Leland Teschler
Editor
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The first generation of wireless USB interfacing is likely to take the format depicted here. The wireless USB connection is between a wireless hub and dongles plugged into USB-certified devices such as cameras, mice, PDAs, or display terminals. Belkin Corp. recently developed hub/dongle connections for the Cable-free USB format. An alternative approach comes from Gefen Inc., which makes small sender/receiver units that fit in an ordinary USB port to provide a Cable-free USB connection. |
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Billed as the world's first UWB image-transfer board, this device from FotoNation, San Francisco, permits wireless transfer of photos at 200-Mbit/sec rates between cameras and PCs. It follows the PTP-IP (picture transfer protocol IP) standard and implements WiMedia-style Wireless USB through use of a UWB chip from Wisair Corp. |
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The difference between direct sequence and OFDM transmission techniques can be illustrated by time and frequency domain plots like these devised by authors at Intel Corp. Direct-sequence transmissions use super-short pulses at differing frequencies. The resulting bandwidth is quite wide. OFDM, sometimes referred to as multiband UWB, uses several carriers at different frequencies and initiated at different times. The resulting frequency domain plot consists of frequency bands with minimal overlap. |
January's Consumer Electronics Show marked the first public demonstration of wireless USB equipment. USB, for Universal Serial Bus, is the standard used for connecting computers to mice, scanners, digital cameras, printers, and so forth. The wireless version of USB eliminates the cord between the computer and the peripheral by substituting a radio link.
The use of RF on wireless USB lets the peripheral sit as far as 30 ft away, though the highest transmission speeds come if the separation is only 10 ft or less. And the peripheral needn't be in sight of the computer, a necessity for connections that use infrared beams as a communication medium.
No question electronics makers had consumer users in mind when they devised wireless USB. But equipment on the production floor could benefit from the new spec as well. The reason is wireless USB may be an inexpensive means of beaming massive amounts of data between two points. The USB 2.0 spec which wireless versions implement out-performs 100 Mbit Ethernet. So the wireless spec could potentially handle sophisticated I/O that is data intensive.
Many home video cameras now carry USB ports, so the wireless versions of USB are designed with the bandwidth demands of video data streams in mind. Thus there may be benefits to using wireless USB for industrial cameras mounted on moving actuators or far away from the video processor. Ditto for other kinds of sensors that sit on many kinds of equipment and generate an appreciable amount of information.
Moreover, wireless USB employs a technique called ultrawideband (UWB) for its radio link. This transmission scheme tends to work well even in RF environments that include a lot of electrical interference and noise.
Thus the wireless USB link between a computer and an industrial peripheral is likely to maintain itself despite the impulse and broadband-radiated interference that characterizes industrial plants. And wireless USB transmissions use a level of security encoding that is comparable to that of wireless Ethernet protocols.
Wireless USB equipment is also likely to be relatively inexpensive because consumer electronics is its main focus. Industrial users will likely benefit from the economies used to reach price-conscious applications in the home.
There are, of course, caveats to be aware of. Wireless USB is strictly point-to-point from peripheral to controller. Unlike in WiFi setups, wireless USB devices cannot communicate among themselves so there is no opportunity for mesh networking or other sophisticated interconnections.
Potential users should also be aware there are two different camps of wireless USB devices, each using incompatible transmission schemes and host software. Wireless USB products that made a debut at the Consumer Electronics Show use what's called Cable-free USB. It is promoted by a group of companies that include Freescale Semiconductor, Belkin Corp., Icron Technologies Corp., and Gefen Inc. Products designed to this spec support existing USB 1.1 and USB 2.0 devices. Practically speaking, that means you needn't change the driver software of a hardwired USB device to make it work over the Cable-free version. It should be plug-and-play.
Cable-free USB setups convert existing USB devices to wireless mode through use of a wireless hub and dongles. The dongle plugs into a normal USB port on a peripheral such as a camera or mouse. It is actually a UWB radio that talks with a wireless hub, which is plugged into the USB port on the host computer. Any equipment that uses USB 1.1 or 2.0 can hook up to the wireless gear with no changes needed in software drivers.
Vendors of Cable-free USB equipment say the wireless link will operate at the same 480-Mbits/sec rate as ordinary 2.0 USB. This rate is fast enough to handle a full-motion video stream. It compares favorably to the 11 Mbits/sec available on 802.11b WiFi networks and 54 Mbits/sec on the more-recently defined 802.11-g spec. (The scenario assumes a USB transmitter and receiver are no more than 3 m apart from each other. The effective transmission speed depends on distance and drops to about 110 Mbits/sec at 10 m.)
The second type of wireless USB format is called WiMedia. End products incorporating Wi-Media are just starting to appear. Companies that include Dell, Intel, Microsoft, Samsung, and Toshiba say they aim to offer Wi-Media in the second half of this year. These firms helped define the WiMedia spec through an industry association called the Wireless USB Promoter Group.
One of the first firms to release a WiMedia product is FotoNation, a research and development house in San Francisco that concentrates on digital photography. Its UWB image-transfer board is designed for cameras. "We think the digital-camera market is moving toward wireless connections. You will probably see wireless USB cameras relatively soon from leading camera companies," says FotoNation CEO Eran Steinberg. "We think almost all camera makers will have introductions in this area next year."
Makers of industrial equipment are now evaluating both Cable-free and WiMedia standards. One firm in this category is National Instruments Corp., a maker of virtual instrumentation and controls. Chris Rake, manager of NI's Portable DAC Group, says developers there are "still trying to answer the question about what applications would work with wireless USB. One might be handheld data monitoring and collection. The nice thing about wireless USB is that you could just walk up to a device on the factory floor and start communicating with it."
Rake points out that there are technical challenges to be over-come before this idea makes sense. Perhaps the most obvious is that PDAs and similar devices used as portable instrumentation monitors aren't powerful enough to make use of wireless USB's high-speed streaming.
Another difficulty is in how the first Cable-free USB devices communicate. At least initially, the wireless link will only be through Cable-free USB hubs. "That would be challenging for a handheld device because it would have to talk to a machine through two different wireless nodes," explains Rake. "You'd like to have the ability to incorporate the wireless technology into the device itself." Other trends could impact industrial applications. There is talk that cell phones, for example, could begin incorporating wireless USB as a means of receiving movies and video content. These same capabilities could let ordinary cell phones serve as portable terminals able to view diagnostics or status updates from industrial processes and machines.
UWB FOR USB
The idea behind UWB transmission is to use a large bandwidth to send information. In so doing, the technique reduces the impact of localized interference generated by such common sources as microwave ovens and medical gear.
But though the two wireless USB formats each use UWB techniques, that is the extent of their commonality. Their transmission modes are incompatible with each other. (There is no standard for UWB transmissions. Nor is there likely to be one anytime soon. The committee trying to develop UWB standards disbanded in December after failing to agree on numerous issues surrounding the format.)
Cable-free devices transmit according to a technique called direct-sequence code-division multiple-access (DS-CDMA). It spreads data transmission throughout a 3.1 to 10.8-GHz spectrum in the form of numerous brief, low-duty cycle pulses. It encodes information onto the radiated signal by multiplexing the digital information to be transmitted with a digital code called a signature. Each bit in a data packet is modulated by a code pattern through a technique called bi-phase shift keying. In the receiver, the original signal is recovered by demodulating out the same code pattern.
This technique tends to minimize interference because interfering signals are generally much narrower than the spread-spectrum signal. That means the received energy of the interference appears to be much weaker than that of the spread signal.
The point to note about the Cable-free USB approach is its use of a single band of frequencies. In contrast, the approach employed by the WiMedia specification is to divide the frequency spectrum ranging from 3.1 to 10.6 GHz into multiple smaller and nonoverlapping subbands with bandwidths greater than 500 MHz. Partitioning the spectrum this way offers the advantage of avoiding transmission at frequencies that may be occupied by other transmission systems such 802.11a LANs, which work at 5 GHz.
WiMedia devices use a technique called orthogonal frequency division multiplexing, or OFDM. Consecutive symbols are modulated over different subbands with a frequency-hopping scheme, rapidly switching a carrier among the different frequency channels. Orthogonality in OFDM refers to the fact that it employs carriers spaced apart at precise frequencies picked so they do not interfere with one another.
This scheme promotes the use of high data rates by compensating for delays in transmitted signals, which can differ depending on the frequency. The bit rate and signal strength of each carrier can adapt to conditions hampering transmission at that frequency, so more bits get sent on good channels,fewer on those experiencing-interference.
Both Cable-free and WiMedia UWB methods provide the same range and both tolerate a certain amount of electromagnetic interference. They each have strong points that are comparable. All in all, industry observers think marketing and merchandising may trump technical considerations in determining which version becomes more widely adopted.
Label printing goes wireless
The move to RFID tags is having an unintended consequence: It is promoting the use of wireless networks both in factory operations and retail stores. Bar-code printer maker Avery Dennison, Philadelphia, says the RFID movement has led many manufacturers to set up WiFi networks in their production facilities partly as a means of slapping RFID tags on outgoing cartons of product. Because only a few retailers demand that suppliers deliver RFIDtagged merchandise, the tagging tends to take place at the loading dock as manufacturers get ready to ship, says Avery Dennison. A printer equipped with a WiFi card sits at the dock to produce the tags . The WiFi connection lets the printer get instructions from a central computer about what to put on the tag. The WiFi facility comes in handy because few loading dock areas have hardwired Ethernet connections nearby. Recently developed versions of bar-code and RFID-tag printers have the WiFi card built in. Though they have found use almost exclusively in shipping areas, Avery Dennison says retailers also are expressing interest in WiFi printers for similar reasons: Ethernet connections in retail stores tend to be up front at the cash registers. WiFi printers can sit anywhere. Wireless connections could let them crank out price tags in a back room to quickly handle situations such as mark-downs and in-store promotional pricing. |
MAKE CONTACT
Avery Dennison, Printer Systems, www.ris.averydennison.com
Belkin Corp., belkin.com
FotoNation, fotonation.com
Freescale Semiconductor, freescale.com
Gefen Inc., gefen.com
Icron Technologies Corp., icron.com
National Instruments Corp., ni.com
Wimedia Alliance, wimedia.org
Wisair Corp., wisair.com