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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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Data are modulated using Gaussian Frequency-Shift Keying (GFSK). A fast Frequency-Hopping Spread Spectrum (FHSS) is used over 50 kHz bandwidth channels. Unlike other spreading techniques, this solution allows transmissions to be performed in a narrow band with improved sensitivity and, in consequence, it offers long range link capabilities. In fact, Wavenis features an indoor range of 200 m and an outdoor line-of-sight range of 1 km. Other data processing techniques are BCH (32,21) Forward Error Correction (FEC) encoding, data interleaving, scrambling and, optionally, data encryption.

Wavenis devices are identified by 6-byte MAC addresses. Unicast, broadcast and multicast communications are supported. The Wavenis MAC sublayer offers synchronized and non-synchronized schemes. Synchronization is generally performed at preselected times that depend on the application.

For example, lighting applications may require more frequent synchronization than metering applications.

In a synchronized network, nodes are provided with a mixed CSMA/TDMA mechanism if feedback is requested after a broadcast or multicast message. In such a case, a node allocates a timeslot which is pseudorandomly calculated according to its address. Before transmission in that slot, the node performs Carrier Sense (CS). If the channel is busy, the node computes a new timeslot for the transmission. For non-synchronized networks, in applications where reliability is a critical requirement (e.g. alarms, security, etc.), CSMA/CA is used.

The Wavenis Logical Link Control (LLC) sublayer manages flow and error control by offering per-frame or per-window ACKs. Unreliable communication (i.e. without ACKs) is an available option.

In contrast with other technologies, Wavenis defines only one type of device. (For example, IEEE 802.15.4 defines two device types, RFDs and FFDs; Z-Wave defines two device types also, which are controllers and slaves).

Chapter 11. Wireless sensor network architectures and technologies

Wavenis network layer specifies a four-level hierarchical tree. The root of the tree may play the role of a data collection sink or a gateway, for instance. A device that joins a Wavenis network broadcasts a request for a device of a certain level and a sufficient QoS value. The QoS value is obtained by taking into consideration parameters like Received Signal Strength Indicator (RSSI) measurements, battery energy and the number of devices that are already attached to this device.

Wavenis systems support self organising and self healing networking capabilities. In addition peer to peer (P2P), star, tree, mesh, broadcast and repeater operating modes can be enabled [39].

In mid-2009, members of the Wavenis-OSA board of directors declared that IP was being considered as layer three for future Wavenis specifications.

Wavenis supports various encryption algorithms, such as Triple Data Encryption Standard (3DES) and 128-bit AES.

–  –  –

11.2.3. EnOcean EnOcean is a proprietary technology developed by a homonymous company [16], which is being promoted by the EnOcean Alliance [17]. EnOcean is based on a self-powered WSN solution for various environments and has been successful in the building automation area. The energy used by EnOcean products is mainly harvested from motion, light and thermal sources [40] (see Chapter 14 for details about energy harvesting).

Sensors Everywhere

EnOcean uses the 315 MHz and 868 MHz bands, which are available in North America and Europe, respectively. Data are modulated using Amplitude Shift Keying (ASK), which means that energy is only consumed for transmission of the “1” symbol. Data rates are 125 kbps and 120 kbps, respectively, while the transmission range is 200 and 300 meters in free field, respectively. The packet size is 14 bytes, and the payload size is 4 bytes.

EnOcean devices are identified by 32-bit serial numbers. Typical EnOcean deployments have been point-to-point or point-to-multipoint, although it is also possible to use repeaters or a simple proprietary mesh networking solution.

Some of the main applications for EnOcean include lighting, shading, presence detection, window monitoring, and room temperature sensing and measured data acquisition.

11.2.4. INSTEON

INSTEON is a protocol architecture developed by SmartLabs for home automation [18]. INSTEON is promoted by the INSTEON Alliance. One of the distinctive features of INSTEON is the fact that it defines a mesh topology composed of RF and Power Line Communication (PLC) links. Devices can be RF-only, power-line-only, or can support both types of communications.

INSTEON RF signals use Frequency-Shift Keying (FSK) modulation with a 64 kHz deviation frequency at the 904 MHz center frequency, with a raw data rate of 38.4 kbps. The typical range in outdoor environments is roughly 45 m.

INSTEON offers unicast, multicast and broadcast communications services. Messages are protected with an 8-bit CRC. ACKs (either positive or negative ones) are sent in response to unicast messages and may piggyback information about the status of the acknowledging device. The devices are identified by 3-byte unique numbers.

INSTEON devices are peers, which means that any device can play the role of a sender, a receiver, or a relayer. Communication between devices





Chapter 11. Wireless sensor network architectures and technologies

that are not within the same range is achieved by means of a multihop approach, which differs in many aspects from more traditional routing techniques used in other technologies. All devices retransmit the messages they receive, unless they are the destination of the messages.

The source specifies the maximum number of hops for each message, which cannot exceed four hops, as in Z-Wave. The multihop transmission is performed using a timeslot synchronization scheme, by which transmissions are permitted at certain timeslots and devices within the same range do not transmit different messages at the same time. These timeslots are defined by a number of powerline zero crossings37. RF devices not attached to the powerline can transmit asynchronously, but the messages will be retransmitted synchronously by RF devices attached to the powerline. In contrast with classical collision avoidance mechanisms, devices within the same range are allowed to transmit the same message simultaneously. This approach, which is called simulcast, relies on the very low probability of multiple simultaneous signals being cancelled, either using the powerline or RF media. In fact, simulcasting increases communications reliability. The source of a message may perform up to five retransmissions if an ACK from the destination is not received in response to that message.

INSTEON messages contain two bytes that codify the command to be executed by the receiver. INSTEON defines 16 different device categories, and for each category there may be up to 4096 devices. Two types of messages are defined: 10-byte standard messages and 24-byte extended messages. Extended messages can contain encrypted payloads.

INSTEON was developed by keeping compatibility with X-10 systems38, which gives INSTEON the possibility of being installed in many residential environments where X-10 devices are already deployed.

Powerline zero crossings are the periods whereby the electrical signal value is equal or very close to zero. This feature helps to keep compatibility with the X-10 home automation system (see next footnote).

X-10 is an old home automation system with limited functionality that uses the power lines to communicate between nodes.

–  –  –

ONE-NET has been developed by a group of companies to offer an open standard for low power wireless connectivity [19]. The protocol is intended for developing control applications for home and small office environments.

ONE-NET can be used in the 868 MHz and 915 MHz bands (any of the 25 channels available in the US can be used). The nominal data rate is

38.4 kbps, but throughput is limited to 6 kbps. The distance between two nodes is up to 60 – 100 m indoors. CSMA is used as the MAC mechanism.

The devices are assigned 48-bit identifiers, which are called system IDs, and are compose of a 36-bit network ID and a 12-bit device ID.

ONE-NET offers several topologies, including star, peer-to-peer and mesh. Security is supported by means of the Extended Tiny Encryption Algorithm (XTEA), which is a cipher that uses a 128-bit key. Data transactions are protected against spoofing and replay attacks by means of nonces39.

11.3. Industrial automation networks

This section focuses on the main WSN solutions for the Industrial Automation domain. An overview is presented for the Time Synchronized Mesh Protocol (TSMP), a protocol developed by Dust Networks, which has been used as a core component of WirelessHART and ISA100.11a.

Subsection 11.3.1 focuses on TSMP, while sections 11.3.2 and 11.3.3 present WirelessHART and ISA100.11a, respectively.

11.3.1. TSMP

TSMP [20, 21] is a protocol developed by Dust Networks [22], which provides services with the aim of contributing to end-to-end reliability at the MAC, network and transport layers, particularly for industrial automation networks. TSMP also provides physical layer enhancements.

A nonce is a number or bit string used only once, in security engineering.

Chapter 11. Wireless sensor network architectures and technologies

TSMP has been implemented in the 2.4 GHz band on IEEE 802.15.4 radios and in the 900 MHz band on proprietary radios. TSMP assumes the use of FHSS in conjunction with DSSS in order to achieve greater robustness against interference than when DSSS alone is used. Due to the use of FHSS, consecutive transmissions between a node and its next hop are performed by using different frequencies.

TSMP, on the other hand, is based on a TDMA approach. Hence, consecutive transmissions between two nodes take place in different frequencies as well as in different timeslots. A node can participate in different frames (which comprise a number of timeslots) of different sizes for performing different tasks at once. For a given task, the transmission rate of a node depends on the corresponding frame size. Note that TDMA naturally allows duty cycling. In TSMP, nodes are synchronized with a scheme that is based on message exchanges between a node and its neighbours.

In TSMP, all devices are capable of routing data. A TSMP node joins a network by discovering neighbours, which requires the node to listen at the frequencies available and to measure the RSSI corresponding to each neighbour. The node obtains synchronization and frequency hopping information, and subsequently establish links with its neighbours.

TSMP relies on spatial and temporal diversity to assure data delivery.

Spatial diversity is achieved by the use of mesh topologies, which offer path redundancy between a source and its corresponding destination (which is in general a sink node). Temporal diversity is achieved by the use of link layer ACKs and NACKs (which are generated when the receiver’s buffer is full). If an ACK is not received after the transmission of a frame, a retransmission attempt takes place in the next available slot.

The routing paradigm used in TSMP is called graph routing. A graph is a structure that represents the nodes of a network and the connections among them. TSMP uses directed acyclic graphs towards certain destination nodes (e.g. a special node called a network manager). A network may have multiple graphs if several destinations are defined. A node includes the identifier of a graph in the packets that it transmits. This identifier is used by intermediate nodes for retransmitting the packets towards their next hops.

Sensors Everywhere

TSMP offers encryption and authentication/integrity at the network layer, as well as frame authentication/integrity at the link layer. The cryptographic operations employ the 128-bit key AES algorithm in CCM* mode (see chapter 9, section 9.2.2).

The following types of keys are used:

• Join keys, which are assigned to each device prior to deployment, and are used in the process of authenticating the joining device with the network manager, which maintains an access control list.

• Network keys, which are shared by all network devices and are used for MAC layer frame authentication/integrity.

• Session keys, which are used to provide end-to-end confidentiality and data integrity.

When a node joins a network, it sends a join request message encrypted with its Join key. The network manager checks the identity and the key used by the new device. If these parameters correspond to a valid device, the new device is admitted and the network manager sends a Session key and the shared Network key to the device. The dialogue between the network manager and the new node is encrypted with the node’s original join key.

Dust Networks currently offers the family of SmartMesh products [42], which were developed for various applications, with a special focus on industrial automation. These products use an IEEE 802.15.4 radio and have a 200 m range in an outdoor environment, with a consumption of 50 mA.

The manufacturer also offers WirelessHART compatible devices based on the SmartMesh platform.

11.3.2. WirelessHART

WirelessHART is a wireless communication technology designed for industrial plant applications, which is included in the Highway Addressable Remote Transducer (HART) 7 specification. It is developed by the HART Communication Foundation, which specifies the standards for the HART communication protocol [23]. This protocol is a bi-directional communication protocol that offers communication between intelligent field instruments and host systems [24].

Chapter 11. Wireless sensor network architectures and technologies

WirelessHART is compatible with 2.4 GHz IEEE 802.15.4-2006 physical layer and MAC frame format. The MAC mechanism used is a combination of CSMA and TDMA, where a timeslot has duration of 10 ms. Frequency hopping is used on a message by message basis. Channel blacklisting is supported in order to avoid the use of interfered channels. Various types of superframes can be defined, which support fast (i.e. transmission every second), slow (i.e.



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