A WHAN for smart surveillance and intrusion detection has many peculiarities that are common to a broader class of industrial automatic control systems. The physical (PHY) layer radio characteristics mostly common to all these systems are low data rate (below 500 kbps), carrier frequency at 2.4 GHz or in the 868/915-MHz ISM bands, and receiver sensitivity above -110 dBm[4]. Most of the PHY layer access schemes are based on offset quadrature phase-shift keying (O-QPSK) or frequency-shift keying (FSK) combined with direct-sequence spread spectrum (DSSS) transmission which allows the network susceptibility to interference to reduce, providing a few dBs of link gain and some improvements over harsh environments characterized by multipath fading. Multi-channel radios are also adopted to efficiently manage the co-channel interference and to reject any external disturbance through dynamic scheduling and channel/frequency hopping.
The basic requirements that need to be considered for WHAN protocol design are listed in the following.
-
Network services. A wireless network for smart surveillance must support different classes of traffic patterns with related quality constraints. The network should provide both a periodic low-power monitoring service and a real-time alarm propagation mechanism which must be robust enough to cope with inherently unreliable wireless links characterized by signal power fluctuations. Intrusion detection systems have stricter reliability and delay requirements compared to conventional home automation services. Reliable communication occurs only if both sensor observations and feedback from the AP controller are decoded by the respective parties within specified deadlines defined by the controller policy. This hard constraint calls for an advanced wireless link-layer protocol management to provide an optimal trade-off between reliability and real-time communication.
-
Indoor radio planning. Indoor home environments are typically characterized by severe multipath due to the presence of reflective surfaces (e.g., walls, floor, and furniture)[4]. Radio planning is a useful tool which relies on the prediction of the wireless link quality. Prediction can be supported by independent radio measurement campaigns over typical indoor buildings and/or by empirical propagation models. A low accuracy in the radio planning design phase will turn into high logistic costs: adding new wireless REs to improve the coverage as well as moving them around the environment may become unacceptable and highly time consuming in some cases.
-
Low duty cycling operation. Wireless autonomous devices are battery powered. EDs are usually deployed in predefined spots and must remain active for 3 to 5 years. This poses stringent constraints on the sensor and the radio transceiver design for minimizing the energy consumption. The MAC sub-layer protocol and the application software need to be jointly optimized to preserve the battery. Energy harvesting techniques also provide a powerful tool for lifetime maximization. Some of the techniques employed to reduce power consumption include: (1) dynamic sleep mode activation (with fast wake-up times) to shut-down devices when not transmitting or receiving and (2) low duty cycling design to minimize ED activity cycles. Being the network almost static, the adoption of guaranteed (interference-free) time-division multiple access (TDMA) and beacon-enabled network designs[5] are to be preferred compared to random access strategies to minimize idle listening.
2.1 Indoor surveillance systems: EU specifications
European wireless intrusion-detection systems are governed by the EN 5013-5-3[3] law that disciplines several aspects like the immunity against channel variations and transmission collisions, the detection of device substitutions, and the robustness to radio interference. The same rules explain how to test every single device to ensure the compliance of the intrusion-detection system to the requirements. The EN 5013-5-3 specifications identify four levels (or grades) of security and specify for each level the requirements on some key system parameters. Below, we briefly describe these parameters and the related requirements for each level of security, ranging from 1 to 4 with increasing security degree.
-
Channel immunity. This property is related to the sensitivity of the system to channel variations and particularly to any attenuation increase (i.e., due to deep fading, interference etc.): grade 1 service can support an attenuation increase of up to 3 dB, grade 2 to 6 dB, grade 3 to 9 dB, and grade 4 to 12 dB.
-
Transmission collisions. The objective of the collision rate requirement is to ensure a high level of confidence during the transmissions of alarm and monitoring messages to avoid auto-interference (interference among devices of the same system). The collision rate depends on the channel occupation time (or duty cycle) of the devices. Grade 1 service is characterized by a duty cycle larger than 10% measured over 240 min, duty cycle for grade 2 is lower than 5% over a period of 240 min (or 10% over a period of 120 min), while duty cycles for grades 3 and 4 are lower than, respectively, 10% and 1% measured over a cycle time period of 100 s. Notice that a more stringent 1% duty cycle limitation is applied to low-power wireless communications over ISM frequency bands.
-
Link reliability. The link reliability measures the probability of information loss during the communication. It is set to 10-3 for grades 1 to 2 (corresponding to 999 correctly interpreted messages out of 1,000[3]) and to 10-4 for grades 3 to 4 (corresponding to 9,999 correct messages out of 10,000). For wireless communication over harsh indoor propagation environments, the use of direct/indirect retransmissions (if allowed) can be optimally designed to comply with such specifics.
-
Security. In order to prevent both unintentional and intentional device substitution, each transmitter shall be identified by an identification code. The security levels are characterized by the probability for an intruder to discover the identification code in less than 1 h: this is 5% for grade 1, 1% for grade 2, 0.5% for grade 3, and 0.05% for grade 4.
-
Cross-tier interference. The system robustness against cross-tier radio interference is measured in terms of in-band and out-of-band interference. Let Fmin and Fmax be the minimum and maximum carrier frequencies: the out-of-band carriers F1 and F2 are defined as F1 = 0.95 · F
min
and F2 = 1.05 · F
max
. Grades 1 and 2 compliant systems are robust against an interferer centered on F1 and F2 with an intensity of 10 V/m, while grades 3 and 4 systems are also robust to an in-band interferer centered on the carrier frequency F
t
= (F1 + F2)/2 with an intensity of 10 V/m.
-
System monitoring. The measurement of the noise and the interference level is implemented by a periodic message exchange. The period interval for link quality sensing depends only on the transmitter role in the system (e.g., ED or RE devices) and on the network topology. From grade 1 to grade 4, the system has to guarantee that the time interval is not greater than 60 min, 20 min, 100 s, and 10 s, respectively.
-
Antenna protection. Grades 1 and 2 are assigned if the antennas cannot be removed without opening the housing, while grades 3 and 4 are assigned if the antennas fulfill the same tamper protection requirements valid for the corresponding devices.
2.2 Wireless protocols for home and building automation
In the following, a review of the most suitable commercial systems for wireless networking in home and building automation is presented. The selection criteria include frequency bands, data rates, modulation techniques, routing schemes, topologies, interoperability, openness of the software architecture, standardization, and general suitability to support critical home automation applications and security[6].
Bluetooh (Bluetooth Special Interest Group, Kirkland, Washington, USA) and ZigBee (ZigBee Alliance, San Ramon, CA, USA) have been recently investigated in the literature for WHAN applications. A Bluetooth WHAN has been introduced in[7] using a primary network controller and a number of sub-controllers connected by star topology. However, the wireless architecture does not completely replace cabling, and the use of the Bluetooth technology shows disadvantages in terms of access delay. A ZigBee-based WHAN has been proposed in[8]. Although ZigBee interface based on the IEEE 802.15.4-2006 standard[9] provides an effective network solution for low-power wireless sensing, the overall size of the radio stack (between 45 and 100 kb) limits its applications to a small subset of smart home automation scenarios. ZigBee is briefly reviewed in the following together with a selection of wireless technologies that present interesting characteristics for WHAN applications. The comparative analysis is also summarized in Figure2.
ZigBee is a wireless networking technology developed by ZigBee Alliance for low data rate and short-range applications[8]. Protocol stack is composed of four main layers. The first two layers (PHY and MAC) are defined by the IEEE 802.15.4 standard, while the network (NWK) and application (APL) layers are defined by the ZigBee specifications. The standard IEEE 802.15.4[9] is a specification for low-power WSN originally designed for the frequency bands 868 to 868.6, 902 to 928, and 2,400 to 2,485 MHz. For the 2,400 - 2,485 MHz band, the PHY layer transmission maps any 4-bit codeword into a 32-chip sequence using DSSS with a bandwidth expansion factor of 8. The chip sequences are concatenated, modulated, and translated to radio frequency (RF) using O-QPSK modulation. Today, commercial battery-operated systems enable data to be transmitted at a rate of up to 250 kbps, while a maximum power of 12 dBm guarantees a reasonably high channel immunity against deep fades (up to grade 4). In critical environments, the transmit power could not exceed 12 dBm to meet the RF regulations for the use of unlicensed spectrum in hazardous environments. ZigBee upper layers support two methods for channel access, the beacon-less and the beacon-enabled access. In beacon-less mode, devices employ a plain carrier sense multiple access with collision avoidance (CSMA/CA) scheme based on the low power listening principle. The use of CSMA as access technology for all sensors is unsuitable for critical delay-sensitive applications such as intrusion detection systems subject to real-time constraints. On the other hand, in beacon-enabled mode, a coordinator node (i.e., the personal area network coordinator) acts as a clock distributor to provide a framing structure by periodically transmitting beacon frames. The frame is the time between two beacons, and it is divided into three parts: a contention-access period for CSMA/CA, a contention-free period for TDMA, and an inactive period to power-off devices.
The Z-Wave protocol (Zen Systems, Hillsborough, NJ, USA)[10] was developed with an explicit focus on home control applications. Z-Wave operates at 908 MHz in the US and in the ISM band of 868 MHz in Europe, using FSK modulation with data rate 200 kbps. Z-Wave uses a mesh networking approach with source routing, which means that the whole route is determined already at the creation of the frame in the sender. Therefore, only devices which are aware of the entire network topology can send ad hoc messages to any destination. Z-Wave consists of several types of nodes that can be clustered into two main classes, controllers (nodes that create and send control messages) and slaves (nodes that receive and execute the commands). The standard is specifically tailored for remote control of devices used in both residential and commercial buildings. However, the protocol has not been designed to transfer large amounts of data, and it is not suitable for real-time critical data transmission.
EnOcean is a proprietary environment not yet standardized at international level[11]. EnOcean offers its technology and its licenses through the EnOcean Alliance (San Ramon, CA, USA). The objective is to provide self-powered wireless devices, such as piezoelectric or mini solar panels, highly optimized for energy saving for the automation of homes and buildings. Messages are only a couple of bytes long (with a maximum payload of 6 bytes) and are transmitted using amplitude shift keying (ASK) modulation at the data rate of 125 kbps. Packet transmission takes less than 1 ms. The EnOcean protocol cannot increase the transmission reliability by means of end-to-end acknowledgments since its battery-less transmitter modules do not contain a RF receiver. No security mechanisms appear to be included.
Wavenis is a wireless protocol operating at 868, 915, and 433 MHz, developed by Coronis System (Pérols, France) for monitoring and control applications in several environments such as homes and buildings[4]. The standard Wavenis, currently promoted and managed by Wavenis Open Standard Alliance, supports data rate up to 100 kbps and adopts Gaussian frequency-shift keying (GFSK) modulation in conjunction with fast frequency hopping spread spectrum (FHSS). It defines the operations at the PHY, data link and NWK layers, delivered through proprietary APIs.
MiWi wireless protocol[12], developed by Microchip Technology (Chandler, AZ, USA), uses low-power radio systems based on IEEE 802.15.4 for short-range transmissions. Given the small size of the protocol stack, MiWi-based solutions are an alternative to ZigBee for low-cost applications requiring small memory space and able to operate on simple low-cost micro-controllers. The system is based on the IEEE 802.15.4 recommendations for wireless personal area networks (WPAN). It supports a smaller number of functions compared to ZigBee, and it is meant for simple networks with either peer-to-peer, star or mesh topologies in beacon-less configuration. MiWi provides advanced functionality at PHY, MAC, and NWK levels, all accessible through the use of proprietary APIs.
Insteon technology is developed by SmartLabs, Inc. (Irvine, CA, USA) and promoted by the Insteon Alliance for the field of home automation[13]. The system utilizes a dual technology to support communication between devices: it employs both powerline communications with X10 protocol and wireless communications using FSK modulation at 900 MHz. All Insteon compliant devices are peers, which means that each device is able to transmit, receive, and repeat any message compliant with the Insteon protocol, without the need of a master controller or routing software. The powerline communication infrastructure is used to provide synchronization to the wireless system.
KNX-RF (KNX Association cvba, Diegem, Belgium) is a wireless solution specified in Supplement 22 of the KNX specification for cable based systems[14]. Thereby, KNX is not a protocol tailored for radio-frequency communication, but rather a home and building automation standard based on wired media that has been extended to support wireless communications. KNX RF operates at 868 MHz using FSK modulation at 16.4 kbps. KNX RF allows unidirectional (transmit-only) devices in addition to conventional bidirectional ones. Transmit-only devices cannot be configured thorough the network. Data reliability is guaranteed only by APL layer acknowledgements, while link layer acknowledgments are forbidden. KNX RF does not provide any security mechanism. Since the transmitted data are neither encrypted nor subject to integrity check, KNX RF cannot fulfill the high demands of security in critical applications.