Sensors - September 2001 - The Next Step--A Wireless IEEE 1451 Standard for Smart Sensor Networks

The potential market for wireless sensors is almost limitless. Everything from home automation to battlefield coordination could benefit from mature wireless sensor networks.

Michael R. Moore and Stephen F. Smith, Oak Ridge National Laboratory
Kang Lee, National Instutute of Standards and Technology

IEEE 1451 is a family of standards that links sensors to users, similar to the way that IEEE 802 (Ethernet) provides connectivity for information systems. Currently, all working groups under the IEEE 1451 umbrella provide standard interfaces for sensors on tethered networks. But the demand for a wireless physical layer is growing.

A wireless IEEE 1451 standard should provide seamless connectivity among sensors and users, no matter what distance separates them. And it must do this without requiring the installation of new wires and with reasonable cost and size additions at each sensor node.

What Does Wireless 1451 Need to Accomplish?

Figure 1. The U.S. Navy is preparing a list of priority programs that emphasizes information networks and sensors. This is just part of a growing movement calling for well-organized wireless sensor networks.

The military needs cost-effective, reliable, tightly synchronized, secure wireless sensor networks (see Figure 1). According to the May 7, 2001, issue of Defense News, “Senior U.S. Navy officials are preparing a list of new priority programs, emphasizing information networks and sensors…” In hospitals, RF interference, data reliability, and privacy are big issues. Industry must come to grips with a range of reliabiliùy and cost constraints. And home automation applications typically require lower reliability and security but need lower cost. The challenge is to find broadly accepted technology that meets the requirements of as many potential users as possible.

The general requirements of any wireless network include throughput, range, reliability, and power consumption. Some users need tens of megabits per second while others require only a few bits per day. Also, the sizes of the networks vary from a few feet to several miles.

The Wireless Sensing Workshop at Sensors Expo in Chicago on June 4, 2001, tried to determine the interest and requirements for wireless interfaces for sensor-based networks, especially in the industrial community. An informal survey taken at the workshop showed that most industrial users needed 32 or fewer nodes per network and typically <300 bps per node, or an aggregate data rate of <10 kbps for the network. At the same time, the industrial users generally wanted network ranges of a few kilometers, not just tens of meters. While this sampling population was too small to be statistically accurate, the results showed that at least one of the physical layer options must offer long-range operation, but perhaps less throughput would be acceptable (see the sidebar, “Wireless Sensing Workshop in Philadelphia”).

Workshop panelists also discussed reliability. Some asserted that potential users of wireless sensors must sacrifice some reliability to go wireless; others wanted a network protocol that optimized the reliability of the link. One way or another, physical layers chosen for a wireless IEEE 1451 system must address data transmission reliability. Within this context, the concept of reliability has several possible definitions, depending on the application. These include the probability that a message will get through; the probability that a message will get through within a given amount of time; or the probability that errors in messages will be detected.

Requirements of Sensor Networks
Sensor networks have several requirements beyond those needed by generic data networks. They must be able to accommodate the Transducer Electronic Data Sheet (TEDS) associated with each sensor. These networks often have more stringent timing and synchronization requirements. Also, the nodes must have lower power consumption and be smaller in size than the products that support PC-to-PC networking.

The main purpose of a sensor network is to distribute sensor data. For the data to be verified, certified, and coordinated, the user must be able to access the sensor manufacturer’s calibration, correction, and device-identification data. Therefore, any viable sensor network must include mechanisms to identify each sensor and to access associated databases, whether embedded in the sensor (local TEDS) or stored in another location available to the user (virtual TEDS).

It could also be argued that synchronous sampling of sensors (or the determination of when samples were taken) is also a necessary component of a sensor-based network. In general, a sensor network must have a more tightly controlled, more deterministic time-base than random-accessed general data networks can typically provide. Any physical layer suitable for a IEEE 1451–type standard smart transducer network must incorporate a robust mechanism to synchronize various components of the system (e.g., the individual transducer-to-bus interface modules [TBIMs] and the system bus controller [TBC] in the proposed IEEE P1451.3 standard).

Time resolutions required for more precise applications may be in the microsecond range, although many scenarios will be far less demanding. Any new proposed standard will have to at least provide a means of achieving this higher resolution. Two approaches proposed for system synchronization in IEEE P1451.3 include a separate sinusoidal sync signal on the cable and the simultaneous provision for transporting formatted timestamp data from the TBC to the remote TBIMs over the interface. IEEE P1451.3 is designed to use cable as the physical link for communications.

One issue highlighted at the workshop was sensor node power drain. Many participants expressed the need to deploy large numbers of sensors that could be powered by inexpensive batteries for one to five years. Because most of these units would report their readings only a few times a day, the need for a power-efficient wireless transmission scheme coupled with low unit quiescent drain is paramount. The consensus of the group was that an appropriately low-power format still does not exist in the marketplace.

How Do We Develop a Wireless Interface Standard Quickly?
Standards development is a laborious process. But you can accelerate design and adoption by taking advantage of resources already available in the marketplace and by accommodating a variety of networking needs in the standard.

The medium access and physical layer definitions from the IEEE 802 family address some of the needs of a wireless IEEE 1451 network. Adopting parts of these definitions could greatly reduce the time required to develop an IEEE 1451 wireless standard. In addition, the general structure of the IEEE 802 family can also be used as a basis for the structure of the IEEE 1451 family. The IEEE 802 family encompasses several combinations of medium access and physical layers while maintaining consistent network management functions (see Figure 2). The overview, architecture, and management functions are similar to that of IEEE 1451.1.

Figure 2. IEEE 802, which encompasses several combinations of medium access and physical layers while maintaining consistent network management functions, is a good model for the structure of the IEEE 1451family of standards. [Click for larger image]

Leveraging existing products broadens acceptance in the marketplace and has a greater impact on the market. If appropriate product lines can be identified, they should be considered as candidates for adoption. Some products are discussed in the last section of this article.

Like the IEEE 802 family, the IEEE 1451 family will have to accommodate a dynamic market by retaining portions of the network standard that are reusable and by encompassing new technologies, especially at the media access and physical layer levels. Whether one or more physical layers are adopted in the first implementation of a wireless 1451 standard, future technologies and bandwidth needs will require a standard that can grow without throwing out the infrastructure of the rest of the standard.

Technological Realities
Selection of Frequency Band. All wireless standards must comply with government regulations for radio-communications devices, including the guidelines for operating frequency ranges, power output, and technical standards. For example, the U.S. Federal Communications Commission haı established license-free bands (designated ISM, for Industrial, Scientific, and Medical) suitable for wireless-sensor systems. Similar allocations are available in Canada through the Department of Communications and in Europe through the European Telecommunications Standardization Institute.

The most fundamental choice to be made in the deployment of wireless sensor networks is the selection of the frequency of transmission. The options include licensed bands in the VHF and UHF regions, all of which require expensive and time-consuming application procedures through the FCC; leased transmission facilities and/or services from independent providers, such as private industrial radios, shared trunked systems, œellular digital modems (e.g., charge-coupled photodiode or photoconductive decay units), or satellite-based systems (e.g., GOES and Argos); and unlicensed systems restricted to specific ISM and similar bands, including ones centered at
13.56 MHz, 27.55 MHz, 303 MHz,
315 MHz, 404 MHz, 433 MHz, 868 MHz (Europe), 915 MHz (North America),
2.45 GHz, 5.2 GHz (North America),
5.3 GHz, and 5.7 GHz (North America).

Although the first option offers straightforward use and relative freedom from interference, the overall costs and implementation delays are often prohibitive for wireless sensors unless the networks can be shared with other services, such as voice communications. The second option usually incurs high ongoing costs because of periodic provider charges, but it may be useful in applications in which there are widely spaced sensors in remote locations or at a few monitoring points. In general, these systıms are well suited for low-data-rate, low-read-rate applications. The third option affords the greatest implementation flexibility and will usually be the least expensive to operate, although a basic concern of ISM-band systems is the finite probability of experiencing interference from unlicensed and licensed users in those bands. As a result, spread-spectrum technology is generally needed to achieve satisfactory transmission reliability in most wireless sensing applications.

Coexistence with Other Networks/Interferers. As more and more wireless systems are fielded, coexistence becomes a bigger issue. It’s not legal to build a system that overrides or interferes with other licensed systems. In short, it should have minimal impact on the performance of other systems. ©enerally speaking, these requirements favor the use of spread-spectrum technology, although conventional narrowband RF transmissions may suffice for some applications.

FHSS vs. DSSS vs. OFDM. Table 1 compares a few of the physical layers in the IEEE 802 family, as well as those in a couple other standards.

TABLE 1
An Overview of Wireless Standards
Standard	OFDM	FHSS	DSSS	Ghz	Size	Mbps
IS-95			X	1±	Cell	1
Bluetooth		X		2.45	PAN	0.7
P802.15		X		2.45	PAN	0.7
P802.16b	X			5	WAN	54
P802.11A	X			5	LAN	54
P802.11		X	X	2.45	LAN	1, 2
P802.11b			X	2.45	LAN	5.5, 11
OFDM - Orthogonal Frequency Division Multiplex FHSS - Frequency-Hopping Spread Spectrum DSSS - Direct Sequence Spread Spectrum

(IS-95 refers to the cell-phone code division multiple access [CDMA] standard.) There are three common modulation techniques used in wireless information networks, in addition to conventional narrow-band techniques. These are frequency-hopping spread spectrum (FHSS); direct-sequence spread spectrum (DSSS); and the newest contender, orthogonal frequency division multiplex (OFDM). There are also other less common modulation methods, which will be mentioned later.

The familiar frequency-hopping and direct-sequence spread-spectrum formats have been fully covered in earlier issues of Sensors, as well as in many other publications. OFDM has also been discussed, although not as extensively as the first two. Therefore, we’ll highlight only the issues of these three that bear on the planning of a wireless IEEE 1451 standard.

As shown in Figure 3, DSSS (on the left) produces a more noise-like spectrum, and FHSS (on the right) produces narrow-band peaks.

Figure 3. Direct-sequence spread-spectrum modulation is less likely to interfere because its spectrum is more noise-like, whereas a frequency-hopping signal is a frequency-agile narrowband transmission that has good immunity to interference.

The image on the right can also be used to illustrate the spectrum of OFDM, which uses a group of n closely spaced (minimally spaced, orthogonal) carrier frequencies, each of which carries 1/n of the total bits in a message. In FHSS, the carrier hops to each frequency one at a time, while in OFDM, all carriers are used simultaneously. OFDM is not, strictly speaking, a spread-spectrum technique, although it’s sometimes referred to as such.

The better OFDM systems adapt to the channel by avoiding frequencies that exhibit high bit-error rates. Because this method uses all the frequencies (with minimum spacing) all the time, it seems to have the best spectral efficiency (bits/s/Hz) of the three methods, particularly when compared with typical implementations of the other two.

This is true in the case of adaptively modulated OFDM formats (like those used in ADSL wired links, the new IEEE 802.11a protocols, and several wireless versions proposed for the developing IEEE 802.16 Metropolitan Area Networking standard). Here, each OFDM carrier can be individually modulated via binary phase-shift keying (BPSK) or multilevel quadrature amplitude modulation (n-QAM) constellations (usually with n = 4, 16, 64, or 256), yielding even higher spectral efficiency when the link has a high SNR.

In general, direct sequence has the second best spectral efficiency, and frequency hopping is third on this list. A final overall selection consideration is that under current ISM regulations, the FCC doesn’t permit OFDM in the 915 MHz and 2.45 GHz bands, although a possible change in this aspect of the ISM-band rules is under consideration.

Table 2 shows generalizations of the three technologies. Although controversial, these observations are generally true (especially if adjacent-cell reuse is not considered).

TABLE 2
Comparison of RF Technologies
Typical Rank	Spectral Efficiency	Noninter- fering	Power Required	Data Reliability	Effective Range
Best	OFDM	DSSS	FHSS	OFDM	OFDM
Median	DSSS	FHSS	DSSS	DSSS	DSSS
Worst	FHSS	OFDM	OFDM	FHSS	FHSS
OFDM - Orthogonal Frequency Division Multiplex FHSS - Frequency-Hopping Spread Spectrum DSSS - Direct Sequence Spread Spectrum

OFDM is more optimal for a few nodes streaming lots of data, and DSSS is better suited for lots of nodes handling less data per node. FHSS is often preferred over DSSS by the military, mostly because DSSS requires careful management of the transmit power of individual nodes to overcome the near/far problem and FHSS does not.

The ranges are all approximately line of sight because they typically use carrier frequencies at VHF or above. However, their respective ability to adapt to the RF channel and interferers will determine their effective range.

The point of this table is to give general guidance. Tradeoffs between the technologies are application and product dependent.

The Next Step
We must find a well-supported technology that meets the goals of a significant number of wireless IEEE 1451 users and adapt our standard to it. The time and cost for loosely coupled groups of supporters to develop new technologies from scratch for a standard are too high. Therefore, we must consider technologies/products that are available or that will be so within the next year. Popular and cheap is a good combination for most of us. At the present time, Bluetooth is probably the most visible candidate, but there are others worthy of consideration.

Figure 4. The proposed wireless IEEE 1451 standard must address various sizes of networks, as does the IEEE 802 family.

While considering candidate technologies, we should also be mindful of the size of the area the network must cover. Wireless networking technologies are targeted at PANs, LANs, WANs, and even larger networks (see Figure 4). The battlefield graphic in Figure 1 describes a mixture of networks, but they tend toward wider areas. The audience survey described in the first section tends to call for LANs or WANs. Additionally, there are needs for room-size PANs, which Bluetooth has targeted.

Furthermore, the standard should allow the sensor network to maintain connectivity when nodes or links fail. This feature will be necessary to extend the range and reliability of networks. Examples of implementations that offer this type of connectivity include Scatternet topologies, as described by Bluetooth, and Mobile Ad Hoc Networking (see “Mobile Ad Hoc Networking” in the January 2001 issue of Sensors).

To show the feasibility of pushing forward with a wireless sensor standard, a few product lines have been identified in this article. The inclusion of these products shouldn’t be considered an endorsement. Rather, the availability of this hardware demonstrates that the standard could be immediately useful.

Bluetooth (FHSS)
Bluetooth was originally designed to provide a wireless connection among different devices, including PDAs, palmtop computers, peripherals, and pocket phones. These products have been well described in many articles, including “Bluetooth” by Peter L. Fuhr in the August 2000 issue of Sensors. In the article, Fuhr states that Bluetooth is “backed by more than 800 companies, including makers of components and systems…” and that it “represents a nonproprietary method of connecting multiple devices in point-to-point or multi-point configurations.”

As shown in Photo 1, there are a wide variety of products available from various Bluetooth manufacturers.

Photo 1. The wireless 1451 standard could benefit from existing product lines, such as those available from various Bluetooth manufacturers (Courtesy of Thurston Brooks, 3e Technologies International, Inc.)

These units can be used in a network containing one master and up to seven slaves in a configuration called a Scatternet. The slaved units can be placed in various lower power modes to optimize power consumption while retaining connectivity to the network.

Bluetooth uses FHSS and TDMA to accommodate several sensor nodes located close to each other. It transmits ~721 Kbps of data and was originally targeted at a range of ~20 ft. Higher RF power options have been added to increase the range. Frequency hopping is accomplished using 79 carriers in the 2.45 GHz ISM band. Also, the Bluetooth units use <100 mW in standby mode.

The popularity of Bluetooth can be used to the advantage of a developing standard, assuming it fits the requirements of users. It is already in the process of being adopted as a physical layer option by IEEE P802.15.

Axonn (DSSS)
Axonn (504-282-8119) has developed another product line that demonstrates the type of solutions necessary for a wireless sensor standard. These products accommodate several sensor nodes in a multi-point-to-point topology that can be used with TDMA or CDMA multiple access schemes.

One example is the AX550. According to the company’s literature, the AX550 “is a credit-card sized (2.2 by 3.4 by 0.9 in.) transceiver and has four external digital inputs (5 VDC), one digital output (5 VDC, 15 mA sink, and 1 mA source max.), one open collector output (100 mA sink), and an RS-232 (5 V) or an optional RS-485 communication interface. The transceiver operates over a temperature range of –40°C to 85°C in the frequency band 902–928 MHz (selectable in eight 3 MHz steps). It has a sensitivity of –110 dBm (average power) at 19.2 Kbps. The transmitter has a power output of 100 mW, using BPSK spread modulation, OOK data modulation, and an effective data rate of 19.2 Kbps, consuming a little more than 1 W (165 mA; ~7 V).

The Axonn transmitter, using the same RF communication features as the AX550, can achieve a battery life performance that is limited by the battery shelf-life (7+ years using 1.3 AH, ²/₃ A size) for transmissions every one minute. The Axonn devices operate over a line-of-sight range of up to 1 mi. Axonn currently has more than 5 million nodes in operation.

Components such as the AX550 can be used in the system architecture shown in Figure 5, which gives an overview of a generic wireless sensor network.

Figure 5. The wireless 1451 network must accommodate a heterogeneous mix of transmit-only units, receive-only units, transceivers, network gateways, and repeaters. (Courtesy Axonn L.L.C.)

In general, these networks will include a heterogeneous mixture of components. They may include transmit-only units, receive-only units, transceivers, network gateways, and repeaters. There also exist point-to-point and multi-point-to-point network topologies. Physical layer options of a future wireless IEEE 1451 protocol should address all these types of components and topologies.

Summary
There is a great need for a standard protocol for wireless sensor networks. The IEEE 1451 family provides a useful framework for these types of applications. By leveraging emerging technologies and existing hardware and standards, a wireless IEEE 1451 standard can be developed in a timely manner to meet industry’s needs.

SIDEBAR:

Wireless Sensing Workshop in Philadelphia

The Wireless Sensing Workshop held at Sensors Expo in Chicago, IL, on June 4, 2001, had such an overwhelming response from industry that the participants urged organizer Kang Lee of the National Institute of Standards and Technology (NIST) to stage a follow-up workshop. The second workshop is scheduled for Sensors Expo in Philadelphia, PA, on October 4, 2001.

During the Chicago workshop, various standards accommodating wireless technologies were proposed and intensely discussed. These included Bluetooth (IEEE P802.15), Wireless Ethernet (IEEE 802.11b), the Smart Transducer Interface Standard (IEEE P1451), and the idea of a wireless interface for sensors, IEEE P1451.x. The purpose of the Philadelphia workshop will be to further examine these and other technologies that industry deems important enough to be included in a wireless IEEE P1451 proposal and discussion.

This workshop could lead to the formation of a wireless IEEE P1451 Study Group and the development of a formal project proposal to IEEE. Those interested in presenting particular wireless technologies for discussion should contact Kang Lee at kang.lee@nist.gov to be included in the workshop agenda. Everyone is invited and encouraged to participate in the workshop.

Kang Lee is a group leader of Sensor Development and Application Group, the National Institute of Standards and Technology. He can be reached at kang.lee@nist.gov.

A paragraph on page 39 of the September issue of SENSORS discussed which wireless frequency technology to use. The second sentence indicated that FCC licensing for VHF and UHF licensed frequencies were "expensive and time consuming" (my paraphrase). That is an inaccurate statement on both counts.

Acquiring a license through the FCC is not a difficult task at all. As a manufacturer of wireless modems, our sales staff and business associates routinely get licenses for our customers within a very few weeks. The application is simple and easy to fill out. It can be done on line with the FCC or with assistance from our staff. Second, the licenses are often issued within a very few weeks, usually no more than four weeks total.

What one gains from a private license is 1) exclusive use of that frequency; 2) the ability to have higher output power ratings; and 3) the ability to transmit data much farther than can be accomplished with the nonlicensed products.

There seems to have been a concentrated effort to make the using community believe there is a shortage of frequencies and that acquiring a license is difficult and expensive. This simply is not true. Perhaps it has been fostered to direct people to unlicenced products.

Generally speaking, narrowband licensed FM wireless modems do not have the throughput of the wider bandwidth unlicensed products. The 25 kHz channel devices typically can reach speeds of 19,200 bps and the 12.5 kHz bandwidth devices can attain speeds of 9600 bps. Granted, these speeds are not the IEEE standard 802.11b addressing wireless networking. However, we have interfaced products that link via an Ethernet connection in full duplex mode at the lower speeds and they have worked acceptably. What I see here is making something fit into a realm that it really doesn't fit. Short distances may be fine for the wireless networking but that will not always work for longer distances and more disparate equipment. We routinely transmit data over 30 miles and through multiple repeater hops.

Bottom line is this, wireless networks have their place but they are not the be-all and end-all that we are led to believe. I work with customers on a continuing basis who have expectations that will not be met because they apply the same rule set to all RF approaches. It would be a great favor if those who write about the wireless modem would research a bit deeper, talk to the other manufacturers and report a more fair and balanced story.

Thank you for your time and attention.

Harry Ebbeson
Manager Technical Services
Dataradio COR Ltd.
507-833-6706

Mr. Ebbeson,

I apologize if I have misrepresented the narrowband, liscensed community. I can certainly guarantee you that neither I nor either of the other two authors have a vested interest in any particular product or technology. The particular statements that troubled you were based on my understanding (right or wrong) that companies such as Qualcomm (liscensed, wideband) spent many millions (if not billions) of dollars to buy appropriate amounts of bandwidth at FCC auctions. Since I am not in the business, I do not have a clear picture of the business model or the methods by which these costs are distributed to the customer. When I next have opportunity, I will go to your Web site and others like it, so that I will do a better job of representing your community.

As you correctly surmised, I am accustomed to dealing with wider bandwidths, higher throughputs, and shorter distances (in addition to multi-node networking) and this is what probably led to my inadvertently slanting the article.

On your last point about customer expectations: this issue cannot be overstated. I just gave a presentation at the ISA show in Houston on Monday 10 September 2001). There, I made a statement that was echoed by several people, that customers' expectations about RF in general and the tradeoff between various RF technologies in particular must be managed. I told them that we are attacking a two-headed beast. One is making our technologies more robust (e.g., dealing with interference from the plethora of RF products hitting the plant floor). The other is managing the expectations of what RF can do (e.g., customers putting large metal objects very near the antenna and expecting no effect.)

Back to your main point. I will try to do a better job in the future. I can assure you that the article reflected the job experiences and data (both technical and anecdotal) that have come my way and not any intent to push customers away from whatever technology best suits their need.

Mike Moore

The authors of this article are scheduled to participate in the "Forum on Wireless Technology" at Sensors Expo & Conference on Thursday, October 4, 2001.