Wireless-sensor networks find a fit in the unlicensed band
( 01 Jul 2006 )
by Margery Conner, Technical Editor, EDN
Low-power, short-range, low-data-rate wireless networks use the unlicensed RF band. In addition to their overriding need to operate on severely constrained energy sources, such as using one battery for their entire lifetimes or employing energy they scavenge from the environment, these networks must contend with interference from sources such as Bluetooth headsets and microwave ovens. These networks use network protocols that largely determine the networks’ efficiency, robustness, and security. As such, network designers need to first determine which sub-band is best for their application: 900MHz or 2.4GHz. In addition to selecting the frequency band, they also need to decide on a hardware-transceiver scheme: a roll-your-own or an SOC (system-on-chip) approach. They must also decide which network protocol to use.
Both the 900MHz and 2.4GHz bands have advantages. Both reside in the ISM (industrial/ scientific/medical) band, an unlicensed frequency band of 902 to 928MHz, 2.4 to 2.483GHz, and 5.725 to 5.875GHz. Almost all of the transceiver and SOC products targeting wireless-sensor-network applications use the 900- to 928-MHz and 2.4- to 2.483-GHz bands. The 900MHz band touts long broadcast range because of its relatively longer wavelength and its correspondingly longer battery life. However, lower frequency means the use of a larger antenna than a 2.4GHz system requires. And, if you plan to sell your system into a global market, you will quickly encounter a lack of standardization in the 900MHz range. For example, in Europe, you cannot use the 900-to 928-MHz band because it is part of the GSM (Global System for Mobile communications) network for cell-phone communication and, thus, is unavailable.
Wireless-sensor-network-system pioneers, such as Aerocomm, Dust Networks, and Crossbow Technologies, take different approaches to these constraints. For example, Aerocomm sells wireless-network systems into the global market, offering different radios and frequencies for different regions. Randy Macke, international marketing manager at the company, explains, “We typically use 2.4GHz for higher-data-rate applications at short range. The flip side is when you go to a 900MHz product family, there is better range performance but typically a slower data rate. These are the basic trade-offs.” He says that the company aims at making its products global by using one proprietary protocol and interface on every radio it sells; hence, a customer can base a product on Aerocomm’s 900MHz product and then sell into another country that doesn’t accept 900MHz just by swapping in a pin compatible radio.
David Boylan, marketing manager for Analog Devices, also recommends this approach. Boylan sees companies opting for the 900MHz or the 2.4GHz frequencies because of the technical attributes of the two bands and whether these companies need to adapt to a proprietary or a standard network. According to Boylan, vendors often choose the 2.4GHz band because either the vendor or its customers like the security of standards. But this preference is not universal. Other companies prefer to create their own proprietary networks, he says. Reasons for this preference are the value-added and security aspects of proprietary networks. “If they have their own proprietary network, they can implement security features that wouldn’t be available in a published standard,” Boylan says. These vendors typically prefer to use frequencies of less than 1GHz, such as 900MHz (Table 1).
He cites as an example a popular 900MHz application of an automatic meter reader. “You usually have a central node, which might be the utility employee with his handheld unit pinging several units. The 900MHz band is popular because range and battery power are key requirements,” says Boylan. For networks relying on tiny sensor nodes, the 900MHz antenna of approximately 8cm is probably too large. In this case, even if your market is 900MHz-friendly North America, you may need to consider 2.4GHz because of its smaller antenna (see box “Antenna technology feels the squeeze”). Airtime also affects power consumption. Wireless-sensor networks conserve power by spending 99.99% of their time asleep, and they wake up only to contact their network and briefly transmit their information, thus achieving their goal of consuming an average of less than 1μA of current (Reference 1). Therefore, the faster they can transmit their data, the more quickly they can go off the air. This point favors the use of the 2.4GHz spectrum.
Dust Networks develops both the hardware and the software for companies needing wireless-sensor networks. Dust makes 802.15.4-compliant products in the 2.4GHz band and in the proprietary network, 900MHz band. Many customers use the 900MHz network devices for building automation. For this application, says Rob Conant, Dust’s vice president of marketing and business development, the 900MHz band presents a technical advantage because of its longer range and better penetration in buildings, which allows lower power consumption (Reference 2). As for the marketing side of the equation, Conant believes that the 2.4GHz band has an edge. “The global ISM band is the only way to go for companies that want to bring out global products,” he says. “Although 2.4GHz products suffer a penalty in power consumption to get the same range [as 900MHz], 2.4GHz is a standard, and customers know it’s going to be around for years.”
Karl Torvmark, strategic product-marketing manager for Chipcon, agrees. Chipcon’s 2.4GHz ZigBee devices reach 100m or more in good conditions, he says. For a longer communications range, you can increase the output power beyond 1mW, but doing so directly affects the power consumption. If you go below 1GHz with the same output power, you reach several hundred meters, but then an antenna issue arises. “In the hundreds of megahertz, you like to have a whip antenna, usually mounted externally ... if that’s compatible with your overall product design,” he says. Although you can use pc-board antennas operating at less than 1GHz, they can be impractically big,” he states.
Torvmark sees customers leaning toward custom protocols selecting 900MHz, just as ADI’s Boylan does. Chipcon’s customers want to customize their networks, which provides advantages and trade-offs in power consumption, he says. For example, Chipcon’s 900MHz band products, the CC1100 and the higher performance CC1020, don’t offer the total network stack that the 2.4GHz ZigBee products have.
Although Dust’s Conant is a steadfast proponent of the 802.15.4 standard, he’s not jumping on the ZigBee protocol bandwagon: He sees severe limitations to ZigBee for ultralow-power wireless-sensor networks. He gives an example of one instance in which the ZigBee standard uses a less reliable approach than he claims Dust can achieve using its own 802.15.4-compliant network, SmartMesh. “An 802.15.4 radio has 16 channels. ZigBee uses only one of those channels at a time. It’s configurable which channel you use, but, you set it up, and it uses only one of those channels,” he says. And that ch nnel may be experiencing interference from local devices, such as a nearby Bluetooth headset or a microwave. Dust’s network hops between all 16 channels based on external interference, which the company claims results in as much as 99.99% reliability.
Clearly, a range of opinion exists about the best way to handle network protocols for 802.15.4 networks (see box “A brief guide to 802.15.4 and network protocols”). The benefits of being a standard-compliant technology combine with the approach’s global reach and its use of the 2.4GHz band to make ZigBee a likely winner. IC vendors Freescale, Ember, and Chipcon have all introduced ZigBee-compliant SOC or SIP (system-in-package) products.
Freescale was an early proponent of 802.15.4 and ZigBee, releasing its 802.15.4 transceiver on a chip the day after the IEEE ratified the specification. However, Jon Adams, director of radio technology and strategy forFreescale, claims that system designers don’t want to know how to design a network and applications protocol; they just need the basic digital-radio function. So, the company introduced a software-based configuration tool that lets them develop applications using the 802.15.4 stack. Once ZigBee came along last year, he says, the company recognized that designers also need a function for using that ZigBee stack, so that they could quickly build a ZigBee process without reading 700 pages of the spec. “All of our efforts have been toward creating a one stop shop,” he says. Ember and Chipcon have embraced the same approach.
Ember started out developing wireless-sensor-network systems, such as its proprietary EmberNet, before ZigBee existed. Thecompany was also an early member of the ZigBee Alliance. Vice President of Engineering Skip Ashton explains the importance of a multicompany synergy in developing such a complex mesh: “With ZigBee, in addition to the network, you need an ecosystem of the things around your network,” he says. Although one company can provide a network stack, a lot of companies are working on other tools and services in a standards based environment. Other advantages of ZigBee include interoperability testing, multiplevendor testing, and the availability of multiple sources. Ember’s EM250 SOC combines a 2.4GHz RF transceiver with a 16-bit XAP core, which Cambridge Consultants developed and Cambridge Silicon Radio uses in its Bluetooth The EM250 also includes two timers, a sigma-delta ADC, a USART, an I2C port, and an integrated encryption core for the ZigBee security algorithms. The company also offers the EM260, which lacks the processor core, allowing designers to choose their own processor and peripheral controller. Ashton agrees with ZigBee’s critics that its developers have yet to address all its deficiencies, but he’s confident that they will over time. “There are always migration and backward compatibility issues,” he says. As the ZigBee standard evolves, users will be able to perform upgrades in the field through software updates.
REFERENCES 1. Conner, Margery, “Run for your life: Ultralow-power systems designed for the long haul,” EDN, May 26, 2005, pg 46, www.edn.com/article/CA601827.
AUTHOR INFORMATION You can reach Technical Editor Margery Conner at 1-805-461-8242 and mconner@reedbusiness.com.
For more information... For more information on products such as those discussed in this article, contact any of the following manufacturers directly, and please let them know you read about their products in EDN Asia.
ANTENNA TECHNOLOGY FEELS THE SQUEEZE
You must budget the cost and size of antennas into your wireless design. Your options include whip, pc-board, and chip antennas. Whip antennas are familiar additions to mobile wireless devices, such as cellphones, wireless handsets, and walkie-talkies. The formula L=7500/frequency (MHz) provides a good approximation for a quarter-wavelength whip antenna for low-power systems, yielding 8.33cm for a 900MHz system (Reference A).
An antenna can also be a trace on a pc board, which has the benefit of adding virtually no cost. However, you must exercise care in using a pc-board antenna, according to George Rueter, senior applications engineer for Atmel. He says that designers often overlook the issue of antenna selection. Selecting the right antenna provides an advantage in both transmitting and receiving. “If you choose the right antenna, you can pick up a couple of decibels of gain,” he says.
Chip antennas can provide excellent reception and transmission capabilities but at an additional cost of approximately $1. Chip-antenna vendors includeFractus, gigaAnt, and Murata.
A BRIEF GUIDE TO 802.15.4 AND NETWORK PROTOCOLS'. IEEE 802.15.4 governs the low-level layers of wireless short-range PANs (personal-area networks) in the unlicensed RF band for low-power, lowband width applications, such as wireless-sensor networks, interactive toys, and home automation. “Low-level” refers to the lowest two levels of the OSI (open systems interconnection) networking-reference model: the MAC (media access control) and PHY (physical) layers. The specification stipulates such features for the communication scheme as 250-, 40-, or 20-kbps data rates, depending on bandwidth; 16, 10, or one channel; and the handshaking protocol. These specifications allow for an effective communication method between two compliant devices.
However, networks of devices need a higher level networking protocol. Almost all RF transceivers in the 2.4GHz band and many in the 900MHz band claim to comply with 802.15.4, but a controversy exists concerning the selection of a higher level protocol: proprietary, ZigBee, and open-source TinyOS.
Many networking system companies have developed their own proprietary protocols, both as value-added extensions and to allow for unique features, such as frequency hopping. Dust Networks chief executive officer Rob Conant claims that the company’s SmartMesh-XR platform delivers 99.99% reliability due to its frequency hopping capability. “If you rely on a single frequency, and the convenience store next door turns on a Wi-Fi-access point, your system will fall apart. We use frequency hopping to spread out the signal over the entire band.”
The ZigBee Alliance, a group of member companies that work to define the network-, security-, and application software layers for 802.15.4 networks, developed the ZigBee protocol. Although the specification is freely available, only ZigBee Alliance members can use it for commercial purposes.
The University of California-Berkeley leads the consortium that developed the TinyOS event-based operating environment for wireless sensor networks. TinyOS works with ZigBee, because ZigBee defines specifications only down to the network layer. For example, a ZigBee network layer could reside on a TinyOS MAC layer. Some companies, such as wireless-networksystem vendor Crossbow Technology, use TinyOS for both an operating environment and a network protocol. TinyOS has the added advantage of being open-source software. Table A summarizes the higher level protocols associated with the IEEE wireless communication standards for the unlicensed band.
