The advent of wireless sensor networks has made it possible for many cable-free, easily deployable residential applications. Several different kinds of wireless home-networking products have become available. However, challenges like reliability, coverage area and ease of use have continued to prevail.
In Europe these concerns are augmented by factors like hard to penetrate concrete walls and limited available bandwidth in 868 MHz security band (reserved for alarm systems). Bosch Research group has focused on these issues and came out a resultreliable, two-way wireless system. Wireless Local Security Network (wLSN) provides a combination of performance, reliability and ease of use.
wLSN is proprietary solution based on technology and designed for accurate intruder detection and reliable alarm communications. The system consists of battery operated wireless detectors, an intrusion control panel, RF hub, sirens and output modules. In addition to enabling communication among stationary devices, the wLSN supports mobile devices such as key fobs. They can be used to remotely arm or disarm the system and inform the user of the system status. The key fob has also two additional buttons that can be programmed for comfort applications such as opening and closing garage doors, operating window blinds or switching lights on and off.
The communication and networking protocol for this product incorporates a combination of duty cycling techniques, precise time synchronization, and adaptive frequency selection to achieve high levels of reliability while operating on batteries for many years. With the cutting-edge technologies of Stanford, Berkeley and Carnegie Mellon universities, there are currently 59 employees focusing on topics such as wireless technologies for automotive, building technology, consumer goods and industrial applications, sensor and communication technologies including integrated circuits, MEMS, RF applicat ions, sof tware engineer ing and software intensive systems, etc.
Wireless Security Network Design
The security network design, like a typical sensor network, involved choice of operating frequencies, hardware components and network protocol design. A typical sensing device consists of a sensor, microcontroller, transceiver and power source. The design goals were simple and clearreliability, long communication and installation range, easy and unobtrusive installation, extended battery life and competitive cost. However the resulting challenges were numerous. This was accentuated by the diverse requirements of global operation- -different frequency bands, regulatory mandates and market requirements. And protocol design aspects are the most critical and challenging.
Easy In stallation and System Initialization
Wireless technology enables the users to deploy the sensors almost anywhere . Automated device discovery and system auto-configuration further facilitate easy, fast and non-invasive installation. Devices in the network were designed with built-in RF signal strength indicators to help the installer position them in the optimal locations. An optional installation tool makes the process even easier by providing more information to the installer on the signal strength and noise level, and relative signal to noise ratio.
In addition to device discovery and configuration, practical deployments need to adapt to different RF environments and also coexist with other adjacent wireless deployments. This is done by an elaborate initialization scheme that learns about the environment and makes intelligent choices for network parameters like frequencies and thresholds that would ensure optimal and reliable network operation. The initialization scheme requires no user involvement, however allows setting of parameters by user when ever desired.
Reliability and In terference Immunity
Ensuring reliability is the one of the most important elements for a wireless system. Clearly, wireless as a physical medium of communication is more prone to errors than its wired counterpart. However, the overall reliability of the system can be greatly enhanced through careful and good design of the higher layers of protocol stack.
In a wireless system, packets can be missed due to many different reasons such as noise, fading, collision, or interference from other transmitters or devices. The network protocol des ign should address all these problems.
Interference Immunity and Coexistence via Operation in Reserved Alarm Bands
In Europe, a small set of channels in the 868 MHz frequency range have been reserved for alarm applications. The individual channels are only 25 kHz wide and on-air time is highly restricted (0.1percent duty cycle), which also prevents collisions with other security systems. These reserved bands provide immunity to interference from other wireless applications.
However, the communication data rate is very limited (4.8kbps,GFSK) due to the narrow band operation. This poses significant challenges as the duration for communication increases dramatically and thereby the energy per bit and on-air times go up. It is therefore extremely difficult to design a low power, latency critical system that complies with the stringent regulations of these alarm bands.
wLSN system has used techniques from ad hoc and wireless sensor network design theories to make this possible. Duty cycling techniques, accurate time synchronization methods, smart channel access schemes have been used to handle these challenges. The system incorporates a combination of Carrier Sense Multiple Access (CSMA) and Time Division Multiple Access (TDMA) schemes to ensure long battery life while avoiding collisions.
wLSN is a fully supervised two-way wireless network technology. Every communication in the system is acknowledged. Whenever the acknowledgement to a message is not received, it is retransmitted. Each message has several retry opportunities. The retries are designed to be in different frequencies so that if a particular frequency is being interfered, the retries are unaffected. To demonstrate how the reliability increases by having many retransmissions, consider the following simple example: If we assume that the packet loss rate of a wireless link is at most 10 percent which is reasonable for a node installed within the maximum specified installation range from the Hub, with five retries, the chance of missing a message on this link over all the retransmissions will be as small as 0.001 percent.
In stallation Range
The wLSN transceivers use advanced error correction and detection methods for better reception of messages with low signal to noise ratio. Narrow band operation in 868 MHz security band enables the receiver to achieve high sensitivity and detect marginal signals. This consequently leads to an outstanding wi reles s communicat ion range. Experiments and range measurements demonstrate an outdoor communication range of more than 1,000 meters in open fields.
Battery Life, On -Air Collision and Support for High/Low Priority Data
The bat tery-operated devices in the network are mostly in low-power (energy-saving) mode, and they become active only for very short durations for exchanging information with other devices. A synchronized time-slotted protocol handles all the periodic communication among the devices, such that each device gets its own allocated time slot to communicate. This rules out any on-air collisions of the wireless network devices. The system maintains a accurate network-wide time by exchanging periodic time beacons between a hub and remote devices.
In the wLSN protocol, the time is divided into frames which are further split into time slots. Time slots are assigned to different operation of the network such as alarm, command, synchronization, etc. The last time slot in each frame is allocated for low-priority data.
The devices in the network need to receive user commands with a short latency. However, it is very energy intensive to keep the nodes in the listen mode. Therefore the protocol employs a periodic wakeup schedule. This is performed by listening to the channel for a very short period of time and measuring the received signal strength. If the signal strength exceeds a pre-determined threshold, the node remains in full receive mode in an attempt to receive a possible command, otherwise it powers itself down to energy-save mode. In order to transmit a command to a node, the Hub first awakens the node by sending a tone at the wakeup sample time of the node, and then transmits the command message to it. This yields a fast mechanism to send urgent commands while keeping the average energy consumption rate of devices low.
The protocol also uses a prioritizing channel access mechanism that ensures timely message delivery for high priority alarm messages while the less urgent messages may incur higher latencies. The energy-efficient synchronized time-slotted protocol along with the optimized wake-up method, enable a typical battery lifetime of up to five years for battery-powered wireless devices with easy available and inexpensive AA alkaline batteries.
The wLSN system is an option for residential and small commercial systems. It has been designed to take the user experience with regard to installation ease, reliability and maintenance-free operation to new levels. Bosch hopes to reinvest the knowledge and experience gained into building scalable and large systems in future.