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«Carles Gómez Josep Paradells José E. Caballero Edita: Fundación Vodafone España Autores: Carles Gómez Montenegro* Universitat Politècnica de ...»

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iii) IPv6 address auto-configuration and iv) IPv6 Neighbour Discovery for LoWPANs. There also exist open and publicly available 6LoWPAN implementations [9, 10].

Moreover, different types of 6LoWPAN devices exist. An Edge Router interconnects a LoWPAN with another network. A Mesh Node and a Router perform routing tasks in the “mesh under” and “route over” configurations, respectively. A Host is a device that only sources or sinks IPv6 packets.

If a LoWPAN follows the mesh topology, a routing protocol is needed. Two schemes are envisaged for routing in LoWPANs, namely: mesh under and route over. In mesh under (see Fig. 11.2.a)), routing is performed below IP using IEEE 802.15.4 addresses. In this configuration, the whole LoWPAN appears as a single IP link. In route over (see Fig.

11.2.b)), every radio hop is equivalent to an IP hop and routing occurs at the IP layer.

As of the writing of this book, the IETF Routing Over Low power and Lossy networks (ROLL) WG is developing the IPv6 Routing Protocol for Low power and lossy networks (RPL), which is a likely candidate protocol for the route over configuration. RPL maintains Directed Acyclic Graphs (DAGs), which may be rooted at sink nodes and naturally support multipoint-to-sink and sink-to-multipoint communications. Point-to-point communications are also supported, but routes between arbitrary nodes may not be optimal, since they are constrained to the DAG structures (see Chapter 7).

–  –  –

Fig. 11.2. 6LoWPAN architecture: a) mesh under, b) route over Application layer functionality, such as commands and attributes, does not currently exist for 6LoWPAN (nor for other types of IP networks with similar characteristics). Moreover, traditional application layer Internet protocols (e.g. HTTP and SNMP) and data encoding formats are not naturally suited to this kind of networks, given the constraints of the devices and the 50-to-60-byte transport layer payloads available in LoWPANs. The new IETF Constrained RESTful Environments (CoRE) WG will develop new or adapted application-layer protocols and data encoding formats [11]. Other areas within the IETF where work should be done are transport and security. It is likely that future working groups will develop protocols and solutions for these areas.

A major deployment of 6LoWPAN and the future CoRE protocols will be the Smart Energy Version 2 (SE 2) effort. SE 2 aims at providing end-to-end connectivity between energy providers and consumers, and it has been recognized as part of the Smart Grid roadmap of the US NIST [12].

11.1.3. Bluetooth Low Energy When the IEEE began the discussion on the technology to be used as low bit rate WPAN, several proposals were presented. One of them Chapter 11. Wireless sensor network architectures and technologies intended to offer the same radio as Bluetooth but required less power offering lower bit rate. This proposal was not accepted, and latterly was pushed in the Wibree Forum as a solution for short distance communication under the name of “Wibree”. The Wibree Forum merged with Bluetooth SIG by mid 2007 and since that date, a low energy Bluetooth wireless feature has been developed as part of the Bluetooth specification with the name BT-LE. A key milestone in the specification process was reached in December 2009, with the announcement of the adoption of this technology feature as part of the Bluetooth Core Specification Version 4.0 [36, 37].

The current version of Bluetooth Core Specification contains two forms of Bluetooth wireless technology systems: Basic Rate (BR) and Low Energy (LE). Both systems include device discovery, connection establishment and connection mechanisms. The Basic Rate system includes optional Enhanced Data Rate (EDR) and Alternate MAC and PHY (AMP) layer extensions33.

The LE system includes features designed to enable lower current consumption, lower complexity and lower cost than the BR/EDR system. The LE system is also designed for applications with lower data rates and has lower duty cycles. In consequence, depending on the application, one system including any optional parts may be more optimal than the other.

There may be devices implementing either BR/EDR and LE systems (dual mode) or only one of them (stand-alone), allowing compatibility and interoperability with existing devices.

The stand-alone LE system is intended for small devices (like watches and sports sensors) and it will enable new button cell battery operated devices. Many applications such as healthcare, sports and fitness, security, and home automation will be enhanced with the availability of small coin-cell battery powered BT-LE wireless technology.

The Basic Rate system offers synchronous and asynchronous connections with data rates of 721.2 kbps for Basic Rate, 2.1 Mbps for Enhanced Data Rate and high speed operation up to 24 Mbps with the 802.11 AMP [37].

–  –  –

The key technical features of BT-LE are shown next [38]. Very short data packets (8- octet minimum up to 27-octets maximum) are transferred at 1 Mbps. All connections use advanced sniff-subrating34 to achieve low active duty cycles. The adaptive frequency hopping common to all versions of Bluetooth technology is used to minimize interference from other technologies in the 2.4 GHz ISM Band. BT-LE places a significant amount of intelligence in the controller, which allows the host to sleep for longer periods of time and be woken up by the controller35 only when the host needs to perform some action. This allows for the greatest energy savings. BT-LE can support connection setup and data transfer as low as 3 ms, allowing an application to form a connection and then transfer authenticated data in few milliseconds for a short communication burst before quickly tearing down the connection. Increased modulation index36 provides a possible range for BT-LE of over 100 meters. BT-LE uses a strong 24 bit CRC on all packets ensuring the maximum robustness against interference. Full AES-128 encryption using Counter with Cipher block chaining-Message authentication code (CCM) to provide strong encryption and authentication of data packets. BT-LE uses a 32 bit access address on every packet for each slave, allowing billions of devices to be connected. The technology is optimized for one-to-one connections while allowing one-tomany connections using a star topology. With the use of quick connections and disconnections, data can move in a mesh-like topology without the complexities of maintaining a mesh network.

Sniff sub-rating is a technique that reduces the active duty cycle, enhancing the powersaving capability of Bluetooth sniff mode [37].

The Bluetooth core system consists of a Host and one or more Controllers. A Host is a logical entity defined as all of the layers below the non-core profiles and above the Host Controller Interface (HCI). A Controller is a logical entity defined as all of the layers below HCI [37] The Bluetooth BR system employs GFSK (Gaussian Frequency Shift Keying) modulation.

The Modulation index can be between 0.28 and 0.35. For LE system, this index increases between 0.45 and 0.55.

Chapter 11. Wireless sensor network architectures and technologies

As mentioned above, BT-LE places a significant amount of intelligence in the controller, allowing the greatest energy savings. The reason why is because the link layer, which is part of the controller function, is in charge of providing ultra low power idle mode operation, simple device discovery and reliable point-to-multipoint data transfer with advanced power-save. The energy saving mainly comes from the ability of this layer to switch off the connection when not in use. Fig. 11.3 shows three possible Bluetooth Host and Controller Combinations: LE Only Primary Controller, BR/EDR only Primary controller and BR/EDR and LE primary controller [37]. The Figure 11.4 describes the protocol stack within the Host and Controller entities for the latest dual mode Bluetooth operation [37].

Fig. 11.3. Three possible Bluetooth Host and Controller Combinations [37]

–  –  –

Fig. 11.4. BT-LE protocol stack overview for the dual mode [37] Finally, note that the Bluetooth specification defines a star topology for the communication between Bluetooth devices. A mesh network composed of Bluetooth devices can be built, but it is beyond the scope of the Bluetooth specification.

Chapter 11. Wireless sensor network architectures and technologies

11.2. Home/Building automation and AMR/AMI solutions This section presents WSN solutions that are mainly used in the domains of home automation, building automation or AMR/AMI. The solutions presented are Z-Wave, Wavenis, EnOcean, INSTEON and One-Net, which are shown in subsections 11.2.1 to 11.2.5.

11.2.1. Z-Wave Z-Wave is a wireless protocol architecture developed by ZenSys (now a division of Sigma Designs) and promoted by the Z-Wave Alliance for automation in residential and light commercial environments. The main purpose of Z-Wave is to allow reliable transmission of short messages from a control unit to one or more nodes in the network [13].

Z-Wave is organized according to an architecture composed of five main layers: the Physical layer, the MAC layer, the Transfer layer, the Routing layer and the Application layer (see Fig. 11.5).

The Z-Wave radio operates in the 900 MHz ISM bands (e.g. 868 MHz in

Europe and 908 MHz in the United States). This feature has two benefits:

i) larger physical propagation range (or smaller power consumed for the same range) than that available when the 2.4 GHz band is used, and ii) avoidance of interference issues with other systems operating in the 2.4 GHz band, such as WiFi or Bluetooth. Typical ranges claimed by Z-Wave are 30 m indoors and 100 m outdoors. Z-Wave allows transmission at 9.6 kbps and 40 kbps data rates using Binary Frequency Shift Keying (BFSK) modulation. The recent Z-Wave 400 series single chip supports the 2.4 GHz band and offers bit rates up to 200 kbps. This chip has a frequency agility mechanism whereby the receiver simultaneously listens on three different channels and the transmitter can use the one with least interference.

The MAC layer of Z-Wave defines a collision avoidance mechanism that allows the transmission of a frame when the channel is available. Otherwise, the transmission attempt is deferred for a random period of time. The Transfer layer manages the communication between two consecutive nodes. This layer provides frame integrity verification by means of an 8-bit

Sensors Everywhere

checksum and an optional retransmission mechanism based on ACKs, which is only defined for unicast transmissions. Multicast and broadcast modes are supported.

Z-Wave defines two types of devices, namely: controllers and slave nodes. Controllers send commands to the slaves, which reply to these commands and execute them. Z-Wave uses a 32-bit unique identifier for each network referred to as Home ID (i.e. one network corresponds to one home).

Each node of a Z-Wave network is uniquely identified by an 8-bit Node ID. A Z-Wave network can be composed of up to 232 devices.

The Z-Wave Routing layer specifies routing operations on the basis of a source routing approach. When a controller transmits a packet, it includes the path to be followed in the packet. A packet can be transmitted over up to four hops. A controller maintains a routing table that represents the full topology of the network. The routing table is a binary bitmap, which is simple and easy to compress. A portable controller (e.g. a remote control), first tries to reach the destination via direct transmission. If that option fails, then the controller estimates its location and calculates the best route to the destination accordingly. A static controller has the advantage of always knowing its own location in the network. Slaves act as routers and have limited knowledge of the network topology. Routing slaves are a particular type of slave storing static routes and are allowed to send messages to other nodes of the network without being requested to do so.

The Z-Wave routing layer is also in charge of ensuring that a routed packet is correctly forwarded along the end-to-end path. For that purpose, the destination sends an ACK to the source, which is forwarded through the path followed by the data packet in the reverse direction.

The Z-Wave application layer is responsible for the coding and execution of commands in the Z-Wave network. This layer defines application layer messages, which codify a command and its related parameters. Z-Wave also defines command classes, which are groups of commands. There can be up to 128 command classes used by applications and 256 commands per class. Security services are supported in some Z-Wave products by the use of encryption engines.

Chapter 11. Wireless sensor network architectures and technologies

While the Z-Wave 200 and 300 series chips do not offer security services (which entail significant implementation size savings), the 400 series chip supports 128-bit AES encryption.

Z-Wave recently announced the launch of the Z/IP program to drive convergence of Z-Wave and TCP/IP, with the goal of facilitating remote home monitoring from Internet enabled devices. By mid-2009, Sigma Designs had introduced the IP-Wave chip, which runs an IP stack on the Z-Wave singlechip solution.

–  –  –

11.2.2. Wavenis Wavenis is a wireless protocol stack developed by Coronis Systems (an Elster Group Company) for control and monitoring applications in several environments, including automatic meter readings and home and building automation [14]. Wavenis is currently being promoted and managed by the Wavenis Open Standards Alliance (Wavenis-OSA) [15].

Wavenis defines functionality of physical, link and network layers.

Wavenis services can be accessed from upper layers through an application programming interface (API) (see Fig. 11.6).

Sensors Everywhere

Wavenis operates at 433 MHz, 868 MHz and 915 MHz bands, which are ISM bands in Asia, Europe and the US, respectively. The 2.4 GHz band is also supported [39]. The minimum and maximum data rates offered by Wavenis are 4.8 kbps and 100 kbps, respectively, 19.2 kbps being the typical value.

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