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A '''mobile wireless sensor network''' ('''MWSN''')<ref name=Hayes>T. Hayes and F.H. Ali. 2016. [http://www.igi-global.com/chapter/mobile-wireless-sensor-networks/136562 "Mobile Wireless Sensor Networks: Applications and Routing Protocols"]. Handbook of Research on Next Generation Mobile Communications Systems. IGI Global. {{ISBN|9781466687325}}. pp. 256–292.</ref> can simply be defined as a [[Wireless sensor network|wireless sensor network]] (WSN) in which the [[Sensor node|sensor node]]s are mobile. MWSNs are a smaller, emerging field of research in contrast to their well-established predecessor. MWSNs are much more versatile than static sensor networks as they can be deployed in any scenario and cope with rapid [[Topology|topology]] changes. However, many of their applications are similar, such as environment monitoring or surveillance. Commonly, the nodes consist of a [[Social:Radio|radio]] [[Transceiver|transceiver]] and a [[Engineering:Microcontroller|microcontroller]] powered by a [[Physics:Battery (electricity)|battery]], as well as some kind of [[Engineering:Sensor|sensor]] for detecting [[Physics:Light|light]], [[Physics:Heat|heat]], [[Physics:Humidity|humidity]], [[Physics:Temperature|temperature]], etc.
==Challenges==
Broadly speaking, there are two sets of challenges in MWSNs; hardware and environment. The main hardware constraints are limited battery power and low cost requirements. The limited power means that it's important for the nodes to be energy efficient. Price limitations often demand low complexity algorithms for simpler microcontrollers and use of only a simplex radio. The major environmental factors are the shared medium and varying topology. The shared medium dictates that channel access must be regulated in some way. This is often done using a [[Medium access control|medium access control]] (MAC) scheme, such as [[Carrier-sense multiple access|carrier-sense multiple access]] (CSMA), frequency-division multiple access (FDMA) or [[Code-division multiple access|code-division multiple access]] (CDMA). The varying topology of the network comes from the mobility of nodes, which means that multihop paths from the sensors to the sink are not stable.

==Standards==
Currently there is no standard for MWSNs, so often protocols from MANETs are borrowed, such as Associativity-Based Routing (AR), [[Ad hoc On-Demand Distance Vector Routing]] (AODV), [[Dynamic Source Routing]] (DSR) and Greedy Perimeter Stateless Routing (GPSR).<ref>[[Biography:Brad Karp|B. Karp]] and [[Biography:H. T. Kung|H. T. Kung]]. 2000. [http://www.dtic.mil/get-tr-doc/pdf?AD=ADA440078 GPSR: Greedy Perimeter Stateless Routing for Wireless Networks]{{dead link|date=June 2022|bot=medic}}{{cbignore|bot=medic}}. In Proceedings of the 6th Annual International Conference on Mobile Computing and Networking (MobiCom '00). pp. 243–254.</ref> MANET protocols are preferred as they are able to work in mobile environments, whereas WSN protocols often aren't suitable.

==Topology==
Topology selection plays an important role in routing because the network topology decides the transmission path of the data packets to reach the proper destination. Here, all the topologies (Flat / Unstructured, cluster, tree, chain and hybrid topology) are not feasible for reliable data transmission on sensor nodes mobility. Instead of single topology, hybrid topology plays a vital role in data collection, and the performance is good. Hybrid topology management schemes include the Cluster Independent Data Collection Tree (CIDT).<ref>R. Velmani, and B. Kaarthick, 2014. [http://downloads.hindawi.com/journals/isrn.sensor.networks/2014/518268.pdf An Energy Efficient Data Gathering in Dense Mobile Wireless Sensor Networks],” ISRN Sensor Networks, vol. 2014, Article ID 518268, 10 pages. doi:10.1155/2014/518268.</ref> and the Velocity Energy-efficient and Link-aware Cluster-Tree ([[VELCT]]);<ref>R. Velmani, and B. Kaarthick, 2015. [https://www.researchgate.net/profile/Dr_Velmani_Ramasamy/publication/273393493_An_Efficient_Cluster-Tree_Based_Data_Collection_Scheme_for_Large_Mobile_Wireless_Sensor_Networks/links/56220ea808ae70315b58d32e/An-Efficient-Cluster-Tree-Based-Data-Collection-Scheme-for-Large-Mobile-Wireless-Sensor-Networks.pdf An Efficient Cluster-Tree Based Data Collection Scheme for Large Mobile Wireless Sensor Networks]. IEEE Sensors Journal, vol. 15, no. 4, pp. 2377–2390. doi: 10.1109/JSEN.2014.2377200.</ref> both have been proposed for mobile wireless sensor networks (MWSNs).

==Routing==
Since there is no fixed topology in these networks, one of the greatest challenges is routing data from its source to the destination. Generally these routing protocols draw inspiration from two fields; WSNs and [[Engineering:Mobile ad hoc network|mobile ad hoc network]]s (MANETs). WSN routing protocols provide the required functionality but cannot handle the high frequency of topology changes. Whereas, MANET routing protocols can deal with mobility in the network but they are designed for two way communication, which in sensor networks is often not required.<ref>T.P. Lambrou and C.G. Panayiotou. 2009. [https://www.researchgate.net/profile/Theofanis_Lambrou/publication/220963697_A_Survey_on_Routing_Techniques_Supporting_Mobility_in_Sensor_Networks/links/00463525325ec67bc9000000.pdf A Survey on Routing Techniques Supporting Mobility in Sensor Networks]. In Proceedings of the 5th international conference on Mobile Ad Hoc and Sensor Networks (MSN'09). pp. 78–85.</ref>

Protocols designed specifically for MWSNs are almost always multihop and sometimes adaptations of existing protocols. For example, Angle-based Dynamic Source Routing (ADSR),<ref>S. Kwangcheol, K. Kim and S. Kim. 2011. ADSR: Angle-Based Multi-hop Routing Strategy for Mobile Wireless Sensor Networks. In proceedings of the IEEE Asia-Pacific Services Computing Conference (APSCC). pp. 373–376.</ref> is an adaptation of the wireless mesh network protocol [[Dynamic Source Routing]] (DSR) for MWSNs. ADSR uses location information to work out the angle between the node intending to transmit, potential forwarding nodes and the sink. This is then used to insure that packets are always forwarded towards the sink. Also, Low Energy Adaptive Clustering Hierarchy (LEACH) protocol for WSNs has been adapted to LEACH-M (LEACH-Mobile),<ref>D. Kim and Y. Chung. 2006. [https://ieeexplore.ieee.org/abstract/document/4673775/ Self-Organization Routing Protocol Supporting Mobile Nodes for Wireless Sensor Network]. In proceedings of the 1st international multi-symposiums on Computer and Computational Sciences (IMSCCS’06). pp. 622–626.</ref> for MWSNs. The main issue with hierarchical protocols is that mobile nodes are prone to frequently switching between clusters, which can cause large amounts of overhead from the nodes having to regularly re-associate themselves with different cluster heads.

Another popular routing technique is to utilise location information from a [[Engineering:GPS|GPS]] module attached to the nodes. This can be seen in protocols such as Zone Based Routing (ZBR),<ref>U. Ahmed and F.B. Hussain. 2011. [https://www.researchgate.net/profile/Usama_Ahmed6/publication/261356420_Energy_efficient_routing_protocol_for_zone_based_mobile_sensor_networks/links/57341e0108ae9ace840744fb/Energy-efficient-routing-protocol-for-zone-based-mobile-sensor-networks.pdf Energy efficient routing protocol for zone based mobile sensor networks]. In proceedings of the 7th international Wireless Communications and Mobile Computing conference (IWCMC). pp. 1081–1086.</ref> which defines clusters geographically and uses the location information to keep nodes updated with the cluster they're in. In comparison, Geographically Opportunistic Routing (GOR),<ref>Y. Han and Z. Lin. 2012. [https://ieeexplore.ieee.org/abstract/document/6204919/ A geographically opportunistic routing protocol used in mobile wireless sensor networks]. In proceedings of the 9th IEEE international conference on Networking, Sensing and Control (ICNSC). pp. 216–221.</ref> is a flat protocol that divides the network area into grids and then uses the location information to opportunistically forward data as far as possible in each hop.

Multipath protocols provide a robust mechanism for routing and therefore seem like a promising direction for MWSN routing protocols. One such protocol is the query based Data Centric Braided Multipath (DCBM).<ref>A. Aronsky and A. Segall. 2010. [https://www.researchgate.net/profile/Adrian_Segall/publication/224208912_A_multipath_routing_algorithm_for_mobile_Wireless_Sensor_Networks/links/00463526baafd028c3000000/A-multipath-routing-algorithm-for-mobile-Wireless-Sensor-Networks.pdf A multipath routing algorithm for mobile Wireless Sensor Networks]. In proceedings of the 3rd Joint IFIP Wireless and Mobile Networking Conference. pp. 1–6.</ref>

Furthermore, Robust Ad-hoc Sensor Routing (RASeR)<ref>T. Hayes and F. Ali. 2016. [https://www.sciencedirect.com/science/article/pii/S1570870516301822 Robust Ad-hoc Sensor Routing (RASeR) Protocol for Mobile Wireless Sensor Networks]. Elsevier Ad Hoc Networks, vol. 50, no. 1, pp. 128–144.</ref> and Location Aware Sensor Routing (LASeR)<ref>T. Hayes and F. Ali. 2016. [http://sro.sussex.ac.uk/61642/1/Location%20Aware%20Sensor%20Routing%20(LASeR)%20Protocol%20for%20Mobile%20Wireless%20Sensor.pdf Location Aware Sensor Routing (LASeR) Protocol for Mobile Wireless Sensor Networks]. IET Wireless Sensor Systems, vol. 6, no. 2, pp. 49–57.</ref> are two protocols that are designed specifically for high speed MWSN applications, such as those that incorporate UAVs. They both take advantage of multipath routing, which is facilitated by a 'blind forwarding' technique. Blind forwarding simply allows the transmitting node to broadcast a packet to its neighbors, it is then the responsibility of the receiving nodes to decide whether they should forward the packet or drop it. The decision of whether to forward a packet or not is made using a network-wide gradient metric, such that the values of the transmitting and receiving nodes are compared to determine which is closer to the sink. The key difference between RASeR and LASeR is in the way they maintain their gradient metrics; RASeR uses the regular transmission of small beacon packets, in which nodes broadcast their current gradient. Whereas, LASeR relies on taking advantage of geographical location information that is already present on the mobile sensor node, which is likely the case in many applications.

==Medium access control==
There are three types of medium access control (MAC) techniques: based on [[Time-division multiple access|time division]], frequency division and [[Code-division multiple access|code division]]. Due to the relative ease of implementation, the most common choice of MAC is time-division-based, closely related to the popular CSMA/CA MAC. The vast majority of MAC protocols that have been designed with MWSNs in mind, are adapted from existing WSN MACs and focus on low power consumption, duty-cycled schemes.

== Validation ==
Protocols designed for MWSNs are usually validated with the use of either analytical, simulation or experimental results. Detailed analytical results are mathematical in nature and can provide good approximations of protocol behaviour. Simulations can be performed using software such as [[Company:OPNET|OPNET]], NetSim and [[Software:Ns (simulator)|ns2]] and is the most common method of validation. Simulations can provide close approximations to the real behaviour of a protocol under various scenarios. Physical experiments are the most expensive to perform and, unlike the other two methods, no assumptions need to be made. This makes them the most reliable form of information, when determining how a protocol will perform under certain conditions.

==Applications==
The advantage of allowing the sensors to be mobile increases the number of applications beyond those for which static WSNs are used. Sensors can be attached to a number of platforms:
* People
* Animals
* Autonomous Vehicles
* Unmanned Vehicles
* Manned Vehicles
In order to characterise the requirements of an application, it can be categorised as either constant monitoring, event monitoring, constant mapping or event mapping.<ref name=Hayes/> Constant type applications are time-based and as such data is generated periodically, whereas event type applications are event drive and so data is only generated when an event occurs. The monitoring applications are constantly running over a period of time, whereas mapping applications are usually deployed once in order to assess the current state of a phenomenon.
Examples of applications include health monitoring, which may include heart rate, blood pressure etc.<ref>H. Yan, H. Huo, Y. Xu and M. Gidlund. 2010. [https://ieeexplore.ieee.org/abstract/document/5681102/ Wireless Sensor Network Based E-Health System – Implementation and Experimental Results]. IEEE Transactions on Consumer Electronics, vol. 56, no. 4, pp. 2288–2295.</ref> This can be constant, in the case of a patient in a hospital, or event driven in the case of a wearable sensor that automatically reports your location to an ambulance team in the case of an emergency. Animals can have sensors attached to them in order to track their movements for migration patterns, feeding habits or other research purposes.<ref>S. Ehsan et al. 2012. [https://ieeexplore.ieee.org/abstract/document/6144759/ Design and Analysis of Delay-Tolerant Sensor Networks for Monitoring and Tracking Free-Roaming Animals]. IEEE Transactions on Wireless Communications, vol. 11, no. 3, pp. 1220–1227.</ref> Sensors may also be attached to unmanned aerial vehicles (UAVs) for surveillance or environment mapping.<ref>B. White et al. 2008. [https://ieeexplore.ieee.org/abstract/document/4631426/ Contaminant Cloud Boundary Monitoring Using Network of UAV Sensors]. IEEE Sensors Journal, vol. 8, no. 10, pp. 1681–1692.</ref> In the case of autonomous UAV aided search and rescue, this would be considered an event mapping application, since the UAVs are deployed to search an area but will only transmit data back when a person has been found.

==See also==
* [[List of ad hoc routing protocols]]
* [[Engineering:List of sensors|List of sensors]]
* [[Cyber-physical system]]
* [[Engineering:Mobile ad hoc network|Mobile ad hoc network]]s
* [[Engineering:Mobile robot|Mobile robot]] and [[Engineering:Mobile manipulator|Mobile manipulator]]
* [[Wireless sensor network]]s

==References==
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