Machine Design

Look Ma – no wires!

The wireless buzz is not just confined to the world of cell phones and pagers. Industrial networking is also jumping on the wireless wagon.

Benson Hougland
Matt Chang
Opto 22 Temecula, Calif.

Edited by Miles Budimir

A wireless LAN network augments existing wired networks by providing network connectivity to places where running cable is either impossible or costly. A single access point links laptops, handheld computers, and wireless I/O modules back to an existing wired LAN or WAN (wide area network).

Spread-spectrum transmissions spread out the signal over a range of frequencies. This increases signal-to-noise performance making the transmission more immune to narrowband interference. On the other hand, narrowband transmissions transmit on one frequency, increasing the likelihood of interception or jamming.

Direct sequence is a spread-spectrum technique that encodes each bit of data and sends it as a bit sequence. The original information is spread out over a wide frequency range using more bandwidth than the original data needed. To a nonsynchronized receiver, it appears as wideband noise.

Frequency-hopping spread spectrum transmits packets of data on different carrier frequencies. To unsynchronized receivers, the transmission appears as a series of noise impulses.

A wireless LAN unit from Opto 22 connects remote I/O hardware back to a wired network. 500 mW of output power allows transmissions of up to 2,000 ft in open environments.

Wireless is where it's at these days, and not just for consumer products. Increasingly, wireless networking is seeing use in industrial areas and displacing or augmenting traditional wired networks.

Where masonry walls make running network conduit difficult or costly, a wireless local-area network (WLAN) can be more practical. For instance, an automated machine can utilize a wireless Ethernet I/O module to connect it to a network.

A WLAN is also well suited for mobile test stands or equipment that moves around on the factory floor, such as bar-code wands or other data-collection devices.

A WLAN is a data-communications network that usually augments an existing wired network. A typical setup consists of a piece of hardware called an access point connected to a wired network. The access point is a transmitter and receiver (transceiver) that receives, buffers, and transmits data between the wired and the wireless network.

A typical setup consists of an access point with a built-in omnidirectional antenna. When mounted on the ceiling or a wall, they provide several hundred feet of coverage. Antennaless access points are also available to allow use of directional antennas for special situations.

At the other end of the wireless link is an interface card in some type of electronic device. Typical devices today include notebook computers, desktop PCs, and handheld devices such as bar-code readers or inventory-control terminals.

Access point systems are available from several different vendors including Symbol Technologies, Intermec, and others. Software tools for installing, maintaining, and upgrading the WLAN are usually included, and software upgrades can be done in Flash memory.

The introduction of a standard called 802.11b made wireless LANs more attractive. One reason was that this standard allowed for a higher data rate. Another was interoperability. When WLANs used protocols that were strictly proprietary, all hardware had to come from a single supplier. There was no guarantee that these systems had staying power.

Today, however, major computer manufacturers including IBM, Dell, and Gateway have announced products shipping with 802.11b capability built into a laptop. Intersil Corp., Irvine, Calif., a company that developed many of the chipsets for WLAN, is predicting that most motherboards in the future will have 802.11b functionality built in.

The 802.11 standard
IEEE 802.11 defines network standards on several levels for wireless communication networks. It defines the physical layer including the modulation schemes and signaling characteristics. The general spec defines three physical transmission methods; two RF methods and one infrared.

RF devices operate in the unlicensed 2.4 GHz ISM (industrial, scientific, medical) band, with 83 MHz of bandwidth between 2.4 and 2.483 GHz. However, the IEEE standard does not specify the maximum amount of radiated RF power that can be transmitted. The FCC sets a limit of 1 W for the U.S..

The FCC also specifies that spread-spectrum transmission methods be used in this frequency band. Spread spectrum is a wide-band RF technology developed by the military. The basic idea behind spread spectrum is that it spreads the transmission of data over several different frequencies, as opposed to narrow band transmission which sends all the data on a single carrier frequency. For military applications, this makes it much harder to intercept or jam a message. But the spreading also helps avoid interference with other narrowband transmissions, opening up an otherwise crowded frequency spectrum.

The general 802.11 standard supports two types of spread spectrum methods; frequency-hopping spread-spectrum (FHSS) and direct-sequence spread spectrum (DSSS).

FHSS works by sending data on a single carrier that changes frequency. That is, it effectively transmits data in small packets at various frequencies. FHSS divides the 83 MHz of bandwidth into 79 onemegahertz channels. The standard further specifies three sets of 26 hopping patterns, with a minimum of 2.5 hops/sec. The pattern of changing frequencies is known to both the transmitter and receiver. But to unsynchronized receivers, the signals appear as noise impulses.

On the other hand, DSSS spreads the data using a modulation scheme called a Barker sequence. Each bit of data is modulated and encoded by an 11-bit sequence. The result is that each data bit is sent as a bit sequence, using more bandwidth than is actually needed to send the individual bit. This reduces the effect of narrowband interference.

With respect to transmission speed, the original 802.11 spec uses FHSS with a maximum data rate of 2 Mbits/sec. The newer 802.11b uses DSSS and supports a maximum data rate of 11 Mbits/sec, with extensions to double the data rate to 22 Mbits/sec.

But these specs don't reflect guaranteed throughput, only the maximum possible data rate. For example, most 802.11b networks typically don't hit throughputs much higher than 5.5 Mbits/sec.

Specified power levels for 802.11 are such that a typical omnidirectional antenna gives a 100 to 300-ft range, depending on obstructions and a clear site line. Directional antennas, of course, can boost the range in one direction, depending on antenna gain, to as much as several thousand feet.

One point to note is that performance degrades with increasing range. Devices built to 802.11b are rated at 300 ft but will not work at maximum speed except when close to the base station. The minimum specified data rate for both 802.11 and 802.11b is 1 Mbit/sec. Compared to the 2 Mbits/sec maximum that 802.11 specifies, that might sound low. However, most industrial applications involve the relatively slow tasks of checking or monitoring status or reading an analog value. For these applications, 1 Mbit/sec is fast enough.

Cut the noise
Electrical noise has always been a concern for wireless networks in industrial environments. However, the widespread adoption of spreadspectrum modulation techniques has made noise much less of an issue. Both FHSS and DSSS transmission techniques can provide a certain amount of noise immunity.

There are two basic types of noise on a plant floor; burst noise and narrow band or constant frequency noise. Burst noise is generated by a contactor sparking or an arc welder and tends to have many frequency components. On the other hand, a common source of narrow band noise is a motor drive emitting harmonics of its fundamental frequency.

Because FHSS transmissions hop and use different frequencies, there is less of a chance of noise interfering with a transmission. For this reason, FHSS is more immune to narrow-band noise. For instance, an harmonic of a given fundamental frequency may be strong enough to cause interference on a particular transmission frequency. However, because FHSS uses different frequencies to transmit data, chances are good that the noisy frequency will not be used. If it is, error correction algorithms will usually recover whatever data was lost.

Broadband noise is more of a problem because it is not confined to a single frequency but is spread out over a range of frequencies. But error correction algorithms also minimize broadband noise effects on transmission.

Speed and compatibility
A wireless Ethernet does not perform as well as wired versions. For instance, compared to a wireless network, hardwired networks can transmit at near the capacity of the network. There is additional message overhead associated with wireless protocols and handshaking between transmitters and receivers.

Wired Ethernet devices respond to a query from a master in substantially less than a millisecond. In contrast, the same operation can take anywhere from 10 to 15 msec in the wireless version. However, data throughput is typically better once the initial connection is made. But the need for additional handshaking will probably always make wireless LANs slower than hardwired versions.

Another concern in the wireless world is compatibility. Because the 802.11 standard only specifies the physical layer and media access control (MAC) layer and not implementation, manufacturers of wireless equipment have interpreted the spec differently. The spec also doesn't contain a compliance test. This results in products from different vendors not being able to communicate with each other.

WECA (Wireless Ethernet Compatibility Alliance) and WiFi (Wireless Fidelity), standards have gone a long way toward addressing the problem of interoperability. All products that are WiFicertified will probably work with one another. Unfortunately, this is only an 802.11b standard. There exists no similar standard for 802.11.

A new spec, 802.11a, is due out later this year. It will operate in the new Unlicensed National Information Infrastructure (UNII) band, with considerably more bandwidth than either 802.11 or 802.11b. The standard spells out a whopping 300 MHz of total bandwidth, 200 MHz between 5.1 and 5.3 GHz, and 100 MHz at 5.7 GHz. The higher bandwidth will make possible throughputs approaching 54 Mbits/sec. This is close to that of hardwired networks. However, lack of affordable chip technology may delay widespread use of the new standard.

Bluetooth — still cutting its teeth

Bluetooth modules such as this one from Ericsson contain the RF circuitry, baseband controller, and Flash memory along with the software needed to embed Bluetooth in a host of products.

Bluetooth is an emerging wireless standard that has created quite a buzz over the last few months. It is designed to be a personal-area network (PAN) for data and voice communication among portable electronic devices and desktop PCs.

Bluetooth operates in the 2.4 to 2.48-GHz ISM band using frequency-hopping spread-spectrum transmission. The specification defines two ranges of operation; a short 10-m range with a maximum power output of 1 mW, and a medium 100-m range with a 100 mW maximum. Data rate for both ranges is set at 720 kbits/sec.

Bluetooth hardware consists of an analog transceiver and a digital host controller. A DSP and a CPU core for baseband processing, encryption, and coding, and the software stack all fit on a single CMOSchip. The single-chip implementation lowers power consumption, making it suitable for mobile devices.

The novelty of Bluetooth is its ability to form ad hoc networks. All Bluetooth devices are identical except for a 48-bit device identifier. This means that any Bluetooth device can be a master with up to seven simultaneously active slaves. Such a setup is called a piconet and is the basic Bluetooth network topology. A set of piconets is known as a scatternet.

Bluetooth does have some competition, though. The infrared standard IrDA (Infrared Data Association) is one. However, it's limited to point-to-point and line of sight communication.

Another is the HomeRF standard. It too operates in the 2.4-GHz ISM band and supports ad hoc networks. However, it has a hop rate of 8 Hz compared to Bluetooth's 1.6 kHz, and only supports data communication, not voice.

The 802.11 standard has a higher data rate. But it is generally more expensive to implement and consumes more power. Another problem is interference. Tests have shown that Bluetooth transmissions significantly degrade 802.11b transmissions nearby. This is because both operate in the 2.4-GHz ISM band. However, Mobilian Corp., Portland, Oreg., has recently introduced a two-chip transceiver set that purports to eliminate this problem, allowing simultaneous unimpeded transmission.

Another problem is that the Bluetooth specification is not yet finished. This means, for example, a Bluetooth product from Nokia may not work with one from Ericsson.

For now it remains unclear how Bluetooth will fit into the wireless landscape. Experts think the technology will catch on in the consumer market within 18 months and move into industrial uses thereafter. One likely scenario is that Bluetooth devices may bridge the gap between wireless PANs and industrial wireless LANs. — Miles Budimir

Wireless on the Web — White paper on technology underlying the 802.11 standard. — White papers on the coexistence of 802.11b and Bluetooth. — The official Bluetooth site contains an overview of Bluetooth including technical specs, white papers, and information on the Bluetooth special interest group. There's even some background on the 10th-century Viking king who unwittingly lent his name to the new standard. — Home page of the Wireless Ethernet Compatibility Alliance (WECA). Check here to see which 802.11b products are compatible. Also, check out technical information like white papers and a wireless glossary. — White papers on a variety of WLAN topics including a paper on RF site surveys and antenna selection. — The official Web site of the Wireless LAN Association. Here you'll find the latest WLAN news, white papers, and announcements on industry events. — Spread Spectrum Scene offers useful info on wireless and RF topics, including some in-depth tutorials on spreadspectrum techniques.

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