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A sibling node can be defined as a neighbour of a specific node, which has the same rank in the topology but does not necessarily have the same parent as this node.
Finally, note that the star topology is a particular case of a tree topology where all nodes are one hop away from the tree root.
18.104.22.168. Mesh topology In the mesh topology, nodes can communicate with all their neighbours (i.e. the nodes that are within the coverage range of a certain node) and any node can communicate with any other node if a path exists. Hence, this topology exploits all the communications links that exist in a network (see Fig. 1.3.b).
The mesh topology is suitable for applications where any node can communicate with any other node. In contrast with the star and tree topologies, the mesh topology does not require the presence of special nodes.
1.4.2. The layered protocol stack approach Typically, many computer network protocol architectures follow the layered approach, whereby a layer offers a set of services to the layer on top of it. The well known Open Systems Interconnection (OSI) reference model  proposes a layered protocol stack, and the Transmission Control
Chapter 1. Introduction to Wireless Sensor Networks
Protocol/Internet Protocol (TCP/IP) stack constitutes a well known example that follows the layered architecture .
The use of layers allows one layer to operate independently despite the problems addressed by other layers. The layered approach has some overhead, which is not significant in devices with plenty of resources such as desktops. However, about a decade ago, some researchers believed that the resource constraints of sensor nodes might constitute a reason for abandoning the layered approach in WSNs .
As will be seen throughout the book (and especially in Chapter 11), the main current WSN solutions follow the layered approach. Nevertheless, cross-layer mechanisms, which use information from various layers to operate, have proved to be beneficial in many wireless systems. These include WSNs, where the simplicity of the sensor node platforms makes it easy to implement cross-layer mechanisms.
1.4.3. WSN protocols
Communication among devices in WSNs is enabled by the following
types of protocols:
• Physical layer protocols (see Chapter 3).
• Medium Access Control (MAC) protocols (see Chapter 4).
• Routing protocols (see Chapter 7).
• Transport protocols (see Chapter 8).
• Data encoding and aggregation protocols (see Chapter 12) Additionally, there are important cross-layer services in WSNs such as security (see Chapter 9) and topology control (see Chapter 6).
REFERENCES M. Dohler, T. Watteyne, T. Winter, D. Barthel, “Routing Requirements for Urban Low-Power and Lossy Networks”, RFC 5548, May 2009.
 E. Kim, D. Kaspar, N. Chevrollier, J.P. Vasseur, “Design and Application spaces for 6LoWPANs”, draft-ietf-6lowpan-usecases-05, IETF Internet Draft, November 2009 (Work in progress).
 Smart Dust project website:
http://robotics.eecs.berkeley.edu/–pister/SmartDust/  H. Zimmermann, “OSI Reference Model – The ISO Model of Architecture for Open Systems Interconnection”, IEEE Transactions on Communications, Vol. COM-28, No. 4, April 1980.
 R. Braden, “Requirements for Internet Hosts – Communication Layers”, RFC 1122, October 1989.
 D. Estrin, R. Govindan, J. Heidemann, S. Kumar, “Next century challenges:
scalable coordination in sensor networks”. In MobiCom ’99: Proceedings of the 5th annual ACM/IEEE international conference on Mobile computing and networking, pages 263–270, New York, USA, 1999.
 A. Sheth, “Semantic Sensor Web”, Intelligent Sensors, Sensor Networks and Information Processing – ISSNIP, Melbourne, Australia, August 2008.
 “Technical Document on Sensor Networks (Version 3)”, Technical Document of ISO/IEC JTC 1, Study Group on Sensor Networks (SGSN), December 2009.
Applications of WSNs and standardization initiatives Chapter 2. Applications of WSNs and standardization initiatives
2. Applications of WSNs and standardization initiatives The range of WSN applications is huge. On the other hand, future predictions are indicating that “In the next 10 years, many more devices and machines than humans will be connected to the Internet”; or in other words, "The number of elements to communicate is no longer Billions (people) but Trillions (things) [57, 58]”. This fact has triggered many standardization initiatives, in order to facilitate products interoperability. Firstly, this chapter aims at briefly introducing the main WSN application areas and some of the general characteristics of the network in each case (see Section 2.1). Secondly, the chapter focuses on the main standardization initiatives and organizations in the area of WSNs (see section 2.2).
2.1. Applications of WSNs This section aims at introducing the main application areas for WSNs. The reader may note that some of these areas overlap to some extent.
2.1.1. Environmental monitoring One of the classical WSN applications is environmental monitoring. In this type of application, sensor nodes are spread over a certain area (e.g. an urban area, a forest, a jungle, etc.) for monitoring parameters such as temperature, humidity, quality of the air, pollen concentration etc. These (and
other) data allow studying the environmental conditions of the zone and how they influence people, animals  and/or vegetation of the zone. In rural zones, forest fire control is another area of applicability. The following table shows examples of what can be sensed at present.
In rural scenarios, especially in zones where access is difficult, the nodes may be randomly deployed (e.g. thrown from a helicopter) and remain static. The data is collected by one or more sink nodes, which can be connected to other networks for remote access.
2.1.2. Healthcare, sports and fitness, assisted living Wearable wireless sensors can periodically report the levels of several body parameters , e.g. temperature, blood pressure, ElectroCardioGraph (ECG) (see Fig. 2.1) and insulin for a precise diagnosis. Sensor nodes can be used for patient location, monitoring and indentification in the hospital or at home (see Fig. 2.2). This fact enables remote care, by which patients, disabled and elderly citizens can benefit from at-home medical attention [35, 48]. On the other hand, alarms can be activated if critical events are Chapter 2. Applications of WSNs and standardization initiatives detected (e.g. acceleration sensors suggest that a person has fallen).
At-home medical attention may exploit the presence of a home automation WSN (see 2.1.6) for enhanced connectivity within the home and with remote hosts.
Fig. 2.1. Sensor node from Shimmer with a ECG extension  Fig. 2.2. Wireless sensor node to be fitted at the wrist (orange box) for patient identification, monitoring and location. The black box at the top of the image is used as a wireless router to create a backhaul network used for communication and as a reference for location in the hospital
Some examples of European Projects addressing different aspects of the health care research topic are HeartCycle  and PERSONA . While the scope of the HeartCycle project is the closed-loop management of both heart failure patients and chronic heart disease patients (including diabetes, arrhythmias, etc.), the scope of the PERSONA (PERceptive Spaces prOmoting iNdependent Aging) project is a standard AAL (Ambient Assisted Living) service platform aimed at assisting elderly people in their daily activities.
Another application area that can benefit from wearable sensors is sports and fitness [36, 37]. Heart rate sensors may communicate, for example, either with a watch which can display the result of the measurement, or with a device which can store the main statistics of a training session. The WSN is, in this case, composed of a small number of nodes. In addition, it may also be possible to transmit these data to a remote database (e.g. connected to the Internet) where the recorded data can be processed and analysed, and based on that, a “virtual coacher” can provide feedback and guidance to the user while the training session is in progress.
Finally, in addition to health-related parameters, biometric sensors can report data from which the emotions of a person can be obtained.
Emotional sensing is an interesting new area of research .
2.1.3. Structural monitoring
Buildings, bridges, tunnels, etc., can benefit from periodic monitoring of the architecture health . On the other hand, upon detection of emergency situations (for example testing of earthquake or structural damages), the sensor nodes can inform quickly about the event, so that appropriate actions can be taken in response.
Another area of application may be internal pipeline corrosion monitoring in the fuel and gas industry.
The nodes in these environments are manually deployed and remain static. The data is collected by one or more sink nodes, which can be connected to other networks for remote access.
Chapter 2. Applications of WSNs and standardization initiatives 2.
1.4. Automotive There are a wide range of applications based on sensors already working within a car, like for example environment control (indoor and outdoor temperatures), windows status, vehicle occupancy, seat belt monitoring, rain brush control, fuel control, tire pressure  or brake assistance (signal information from pedestrian and obstacles sensing), etc. All these sensed data could be arranged based on simple WSN configurations.
WSNs can provide value for logistics (tracking goods, pallets, baggage, etc), supply chain management or dynamic pricing information update in retail stores. For example, product packages can have embedded sensor nodes for detecting unauthorized package opening. These sensor nodes can communicate with static nodes placed in the warehouse for triggering alarms upon detection of unauthorized opening.
On the other hand, the embedded sensor nodes may store information about each product, such as its own identification and additional information, such as the current location, the temperature inside the package, etc. RFID technologies are also suitable for this application category [39, 40].
2.1.6. Home and building automation
WSNs enable a variety of applications in the home for user comfort and
home management. Some examples follow [41, 42]:
• Light control. A new light can be controlled from any switch, which reduces the need for new wired connections. Lights can also be activated in response to a command from a remote control. Furthermore, they can be turned on automatically when presence and luminance sensors detect that people are in a poorly illuminated room.
• Remote control. Infrared technology has been used for wireless communication between a remote control and devices such as TVs, HiFi equipment and Heating Ventilating and Air Conditioning (HVAC) sys
tems. However, infrared requires line-of-sight (LOS) and short-distance communication. RF technology overcomes these limitations.
• Security and safety. Advanced security systems can be based on several sensors (e.g. smoke detectors, glass-break sensors and motion sensors) for detecting possible risk situations that trigger appropriate actions in response. For example, smoke detectors may activate fire alarms.
• Window shades, HVAC, central heating, etc. may be controlled depending on the information collected by several types of sensors that monitor parameters such as temperature, humidity, light and presence. Unnecessary waste of energy can thus be avoided.
Fig. 2.3. Motion sensor based on a passive infrared (PIR) sensor providing also temperature and illumination level In a home, the number of nodes may be in the order of tens (and even hundreds in some residences), and the node density can be high. The nodes are mainly static, but some of them (e.g. those attached to remote controls or even to people) may exhibit some degree of mobility. The home automation network can be connected to other networks (e.g. the Internet) via a gateway or a router, depending on the protocols used.
Chapter 2. Applications of WSNs and standardization initiatives
The applications in building automation are similar to the ones in home automation, but the network size and structure may be different. In a building, the number of nodes may be in the order of thousands. In building automation, areas are defined as small physical locales, such as a room .
These areas are managed by controls which obtain the measurements provided by the sensors present in the area (e.g. temperature, occupancy, light, humidity, etc.). On the other hand, zones are defined as extended physical locales (e.g. a floor) which are also managed by controls that receive the data from the sensors within that zone. In building automation, most of the interactions between nodes do not involve sink nodes.
2.1.7. Industrial monitoring and control
In industrial environments, very critical communications are carried out using cables, which are known to be more reliable than wireless links.
However, these environments will benefit from adding wireless sensor nodes for controlling devices which are not currently connected.
The main WSN applications envisioned for process control (where the main product is typically a fluid, e.g. oil, chemicals, etc.) are monitoring applications with various requirements with regard to real time operation and non-critical loop control operations . In factory automation (where the product is a discrete element, e.g. a car, a lamp, etc.), WSNs will allow to carry out monitoring operations that currently are done manually or are not done.