Data communications in most telemetry applications feature periodic or intermittent data, or repetitive exchanges of low latency data. Even though a number of wired and wireless networking technologies have emerged over the years to support a wide variety of data communication needs as well as voice and real-time streaming data, the particular requirements of a broad segment of this market remain unanswered. These include ultra low power operation at several hundred metres' range, low BOM cost per terminal, low operating costs and networks featuring large numbers of terminals. Perhaps the toughest of these is the requirement for ultra low power operation over an extended range. To allow large networks of self-sufficient equipment to be active in the field for periods up to 10 years without replacing batteries, the average consumption of the terminal has to be around 18µA. This estimate assumes the device is powered by a single, 2.7Ah AA-size lithium battery. Allowing for typical losses due to self-discharge and internal resistance, only around 60% of this capacity, or 1.6Ah may be utilised, implying the maximum permissible average current draw of 18µA over the lifetime.

Currentcontenders

Future telemetry applications are unlikely to use established telephony or LAN standards on the grounds of cost and power consumption. Digital Enhanced Cordless Telephony (DECT) has some validity in telemetry, control and supervision applications. DECT's strength lies in its ability to effectively combine high quality voice telephony with data services in the same base station, making it more suitable for applications including paging, short message services, group calls, automatic alarms, or data collection. Some remote data terminals, for example those addressing agricultural applications where data transmission distances can require a range of some tens of kilometres, relay information via GSM. However power consumption is high. Systems fitted with a solar power unit can sustain year-round operation, but add to the per-unit cost of the terminal. The compliance demands placed on equipment for use in the licensed GSM bands also increases production costs by a factor between two and four compared to terminals for use in the unlicensed bands. Finally, recurring subscription and connection charges from the mobile service provider will add to the total cost of ownership of such a solution.

Short range wireless protocols

Bluetooth and ZigBee have been designed to stringent cost targets to enable ubiquitous wireless networking. Ratified in October 2003, the ZigBee standard has been conceived as a wireless network solution optimised for low cost, low data rate applications up to 20kbps or 250kbps. It defines long battery life, robust security, high data reliability, and product interoperability. But ZigBee's nominal transmission range of 10m, with a maximum of 100m will prevent deployment in larger networks serving industrial installations, agriculture, or even forthcoming utility metering applications designed to enhance service delivery to customers. Similarly, even at low BOM costs, the higher data rates of Bluetooth are subject to the same range restrictions that apply to ZigBee. Off the shelf Bluetooth solutions also generally support profiles for voice applications as well as data, making them too complex and expensive for direct use in telemetry. By trading-off some of Bluetooth's generous 1Mbit/s data rate in favour of longer range and lower overall power consumption, an extension to the Bluetooth standard, capable of satisfying the requirements of a variety of such applications, is possible.

Extending Bluetooth

Working in that direction, French company Coronis Systems developed Wavenis, defining ULP hardware and firmware technology with PHY and MAC layers very similar to Bluetooth specifications. Consider a link budget of, say, 125dBm, which is usually adequate for metering, security, healthcare, or harsh environments. Assuming 15dBm output power, the receiver sensitivity must be improved to -110dBm in order to meet the target link budget. This is 20dBm better than the most sensitive Bluetooth receivers currently available. But sensitivity is not only related to receiver technology in terms of the noise figure and signal to noise demodulation technique. It is also a function of the noise bandwidth, which can be reduced by a corresponding reduction in data rate. Using the same technology as the existing Bluetooth receiver, therefore, the required 20dB improvement can be achieved by reducing the data rate by a factor of 100. Data rates up to 10kbps over 1km are then possible. Since Bluetooth has been conceived from the outset for low power operation it already features power saving modes including Hold, Sniff and Park. By using these modes carefully, and avoiding more power-hungry modes such as page scans, inquiry scans and periodic inquiries, Wavenis modules can exhibit an average operating consumption as low as 15µA, offering up to 10 years of battery life.

Put in practice

Wavenis nodes can establish a 2-way transmission link with a control point, or can be integrated into a network and communicate with other nodes. It can be programmed to offer services including periodic data transfer to the central node, event triggered warning messages, peer-to-peer communication. Network configuration parameters can also be wirelessly updated. A new class of dual-mode Bluetooth/Wavenis master devices could be created, capable of supporting Bluetooth piconets as well as extended ULP piconets via Wavenis technology.