Wireless cloud
Richard Phelps -- Control Engineering, 12/1/2005
Certainly industrial instrumentation pros are well versed in wired, but not necessarily wireless, technology. Moreover, with endless corporate downsizing, plants are lucky to have specialists with the knowledge and time to just keep 'old' technology running much less simultaneously leading the wireless vanguard.
Nonetheless, applications remain the same—wireless is merely a medium. You want sensors, from whatever source, to be interoperable. And, unlike the endless 'blue yonder,' the wireless 'cloud' has finite proportions; it can be quickly filled up with devices all clamoring for bandwidth as well as—and worse yet—transmitting mechanisms creating interference.
Focus here is on unlicensed, so-called ISM band (instruments, scientific, and medical): 868 MHz, 900 MHz, and 2.4 GHz nominal frequencies, each of which has specific capabilities, bandwidth, speed, etc. Correlatively, one of two spread-spectrum technologies must be used in ISM: direct-sequence- (DSSS) or frequency-hopping- (FHSS) spread-spectrum.
What's your wavelength?
Wi-Fi aka IEEE 802.11 — with suffixes 'a (54 Mbit/s),' 'b (11 Mbit/s),' 'g (54 Mbit/s),' and now 'i'—is perhaps the most ubiquitous wireless standard. Excepting 'a,' all use 2.4 GHz. The most popular 'b' and 'g' standards divide the spectrum into 14 overlapping, staggered channels whose center frequencies are 5 MHz apart. The original standard also included 'carrier sense multiple-access with collision avoidance' (CSMA/CA) as the method to access the media.
Bluetooth, based on IEEE 802.15.1 and using 2.45GHz frequency, provides short distance communications. A pseudo-random frequency-hopping technique combats interference and signal fading on 79 available channels (1 MHz width), doing 1,600 hops/s. Each packet has its own channel, so if a packet is poorly transmitted it will be re-transmitted on another channel. Interconnection is possible, forming a piconet: one master device and up to seven connected slaves. Any device can be a master of one piconet and a slave of another; and any piconet's frequency-hopping sequence is unique.
ZigBee, based upon IEEE 802.15.4, operates in either 2.4 GHz or 900 MHz frequency. Its protocol stack is said to be one-third the size of Bluetooth and 802.11 protocols. Potential network addresses number 264, with support for star and mesh topologies. Bob Heil, chairman of the ZigBee Alliance, likens Zigbee to the 'cockroach that can survive nuclear war...it can sneak into 'quiet' (interference free) spaces.' He says that Zigbee is one of the easiest wireless-entry vehicles, operating in either 2.4 GHz or 900 MHz bands. Heil added, 'A facility might start out controlling lighting with a ZigBee network that can be extended to other plant operations.'
No plant is static, with movement of machinery, walls, etc., affecting wireless signals' strength and integrity. And don't overlook collaborating with your facility's neighbors to minimize interference.
Clearly, plant management has to take the broad view, considering the future as well as the present. Only by driving the vendors to provide value-added in the planning and integration sectors will technical—and capital expense— 'gotchas' be avoided in the future.
Phoenix Contacts' David Matthews, regional business unit manager for wireless, sums up the savvy customer's questions: 'How are you going to help me manage my bandwidth; is your technology appropriate for this application; and have you installed this before? Is an industry-standard solution being proposed, or is it a proprietary technology with future costs—dollar and/or operational?' Expressing a complementary sentiment, Freewave Technology's vice president sales-and-marketing, Colin Lippincott, says, 'A quality supplier will allow a customer to test the equipment for a while to be sure of its appropriateness.'
Richard Phelps, senior editor, Control Engineering richard.phelps@reedbusiness.com
Online
For more information see:www.controleng.com/archives, December 2005, for illustrations
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Wireless standards snapshot |
| Feature | Bluetooth | IEEE 802.11b | ZigBee |
| Power profile | Days | Hours | Years |
| Complexity | Complex | Very Complex | Simple |
| Nodes/master | 7 | 32 | 64,000 |
| Latency | Enumeration up to 10 s | Enumeration up to 3 s | Enumeration 30 ms |
| Range | 10 m or 100 m | 100 m | 70-300 m |
| Extendibility | No | Roaming possible | Yes |
| Data Rate | 1 Mbps | 11 Mbps | 250 kbps |
| Security | 64 bit, 128 bit | Authentication service set ID (SSID) | 128 bit AES and Application layer user defined |
Source: Control Engineering with data from Cirronet Inc. |
Demystifying ZigBee and its Application
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