Develop a Wireless Measurement System

Wireless communication offers many benefits for measurement applications, including lower wiring costs, simple data transfer, and remote monitoring capabilities. There are several ways to take advantage of wireless communication with National Instruments measurement hardware and LabVIEW, as this document details.

Table of Contents:

	What wireless technologies are important for measurement and automation?

Wireless communication technologies have become very popular in the last years. There are hundreds of wireless equipment manufacturers and just as many standards. Understanding the benefits and shortcomings of each can make the selection process easier. This becomes more important when you are considering trusting your measurement data to radio waves. You can use the following wireless technologies for enhancing a measurement system. Each of them offers different advantages and features.

Wi-Fi and 802.11 a/b/g: The IEEE 802.11 standard encompasses a series of specifications for wireless LAN technology. In essence, 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. IEEE 802.11 defines communication rates of 1 or 2 Mbps in the 2.4 GHz band. The transmission methods include frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS).

802.11a is an extension to the 802.11 standard that applies to wireless local area networks (LAN) and provides up to 54 Mbps in the 5GHz band. 802.11a uses a different encoding scheme known as orthogonal frequency division multiplexing encoding, which makes this standard incompatible with the ubiquitous 802.11b and now g standards.

802.11b is the extension of the standard that is typically referred to as Wi-Fi and is the one that made wireless networks popular in homes and offices. This variation provides 11 Mbps transmission rates in the 2.4 GHz band. The main difference between b and g is the higher data rate. 802.11g provides 20 Mbps or higher depending on environmental and noise conditions.

Key Features of IEEE 802.11:

• Operation frequency: b/g - 2.4 GHz, a - 5 GHz
• Data rate: a/b/g – 11 Mbps, a/g – 54 Mbps
• Distance: b/g – 100m, a – 50m
• Networking: Point to multipoint
• Power consumption: high

Bluetooth or 802.1a: This standard was defined by a consortium of companies comprised of Ericsson, IBM, Intel, Nokia, and Toshiba. The main intent of this wireless communication standard was to make it easier for devices to communicate over short distances – typically less than 10m. This standard didn’t quite take off as quickly as 802.11 did, mainly due to distance limitations and price of the radio chips. However, the standard has been frequently used for PC peripheral connection, phone and headset connection, and and PDAs. This wireless standard operates in the 2.4 GHz range and uses Gaussian frequency shift keying (GFSK) to modulate the data. That frequency spectrum in 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 Bluetooth consortium is constantly advancing the specifications and extending the capabilities of the standard.

In fact, new developments might take Bluetooth beyond its original intent, which was consumer-oriented technology, and make it a more viable standard for measurement applications and plant installations. A new standard referred to as Class 1 extends the range for Bluetooth-based communications. The distance at which Bluetooth-enabled devices would be able to communicate could be extended up to 100m, or ten times as much as the original specification. With this extended range, and inherent benefits such as self-identification, you are likely to see it appearing in more and more places.

Key Features of Bluetooth (802.1a):

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

GPRS, GSM: The General Packet Radio Service (GPRS) is a non-voice service intended for information to be sent and received across a mobile telephone network. With GPRS, data can be sent or received 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 of crucial importance. GPRS overlays a packet-based air interface on the existing circuit switched GSM network. The theoretical maximum data transmission rate is 172.2 kbps, 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.

One example of this technology used for measurement systems is the cRIO Gxxx Mobile module released by S.E.A. Datentechnik GmbH. This module offers solutions for measurement and supervision applications which could not be connected to data networks. The module allows control and monitoring of non-accessible and mobile measurement systems remotely 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 National Instruments CompactRIO system. This platform, combined with the cRIO Gxxx Mobile module offers an attractive solution for mobile systems e.g. in automotive, shipping, areospace 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 test (SMS) messages in both directions.

Figure 1. cRIO Gxxx module and NI CompactRIO System
Courtesy of S.E.A. Science & Engineering Applications Datentechnik GmbH

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.

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 kbps
• Distance: b35 km
• Networking: Point to point
• Power consumption: high to low, depends on transmitter complexity

Wireless modems and proprietary networks: There are many companies (e.g. Data-Linc) 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. There are different considerations for each type. 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.

	What are the benefits of wireless?

The main benefit of wireless networks is 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 (e.g. vehicles or cranes), and structures that complicate installation of wiring. More traditional applications that perhaps are not permanently operated benefit from wireless because you do not need to install, remove, and reinstall wiring and cabling.

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 it's certainly true that the initial investment required for wireless networking hardware will initially be higher than the cost of traditional wired hardware, overall installation expenses and operating costs will generally be significantly lower. So over the long term, cost benefits outweigh initial investment. These savings will tend to be greatest in dynamic environments requiring frequent moves and changes in network infrastructure.

In addition to these cost benefits wireless networks offer advantages in terms of improved scalability. You can configure these systems in a variety of topologies to address the needs of specific companies or departments. Because the set-ups are easily changed, they can grow from a simple peer-to-peer environment suitable for a small workgroup, right up to full infrastructure networks supporting thousands of users.

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

Performance:
When considering performance, you will need to look at size of the spectrum, distance, data rate, power, number of users, and even technology compatibility.

The 5GHz bands make available a larger part of the spectrum, allowing the creation of more non-overlapping channels. This means significantly better performance as compared to the 2.4GHz band. The 5GHz band is a better option if high performance is an important requirement.

Even though different standards tout specific data rates, in practice you can only expect to see a data rate of approximately 30% or less of the advertised maximum throughput. 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 will typically be limited by the slower standard. For example, when using 802.11b and 802.11g components on the same network, the 802.11g components will slow to the data rate of 802.11b.

There is a constant trade-off between range and throughput. Your hardware should auto-sense 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 will automatically back the rate down from 11 Mbps to 5.5, 2, and even 1 Mbps. Although 1 Mbps may sound low, many businesses have a T1 as their main wired connection to the Internet. As a T1 only moves data at 1.544 Mbps, this should not be a problem.

Range:
As frequency increases, range generally decreases. As a result, 5GHz systems generally have less range than ones operating in the 2.4GHz band. The selection of a 5GHz WLAN could require a greater number of access points, which can result in higher costs. As a result, you may benefit by deploying 2.4GHz systems in larger facilities unless high performance is critical. Keep in mind, however, that 5GHz systems may have equal or even better range in some situations.

Tests show that 802.11g has the same, or perhaps slightly better, range than 802.11b. On the other hand, 802.11a seems to maintain a higher throughput out to the limit of its range, while 802.11g appears to run out of steam at its extreme range.

There are cards such as the Engenius which are designed to add range to wireless for laptop users by increasing the power of the card past the Wi-Fi certification limit of 100mW. Before purchasing additional access points for your system, consider adding an extended range card.

Directional antennas usually make the most sense in point-to-point use. They basically 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 that the receiver could miss the transmitted data if not aligned properly. Directional antennas are typically sold by their gain rating. The effect of the gain can be observed in the "beam width" descriptions of each antenna.

Thus, very-high gain directional antennas are great for applications where you can carefully aim for a distant antenna. Once properly aligned, the high gain ensures the highest transmission rates possible and degree of security as it becomes more difficult to intercept the signal. Very long distances have been covered in experimentation.

Interference:
2.4GHz WLANs such as Bluetooth and 802.11b can experience interference from cordless phones, microwaves, and other WLANs. The interfering signals degrade the performance of an 802.11b WLAN by periodically blocking users and access points from accessing the shared air medium. If it's not possible to reduce potential interference to an acceptable level, then consider deploying a 5GHz system, which is relatively free from interfering sources.

During installation, roam the site to determine the best locations for access points. If you are experiencing interference, simply remove or change locations of the device that is interfering.

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 transmission of 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 802.11b Ethernet standard includes a security protocol called Wired Equivalent Privacy (WEP), which encrypts data packets well enough to keep out most eavesdroppers. Making WEP’s encryption system 100% secure is the goal of the 802.11 working group, which began redesigning WEP in August 2001 when it became clear that its underlying cryptography, RC4 algorithm, was unsound. Such efforts should improve the security of wireless networks in the future.

Another security measure is to minimize the propagation of radio waves outside the physically controlled area of a facility. The causes the wireless network to be more secure because of the reduction of the potential for eavesdropping and denial of service attacks. As a result, 5GHz systems can inherently provide enhanced security over 2.4GHz systems because of their limited range.

	How can I add wireless capabilities to my NI measurement system?

Wi-Fi and 802.11 a/b/g: There are many products available that you can use to add Wi-Fi capabilities to your NI hardware. Depending on the environment where the system is installed, you can choose between commercially available products (e.g. Linksys) or industrially-rugged products (e.g. Data-Linc).

There are different options depending on what NI measurement and control hardware you are using. All of National Instruments programmable automation controllers (PACs) feature Ethernet ports. You can simply connect an NI PAC such as Compact FieldPoint, CompactRIO, Compact Vision System, or PXI system to a Wi-Fi router. 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 plug-in data acquisition card, you can use the computer’s Ethernet port for connecting to a Wi-Fi router, as well as commercially available PCMCIA and PCI Wi-Fi adapters. Also, if it is a newer laptop, it very likely features built-in Wi-Fi capabilities.

Industrial Wi-Fi devices operate essentially in the same way that commercial routers and access points do, except they are rated for more extreme operating conditions. These include extended temperature ranges (-40 to 70 °C), up to 50 g shock and 5 g vibration.

When using LabVIEW and standard communications protocols such as TCP/IP, your application can simply send data back and forth between devices and it will transparently be transmitted 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 (Figure 2). These functions will work the same way whether the measurement system is connected to the network through a cable or through a wireless modem.

Figure 2. LabVIEW TCP VIs

Many PDAs also come equipped with 802.11 capabilities. Using the LabVIEW PDA module, you can build an application for Palm OS or PocketPC-based handhelds to acquire data and transmit wirelessly over 802.11 to another PC for further analysis or storage. To acquire the data using a PDA , you can use the NI CF-6004 CompactFlash DAQ device or any E Series PCMCIA device (Figure 3). In addition, you can program your PDA to monitor data wirelessly from mulitiple 802.11-based measurement systems.

Figure 3. NI CF-6004 data acquisition device for CompactFlash

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 (e.g. Windows). When using a PC to host the measurement system, you can make use of the LabVIEW built-in Bluetooth libraries (Figure 4). 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 communcation, which the NI LabVIEW PDA module will also allow you to access programmatically.

Figure 4. NI LabVIEW Bluetooth Functions

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

1. Your system needs a terminal that supports GPRS
2. A subscription to a mobile telephone network that supports GPRS
Note: use of GPRS must be enabled for a specific user. Some mobile providers will allow automatic access to the GPRS network, while others might require an explicit opt-in.
3. Some know-how is required on how to send and/or receive GPRS data, depending on the specific hardware used, including software and hardware configuration
4. You also need 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 National Instruments hardware, such as the GPRS/GSM module for CompactRIO. With this system, you can simply add this module to a 4 or 8-slot embedded CompactRIO system, and program you code to transmit data. With this technology built in, you can place your stand-alone system at any location where a GPRS-enabled GSM network is available and send and receive data.


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 allows for real-time data exchange and you typically don’t incur in reoccurring subscription or usage costs.

Depending on your measurement equipment, distances, security, and cost requirements, you can select from multiple types of modems, as is depicted in the following diagram from Data-Linc Group. Configuration for these modems is very simple, and you can simply connect the to your National Instruments hardware through a serial port if available, or through Ethernet. Once configured, the transfer of data will be transparent to the software and will work as if you had a wired network.

Figure 5. Typical wireless measurement system

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 a PC or a laptop with a plug-in or external data acquisition device, or one of National Instruments programmable automation controllers, the necessary connections to make the system wireless are readily available. As the wireless technologies evolve and reduce in cost and complexity, your system will too by simply integrating them into your measurement system.

See Also:
Products & Services: Compact FieldPoint
Products & Services: CompactRIO
Products & Services: LabVIEW PDA Module
Products & Services: PDA Data Acquisition
Products & Services: Compact Vision System
Products & Services: PXI