new node approaching the network senses the channel through the reception of beacons from neighbour nodes. (b) The new node joins the network using a three-way handshake with the node having best characteristics.
Then, the node, hereafter denoted as associating node, chooses the parent node to be associated to among the elements of a set P, which includes the nodes having received a beacon message from. Association proceeds by maximizing the following utility function J: J(i) = αRSSIi + βNi + γHCi ,
(1)
which depends on three factors: i) the Received Signal Strength Indicator from node i (RSSIi ); ii) the current number of children of the i-th candidate parent (Ni ) and iii) the distance of the candidate parent from the PAN coordinator (Hop Count, HCi ). The associating node consequently chooses the parent node i∗ which maximizes the utility function J(i), that is: i∗ = arg. max (J(i)) i∈P
B. Routing HAT is responsible for end-to-end (source to destination) packet delivery including routing through intermediate hosts. This is achieved through the use of routing tables stored on each node and a network header inserted in every message. HAT network header is shown in Fig. 3. It contains four fields, namely Packet Scope, Broadcast Sequence Number, Network Source Address and Network Destination Address. Packet Scope
(2)
The RSSI gives an estimate of the quality of the link and it is used to prevent nodes from associating with a parent through unstable links. The last two factors impact on the target network topology. Intuitively, the larger is the weight β (γ), the deeper (wider) is the resulting tree. The weights of the three factors have been empirically set to α = 1, β = −3, and γ = −6. Upon selection a three-way handshake is made (Figure 2(b)): the associating node sends an explicit association request (REQ) to the selected parent node, which, in turn, responds with a message containing the proposed network address (RES). This response is broadcasted since the requesting node does not have a network address yet. Hence, the requesting node must confirm the association with a confirm message (CFR) to the parent and can then switch to the Associated With Network (AWN) state. At this point the associated node generates a Mote Announcement message (MA) that is forwarded through the whole tree to the PAN coordinator. This is necessary to keep updated the list of connected mote at the root of the tree (that is generally connected to a PC or other gateway devices). A specular message called Mote Loss message (ML) is also provided in case of death of nodes. In this case, when a node recognizes that one of his children is dead, a Mote Loss message is sent to the PAN coordinator, so the list of connected motes can be updated.
Fig. 3.
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Network Header Structure
The routing table contains in each row a node id (MAC address) and the corresponding network address (in the form A.B.C.D.). MAC address is necessary for one hop communication, while network address is used for routing purposes. Upon association, each node stores his parent node addresses in the first row of the table and possibly the addresses of its children nodes. Whenever a node receives a message to forward, it checks the Packet Scope field in the network header. Depending on the value of the packet scope, different routing strategies are adopted: •
•
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In the AWN state, the association is maintained by means of periodic beacon messages sent by parent nodes which carry all the information needed to manage and maintain the association. Namely, each beacon message contains a sequence number (that can be used for synchronization purposes), the current route cost (used by associating nodes to compute the association utility function), and a CHILD MASK used to inform associating nodes about the current number of children associated to a given parent. The CHILD MASK is also used to prevent inconsistencies in the association due to asymmetries in the wireless links between parents and children. Indeed, the aforementioned association phase leverages information obtained through downlink beacons, and it consequently accounts only for the downlink quality of the parent-to-node link; thus an associating node could select a parent with a ”good” downlink channel quality but with an actual ”poor” uplink channel quality. This may lead to situations where the parent node does not receive the beacons from its child, and consequently deletes it from the routing table, while the child remains associated to the parent. To prevent this problems, the CHILD MASK reports the identities of all the associated nodes, such that a node receiving a beacon from its parent can crosscheck if it is still in the parent’s routing table. If this test fails, the node goes back to the Initialization state and a new association procedure is triggered.
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LOCAL BROADCAST: in this case the message is simply sent to all reachable nodes; BROADCAST: in this case the message must be delivered to all nodes in the network. To ensure this condition every node that receives a broadcast message forward it again to the radio. To prevent traffic explosion over the channel every node checks for duplicate messages through the Broadcast Sequence Number and discards those who were already received and forwarded; UNICAST: The forwarding node searches for the next hop in his routing table: if the network destination address is in the routing table, the message is sent using the corresponding MAC address. Else, a comparison operation is performed to forward the message in the proper subtree, using only the first K + 1 fields of the destination address, being K the current hop count towards the PAN coordinator. Referring to Figure 1, assume that node with address 1.0.0.0.0 (K = 1) receives a message destined to 1.2.1.0.0: the forwarding node checks his routing table and select as next hop the child whose first K = 2 fields are 1.2 (i.e. node 1.2.0.0.0). This operation is recursively done performed in each level of the tree, providing correct delivery of packets. TO PAN COORD: in this case the message is destined to the root of the tree, so every node simply forwards it to its parent;
HAT protocol inherently supports node mobility since changes in network topology caused by death of nodes or variations in RF paths that can occur in presence of moving obstacles, but it is not reactive to frequent changes due to mobile nodes. To avoid several data losses in downlink traffic when the receiving node is mobile, we introduce protocol modifications in order to handle high mobility of nodes. IV. E NABLING MOBILITY: HAT-M OBILE In this section we present HAT-Mobile, a modification of HAT protocol that allows nodes to move throughout a specific coverage area without losing connection with network. In HAT-Mobile, nodes are divided in two categories: fixed and mobile nodes. Fixed nodes have no mobility and create the routing tree, while mobile nodes can
move freely within the coverage area. We impose that mobile nodes can join the routing tree only as leaves. In order to maintain connection with network, mobile nodes periodically check their routing tables to evaluate the quality of link with their current parent node and neighbors nodes. If the received signal strength from the current parent node becomes lower than a defined threshold τ1 (equal to -70 dB in our experiments), the node can set up a new association with a neighbor node k∗ among the set P of surrounding nodes with better characteristics, according to the following equation:
•
contained in H UPD. This situation happens when a mobile node perform two handovers with nodes that have the same Root Node. In this case the Root Node overwrites the value of New network address. The MAC address contained in the H UPD is in its handover table and the value of Old network address stored in the handover table is equal to the field New network address contained in H UPD. This situation happens when a mobile node performs an handover with a node that was its parent node previously. In this case the Root Node delete the entry.
k∗ = arg. max (J(k)) k∈P
s.t. J(k∗ ) > J(p) + τ2
(3)
PAN Coordinator
where J is the utility function defined in section III-A, J(p) is the cost function of the current parent node and τ2 is a power threshold that forces the new association to have better characteristics than the previous one (equal to 10 dB in our experiments). Moreover τ2 prevents the mobile nodes to frequently start new association procedures. If a mobile node decides to set up a new association, it starts an handover process that cause a de-association with parent node and an association with the chosen neighbor node k∗ .
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Fig. 4. Handover process. (a) Mobile node M starts an handover procedures. (b) The H UPD message is sent to the Root Node of the two fixed nodes involved in the handover process and the handover table is updated.
A mobile node that needs an handover procedure switches to the Handover state and sends an explicit handover request (H REQ) to the selected neighbor node, which in turn responds with a message containing a new network address (H RES). Then the requesting mobile node sends an update message to his old parent node to communicate his de-association and then switches to AWN state. Figure 4(a) shows an example of handover process.
C. Routing
B. Local management of handover process In order to limit signaling traffic involved in the handover procedure thus saving energy, we propose an approach to handle handover information locally by leveraging handover tables and exchange of messages in sub-tree of the topology tree. Upon handover phase, the old parent of the mobile node involved in the process sends an handover table update message (H UPD) to the Root Node of the two fixed node that have implemented the handover process (namely the old and the new parent of the mobile node). The H UPD message contains the old and new network address of the mobile node, together with its MAC address. These information are stored by the Root Node in an handover table and are used for routing purposes. The structure of handover table is: • MAC Address: MAC Address of mobile node; • Old Network Address: Network address of mobile node before handover procedure; • New network address: Network address of mobile node after handover procedure; Figure 4(b) shows an example of local management of handover process. A Root Node, that receives an H UPD with handover information, can perform different actions: • The MAC address contained in the H UPD it’s not in its handover table; the Root Node stores the three addresses in the first free entry of the table. • The MAC address contained in the H UPD is in its handover table and the value of New network address stored in the handover table is equal to the field Old network address
Whenever a node receives a message to forward, it checks its handover table. If the destination address is contained in the Old network address column of the table, the destination node has performed an handover. In this case is necessary to overwrite the destination address of the message to forward with the new network address contained in the handover table. Successively, the routing procedure described in III-B can be performed as it is. D. Addresses Management During the handover procedure new network addresses are assigned to mobile nodes, and the handover information is stored locally. Whenever a new network address is required, it stands to reason that no one of the addresses present in the handover tables of the fixed nodes can be assigned. Doing the contrary would cause errors in the forwarding of messages, since the same network address would point to two different mobile nodes (one assigned directly, and the other pointed through a handover table). For this reason it is of primary importance to lock network addresses involved in handover procedures. Moreover, since the number of assignable addresses is limited, it is necessary to implement specific procedures in order to unlock network addresses for further assignment. 1) Address release: This procedure is linked to the case of double handover described in IV-B, where a Root Node overwrites the value contained in the New network address field of the handover table. In this case is possible to unlock the overwritten network address, because it becomes out of date. So the Root Node sends an address release message (A R) to advice the old parent node that the specified network address can be unlocked (Figure 5). 2) Address release request: This is an on-demand procedure that a fixed node implements when only 25% of its address subspace is unlocked. When a fixed node is in this situation, it sends an address release request message A R R to its parent node containing a locked
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Fig. 5. Procedure of Address Release. (a) Mobile node M with network address x switches from B to C and receives address y. (b) B sends an update to node A, Root of B and C. A stores handover information in its handover table. (c) M switches from C to D and receives address z, node C sends an update to A. Handover information are updated. (d) Node A recognize a double handover and network address y can be released.
network address that is required to unlock. When a node receive an A R R performs two actions: • If the network address is contained in old network address column of its handover table, it inserts in the A R R message the value of new network address; • It forwards the A R R message to its parent node. When the PAN Coordinator receives the A R R message, it sends an announce handover message (A HAN) with the new network address of the mobile node to the gateway that updates the actual network address of mobile node. In this way, all the entries regarding the locked address in the handover tables are useless and the address can be unlocked. Then, the PAN Coordinator sends an A R message (described in IV-D1) to advice the requesting node that it can unlock the specified network address. 3) Address reset: A fixed node must implement this procedure when the association with a mobile node is lost. In fact it is necessary to delete all the entries in the handover tables that contains information about the disconnected mobile node. The fixed node sends an address reset message (A RESET) that contains network address of mobile node to his parent node. A node that receive an A RESET message, deletes the entry relevant to the specified network address and forward the message to the parent node, so that all the entries relative to the lost node are deleted recursively. V. P ERFORMANCE E VALUATION In order to evaluate the performance of the proposed protocol, we carried out different experiments using MEMSIC IRIS sensor nodes operated by TinyOS. In the first experiment we evaluate the latency of handover procedures. This delay refers to the interval between the transmission of the H REQ message from a mobile node to a fixed node and the update of the handover table of the Root Node involved in the handover procedure (Figure 4). This latency can be computed as sum of two contributes: • Handover message time: interval between the transmission of a H REQ message and the reception of the ACK of H UPD
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Fig. 6. Procedure of Address Release Request. (a) Node A requires to unlock network address x. A R R message is sent uplink. (b) PAN coordinator recognize address x in its handover table. (c) PAN coordinator sends a A HAN to the gateway in order to update node list. (d) PAN coordinator informs node A to unlock address x.
message from a mobile nodes requesting a handover. Note that this contribute is constant and does not depend on the current tree topology. It results from our experiments that the handover message time is on average 60 ms. • Tables update time: time spent to update the handover tables of the nodes involved in the procedure. As explained in section IV-B the H UPD message sent by the mobile node must reach the Root of the fixed nodes involved in the process. So this interval does depend on the topology of the tree, in particular on the number of hops between the mobile node and the Root Node. To evaluate this delay, we implemented a testbed that generates a constant traffic at a rate of 20 Hz from the PAN coordinator towards a mobile node. Since the mobile nodes is unreachable before the update of the handover table, the interval of packet mis-reception provides an estimate of the handover latency. We implemented a timer on the mobile node that starts when the ACK of the H UPD message is received and stops when a new message is received (this means that the handover table was updated at the Root Node). We tried different topologies, varying the hop count between the mobile node and the Root of the old and new parent to estimate the handover latency. Table I shows the total latency obtained from our experiments (including the Handover Message time). Hops count 2 3 4
Total Latency [ms] 172 266 374
TABLE I H ANDOVER L ATENCY
In the second experiment we set up a network composed of 8 fixed node distributed in an area of about 120 m2 , setting up a routing tree as the one showed in Figure 7. We transmitted test packets with different rates from the PAN coordinator to a mobile node, moving with a speed of 1 m/s within the coverage area, following a fixed path (the dotted red line in Figure 7). We evaluated the performance of the
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proposed protocol through the estimation of Packet Error Rate under different conditions, varying both data transfer rate and transmission power levels. In order to emphasize the improvement of HAT-Mobile over HAT, we carried out an experiment with two mobile nodes operating the two version of the proposed protocols.
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Fig. 7. Test with a realistic scenery. The dotted line represents the trajectory of the mobile node during the experiments. The network topology is represented with black lines, and the fixed nodes composing the sensor networks are displayed with blue dots.
Figure 8 shows the Packet Error Rate of HAT-Mobile, varying the data rate and for different transmission output powers. We observe that when the transmission power is set to higher values, the PER decreases. This is due to the larger coverage area of fixed nodes, that allow the mobile node to start less handover procedures.
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Fig. 9. Packet Error Rate of HAT and HAT-MOBILE protocol with different data rate.
we implemented a simulator for HAT and HAT-Mobile in MATLAB. The simulator deploys random network topologies with variable number of sensor nodes, and sets up the routing tree according to the procedures described in Section III. A sample network topology with 100 fixed nodes in reported in Figure 10. An increasing number of mobile nodes moving according to a Random Waypoint model is then added to the topology. Handovers are triggered according to the rules of HAT (Sec. III) and HAT-Mobile (Sec. IV).
6
The simulator then computes the total energy consumption related to handovers. To this extent, we used a reference energy model based on MEMSIC IRIS datasheet, in which the radio dissipates 200 nJ per bit. We assumed that both H UPD message and MA message have equal size (176 bit in our implementation).
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Fig. 8. Packet Error Rate of HAT-Mobile with different data rates and transmission powers. The mobile node is moving at 1 m/s in an area of about 120 m2 .
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Figure 9 shows the comparison between HAT and HAT-Mobile with different data rates and with the same experiment settings of Figure 7. The transmission power of both fixed and mobile nodes was set to -12.2 dBm. Expectedly, the Packet Error Rate increases with the offered data rate for both routing solutions. We further observe that HAT-Mobile protocol achieved clear improvements in Packet Error Rate compared to HAT protocol. Besides reducing the PER, HAT-Mobile protocol allows also to reduce the total energy consumption due to the local management of the handover information. In other words, the local management of handovers allows to reduce the total number of signalling messages to be exchanged to support mobility, thus reducing the overall energy consumption. In order to quantify the reduced energy consumption,
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Fig. 10. 100-nodes random network used in our experiments. The red cross in [0,0] is the PAN coordinator, while the blue dots are the fixed nodes. Associations are represented with black lines.
Figure 11 reports the total energy consumption when varying the number of mobile sensor nodes. Notably, HAT-Mobile improves the energy savings of the updating procedure, achieving between 2x and 3x reduction in energy compared to HAT.
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Energy efficiency of HAT-Mobile over HAT
VI. C ONCLUSIONS In this paper, we have addressed the issue of supporting node mobility in hybrid Wireless Sensor Networks composed of static and mobile nodes. Namely, we have proposed a a tree-based routing protocol able dynamically construct routing topologies as the mobile nodes move throughout the network arena. The proposed approach, named HAT-Mobile, includes proactive techniques to speed up the handover procedures of mobile sensor nodes among different access points, while preserving bi-directional connectivity to/from the root node of the tree (PAN coordinator). HAT-Mobile further features solutions to limit the impact of the signalling overhead required to support the handover procedures, which consequently reduces the energy consumption for the overall process of mobility support. The experiment carried out with commercial hardware have demonstrated the potentials of HAT-Mobile in terms of energy efficiency, handover latency reduction, and reduced Packet Error Rate. ACKNOWLEDGMENTS This work has been partially supported by project LAURA, Localization and Ubiquitous Monitoring of Patients for Health care support financed by the Politecnico di Milano. R EFERENCES [1] I. Akyildiz, W. Su, Y. Sankarasubramaniam, and E. Cayirci, “A survey on sensor networks,” Communications Magazine, IEEE, vol. 40, no. 8, pp. 102 – 114, aug. 2002. [2] M. Quwaider, J. Rao, and S. Biswas, “Body-posture-based dynamic link power control in wearable sensor networks,” Comm. Mag., vol. 48, no. 7, pp. 134–142, 2010. [3] “http://laura.como.polimi.it.” [4] A. Redondi, M. Tagliasacchi, M. Cesana, L. Borsani, P. Tarrio, and F. Salice, “LAURA - LocAlization and Ubiquitous monitoRing of pAtients for health care support,” in Advances in Positioning and LocationEnabled Communications, 2010. APLEC ’10., IEEE International Workshop on, 2010. [5] D.-S. Kim and Y.-J. Chung, “Self-Organization Routing Protocol Supporting Mobile Nodes for Wireless Sensor Networks,” in Computer and Computational Sciences, 2006. IMSCCS ’06. First International MultiSymposiums on, 2006. [6] G. Santhosh Kumar, M. V. Vinu Paul, and K. Poulose Jacob, “Mobility Metric based LEACH-Mobile Protocol,” in Advanced Computing and Communications, 2008. ADCOM 2008. 16th International Conference on, 2008.
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