Develop a Wireless Measurement System

Wireless communication offers many benefits for measurement applications, including lower wiring costs and remote monitoring capabilities. However, choosing a technology and method of implementation can be difficult without knowing the strength and weaknesses of each wireless standard. This document discusses the various wireless technologies available on the market and shows how to take advantage of wireless communication with National Instruments measurement hardware and NI LabVIEW.

Wireless Technologies for Measurement and Automation

Over the past several years, wireless communication technologies have become ubiquitous, largely due to consumer electronics. There are hundreds of wireless equipment manufacturers and just as many standards. Understanding the benefits and shortcomings of each technology can make the selection process easier. This is especially important in measurement and automation where measurement data cannot be compromised, even when relying on radio waves. You can use the following wireless technologies for enhancing a measurement system. Each offers different advantages and features.

Wi-Fi and 802.11 b/g: The IEEE 802.11 standard encompasses a series of specifications for wireless LAN technology. The standard refers to an over-the-air interface between a wireless client and a base station, as well as communication between two wireless clients. The original IEEE 802.11 defines communication rates of 1 or 2 Mb/s in the 2.4 GHz band. Transmission methods include frequency hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS).

802.11b is the extension of the standard that is referred to as Wi-Fi, and made wireless networks popular in homes and offices. This variation provides 11 Mb/s transmission rates in the 2.4 GHz band.

802.11g provides 20 Mb/s of bandwidth or higher, depending on environmental and noise conditions. National Instruments chose this standard for Wi-Fi data acquisition because of its reliability, security, and proven track record on the market. You can find more information on NI Wi-Fi data acquisition (DAQ) products later in this document.

Key Features of IEEE 802.11:

• Operation frequency: b/g – 2.4 GHz
• Data rate: B/g – 11 Mb/s, g – 54 Mb/s
• Distance: b/g – 100 m
• Networking: Point to multipoint
• Power consumption: high

Bluetooth (802.1a): A consortium of companies composed of Ericsson, IBM, Intel, Nokia, and Toshiba defined this standard. The main intent of this wireless communication standard was to make it easier for devices to communicate over short distances – typically less than 10 m. However, this standard did not take off as quickly as 802.11, mainly because of distance limitations and the price of radio chips.  PC peripheral connection, phone and headset connection, and PDAs frequently use this standard. This wireless standard operates in the 2.4 GHz range and uses Gaussian frequency-shift keying (GFSK) to modulate data. That frequency spectrum is divided into 79 channels, each spaced 1 MHz from the other. Like 802.11, Bluetooth uses frequency hopping for security purposes, changing channels up to 1600 times a second.

The challenge of Bluetooth for measurements is the limited data rate and limited transmission range. Its configuration and security are also not as extensive as that for 802.11g.

Key Features of Bluetooth (802.1a):

• Operation frequency: 2.4 GHz
• Data rate: 1 Mb/s
• Distance: 10m–100 m
• Networking: Ad hoc
• Power consumption: medium

GPRS, GSM: The General Packet Radio Service (GPRS) is a nonvoice service intended for information to be sent and received across a mobile telephone network. With GPRS, data can you can send or receive data immediately as it is produced, as long as the radio signal is available. Unlike traditional land lines, this system does not require establishing a connection – it is always connected. This is an advantage for applications where time and quick reaction to events is crucial. GPRS overlays a packet-based air interface on the existing circuit switched Global System for Mobile Communications (GSM) network. The theoretical maximum data transmission rate is 172.2 kB/s, but this assumes only one user communicating over the allotted time slots and no error protection. However, the practical rates are usually slower than fixed networks, and depend heavily on surrounding structures, strength of radio signal, and number of users.

Key Features of GSM:

• Operation frequency: GSM-850 uses 824–849 MHz to send information from the Mobile Station to the Base Transceiver Station (uplink) and 869–894 MHz for the other direction (downlink). GSM-1900 uses 1850–1910 MHz to send information from the Mobile Station to the Base Transceiver Station (uplink) and 1930–1990 MHz for the other direction (downlink). GSM-900 uses 890–915 MHz to send information from the Mobile Station to the Base Transceiver Station (uplink) and 935–960 MHz for the other direction (downlink), providing 124 RF channels spaced at 200 kHz. Duplex spacing of 45 MHz is used. GSM-1800 uses 1710–1785 MHz to send information from the Mobile Station to the Base Transceiver Station (uplink) and 1805–1880 MHz for the other direction (downlink), providing 299 channels. Duplex spacing is 95 MHz.

• Data rate: 172.2 kb/s
• Distance: b35 km
• Networking: Point to point
• Power consumption: high to low, depends on transmitter complexity

Wireless modems and proprietary networks: There are many vendors that offer industrial-grade modems specifically designed for rugged environments with extreme temperature ranges and high-shock and -vibration conditions. There are products available that range from narrow band (UHF, VHF) to license-free spread spectrum. Narrow band typically requires a license, offers longer ranges and excellent propagation, supports the ability to transmit even without line of sight, and is appropriate for applications that require low bandwidth. Spread spectrum features include the fact that no license is required, offers short-, medium-, and long-range capabilities, generally requires line of sight, and is appropriate for medium- to high-bandwidth applications.

Benefits of Wireless

The main benefit of wireless technology for measurement applications is the ability to minimize or avoid the use of wires and cables. Depending on the nature of the application and environment, physical wiring can be expensive, inconvenient, or even impossible. Examples include moving/turning platforms, mobile applications (for example, vehicles or cranes), and structures that complicate wiring installation.

Wireless communications also extends the distance, or range, of data acquisition and I/O beyond what is practical with wiring. Therefore, large scale operations, such as water treatment facilities and tank farms, widely use wireless technologies. While the initial investment required for wireless networking hardware may be higher than that of traditional wired hardware, the total system cost including installation expenses and operating costs is generally significantly lower.

When choosing to implement a wireless network, there are several factors that you should take into consideration:

• Performance
• Range
• Security

Performance
When considering performance, it is important to consider the size of the spectrum, distance, data rate, power, number of users, and technology compatibility.

Even though different wireless standards define specific data rates, in practice, you can only expect to see a data rate of about 30 percent or less of the theoretical maximum throughput in a practical application. Factors such as RF interference and the number of users influence the performance of wireless networks. In addition, if you are using multiple compatible standards, the faster standard is typically limited by the slower standard. For example, when using 802.11b and 802.11g components on the same network, the 802.11g components slow to the data rate of 802.11b.

There is a constant trade-off between range and throughput. Your hardware should autosense signal strength (unless you tell it otherwise), and back off the transmission rate if your signal gets weak. If you are using 802.11b for example, it automatically backs the rate down from 11 Mb/s to 5.5, 2, and even 1 Mb/s. For most Internet connections this bandwidth is still sufficient.

Range

Generally, the range of a wireless device decreases as frequency increases, but that is not always the case. Tests show that 802.11g has the same, or perhaps slightly better, range than 802.11b, even though they use the same frequency. There are devices on the market that are designed to add wireless range for laptops by increasing the power of the card past the Wi-Fi certification limit of 100 mW. Before purchasing additional access points for your system, consider adding an extended range card if your local governing body permits it.

Directional antennas usually make the most sense in point-to-point use. They focus the signal into a narrow beam instead of letting it radiate in all directions like the isotropic antenna found in your base station. What you will find is that the higher the gain of the antenna, the narrower the focus of that beam. Thus, as the gain increases, so does the need to properly aim the antenna. This increases the risk of the receiver missing the transmitted data if not aligned properly. Directional antennas are typically sold by their gain rating. You can observe the effect of the gain in the “beam width” descriptions of each antenna.

Security

A primary concern when installing wireless networks is security. The rapid growth and popularity of wireless networks in both the commercial and residential market led to implementation for many diverse applications, including the transmitting private information. The need for privacy drove the development of wireless security protocols and continues to spur efforts to make wireless a more secure technology.

The original 802.11 standard included a security protocol called Wired Equivalent Privacy (WEP), which encrypted data packets well enough to keep out most eavesdroppers but still had some weaknesses. The industry needed a stronger encryption/authentication system, which led to the implementation of IEEE 802.11i (commonly known as WPA2). WPA2 provides the Extensible Authentication Protocol (EAP) and the Advanced Encryption Standard (AES), a 128-bit cryptographic algorithm endorsed by NIST and required in all US government facilities.

Another security measure is to minimize the propagation of radio waves outside the physically controlled area of a facility. This causes the wireless network to be more secure because of the reduction of the potential for eavesdropping and denial of service attacks.

You can find more information on wireless security here.

 

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Adding Wireless Capabilities to a Measurement System

Wi-Fi and 802.11 b/g: NI Wi-Fi data acquisition (DAQ) devices deliver simple, secure measurements with high-performance streaming capabilities on standard, trusted technologies. You can view data in real time, streaming dynamic waveform measurements at up to 51.2 kS/s per channel. In addition, built-in signal conditioning provides connectivity for a variety of sensors, including thermocouples, accelerometers, load cells, and so on. These devices take advantage of NI C Series measurement and control modules, also used for USB data acquisition and NI CompactRIO programmable automation controllers (PACs).

Watch the Wi-Fi DAQ Guided Tour >>

Figure 1. NI Wi-Fi DAQ

Using WPA2, the highest commercially available network security, Wi-Fi DAQ devices protect your system from unwanted access. Authentication ensures that only authorized devices have network access and encryption prevents data packets from being intercepted. Wi-Fi DAQ devices support multiple Extensible Authentication Protocol (EAP) methods that provide for mutual authentication between the DAQ devices and wireless access points. They also support 128-bit AES encryption, endorsed by NIST and required in all US government facilities. With strong security protocols, you can incorporate wireless connectivity with existing enterprise networks safely.

Wi-Fi DAQ devices include NI-DAQmx driver and measurement services software with intuitive application programming interfaces, configuration utilities, I/O assistants, and tools designed to reduce system setup, configuration, and development time.

There are other options depending on what NI measurement and control hardware you are using. All NI PACs feature Ethernet ports. You can connect an NI PAC such as NI Compact FieldPoint, CompactRIO, Compact Vision System, or PXI system to a Wi-Fi access point such as the NI WAP-3701. This immediately makes your system available through the wireless network and communications are transparent, acting as if it were connected to a wired network.

For a desktop PC or a laptop with an NI data acquisition device, you can use the computer’s Ethernet port for connecting to a Wi-Fi access point, as well as commercially available PCMCIA and PCI Wi-Fi adapters. Almost all new laptop and notebook computers feature built-in Wi-Fi capabilities.

When using LabVIEW and standard communications protocols such as TCP/IP, your application can send data back and forth between devices and it transparently transmits irrespective of whether the medium is a physical Ethernet cable or radio waves through the air.

For example, LabVIEW offers many built-in functions for TCP communication (see Figure 1). These functions work the same way whether the measurement system is connected to the network through a cable or through a wireless modem.

 

Bluetooth or 802.1a: Bluetooth technology in its current format (and even with the new extended range Class 1 specification), is mostly appropriate for general-purpose operating-system-based systems (for example, Windows). When using a PC to host the measurement system, you can make use of the LabVIEW built-in Bluetooth libraries (Figure 2). There are multiple serial-to-Bluetooth and USB-to-Bluetooth adapters for adding Bluetooth capabilities to a desktop or laptop computer. Many newer laptops already feature a built-in Bluetooth transceiver. Many PDAs also include Bluetooth communication, which, with the LabVIEW PDA module, you can also access programmatically. 

GPRS, GSM: Configuring your measurement system to send data wirelessly using a GPRS network is likely more complex than using other technologies. The following is a list of requirements to get started: 

1. A terminal that supports GPRS
2. A subscription to a mobile telephone network that supports GPRS (Note: Use of GPRS must be configured for a specific user. Some mobile providers permit automatic access to the GPRS network, while others might require an explicit opt-in.)
3. Some know-how is required on sending and/or receiving GPRS data, depending on the specific hardware you use, including software and hardware configuration
4. A destination to send or receive information through GPRS (Note: A destination can be a URL, another GPRS-enabled device (or software application receiving data), or a phone.)

There are some options specifically designed for NI hardware (see Figure 3), such as the GPRS/GSM module for CompactRIO by S.E.A. Science and Engineering Applications Datentechnik GmbH (SAE). With this module, you can remotely control and monitor nonaccessible and mobile measurement systems by mobile telephone networks. For exact position determination an additional GPS module is available. Distributed systems can be time synchronized by the RCC module. The platform for these measurement data acquisition applications is the CompactRIO system. This platform, combined with the cRIO Gxxx Mobile module offers an attractive solution for mobile systems, for example in automotive, shipping, aerospace, and teleservice applications. The cRIO Gxxx Mobile combo module offers GPRS, GPS, and Radio Clock functionalities. The module allows position determination and the transmission of measurement data or event messages. In addition, small data packets or parameters can be exchanged as text (SMS) messages in both directions.

Typical applications for this technology are appliances, ATM terminals, automotive, remote data collection, gas pumps, industrial and medical remote monitoring systems, remote diagnostics, remote metering, security systems, and vending/gaming machines.

Wireless modems and private networks: There are many companies that offer industrial-grade devices that can operate either in free frequency bands or private licensed frequency bands. The benefit of a private RF network is that you own the system and the frequencies over which the data is transmitted. This provides for real-time data exchange and you typically don’t incur reoccurring subscription or usage costs.  Depending on your measurement equipment, distances, security, and cost requirements, you can select from multiple types of modems, many of which make the wireless connection transparent to the software.

Conclusion

By adding wireless communication capabilities to your existing or new measurement system, you can significantly enhance its reach and flexibility, and even reduce cost. Whether you use NI Wi-Fi DAQ, a PC or laptop with a data acquisition device, or an NI PAC, the necessary connections to make the system wireless are readily available. As wireless technologies evolve and reduce in cost and complexity, your system will too by simply integrating them into your measurement system.

See Also:
Wi-Fi Data Acquisition
PACs
NI LabVIEW for Remote Monitoring

 

Reader Comments | Submit a comment »

I thought it a very informative and clearly written article, thus I gave it a very high rating. What I was looking for is, does LabVIEW PDA support 802.11g
- Robert Kirby, NSWC Dahlgren Div. robert.h.kirby@navy.mil - Dec 7, 2005