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WHAT?
If you’d been off the planet for the last five years and
considered a snake’s pit of cables linking computers and
peripherals as normal, then today’s wireless connectivity
could come as quite a surprise.
C
onsumers hate cables – and
many Y2K era PCs had over a
dozen of them.
Although obviously wire and connector free, wireless linking traditionally involves tradeoffs between power,
range and speed.
Thus low power modules like the
433MHz units we’ve recently featured
have fair range but only slow data
speeds, while faster rates (900MHz
cellular, etc) come with higher power
demands, not easily met by batteries.
by Stan Swan
Much higher frequencies, such as
the license-free 2.4GHz slot, offer
greater bandwidth so more data can
be squeezed into the same signal
spectrum. However microwaves are
very line of sight, so ranges may be
greatly reduced. Phew – it’s all compromises...
Acting in a similar manner to line-
of-sight microwaves, infrared data
remained the only easy wireless technique five years ago, although its need
for clear links meant IR could never
punch data pathways through walls,
filing cabinets . . . or even paper.
To overcome this limitation, WiFi
(IEEE802.11x) – which itself has undergone three recent revamps (a,b,g
with “n” due this year) – and Bluetooth (IEEE802.15.1) evolved.
Bluetooth, although initially a
sleeper, now features in almost all new
Larger-than-life pics
of MaxStream’s XBee (1mW)
and XBee Pro (100mW) ZigBee
devices. Expect prices to be in the
$20-$30 range when they start arriving!
72 Silicon Chip
siliconchip.com.au
Name
WiFi
IEEE title
Speed
Range
Power
Hardware
Cost
Typical use
IEEE802.11g
to 54Mbps
300m
Modest
PC or AP
Modest
PC WLAN
Bluetooth
IEEE802.15.1-2
1-2Mbps
10m
Low
Camera/
Low
cell phone
Consumer PAN
(to 8 devices)
ZigBee
IEEE802.15.4
250kbps
30m
Very low
Microcontroller
Very low
Low duty cycle
(255 devices)
Comparison between WiFi, Bluetooth and now ZigBee wireless systems. As you can see, WiFi is still king of the roost
when it comes to range but ZigBee offers many advantages over Bluetooth – range included. Consume PAN (Bluetooth use)
stands for Personal Area Network.
cellular phones and such consumer
peripherals as headsets, photo printers and PDAs.
The Bluetooth title incidentally
honours tenth-century Viking King
Harald of Denmark, famed for feasting
on blueberries until his teeth apparently were stained blue. His administration skills however were even
more legendary, since for a period (no
doubt in their blueberry off season) he
managed to unify the war-mongering
provinces of Scandinavia to work
together (maybe raiding neighbouring blueberry patches!), much in the
way that today’s Bluetooth seamlessly
links cameras, PCs, headsets, and cell
phones etc.
You may groan with the nearbewildering rate of progress and worry
about security and 2.4GHz “RF smog”
but now there’s a further offering.
ZigBee!
In an attempt to simplify wireless
data communications, yet another
approach has recently emerged and
is showing considerable industry
support and promise. Formally based
on IEEE 802.15.4, it’s better known as
ZigBee.
No, ZigBee is not just a whacky
marketing title dreamt up over a
4-hour liquid lunch. Instead, it
honours the energy-efficient zigzag
“waggle dance” of honey bees, which
directs worker bees to nectar sources.
It was first studied by Austrian zoologist Karl von Frish in 1943.
Available at 915MHz and 2.4Ghz
(with the lower 915MHz being just
20kbps), ZigBee has several power
ratings, with just 1mW the most
common.
It’s somewhat of a Bluetooth “lite”
since, although it has similar crossroom range, it’s slower but far more
energy efficient. Remember that tradesiliconchip.com.au
off above between range, speed and
power?
ZigBee is further able to be implemented with simpler electronics and
(in time) may be so cheap that even
light switches could have it inbuilt,
leading to more flexible and cheaper
control layouts.
Yes, wireless light switches! Given
the obscene costs now associated
with 240VAC fittings, mains cabling,
conduit and redecorating (not to mention the difficulty of relocation), this
may be a very welcome development
indeed.
The table above is a brief feature
summary of these three main wireless data technologies. All are on the
globally license-free (and increasingly
“noisy”) 2.4GHz band.
Power demands of ZigBee devices
are so tiny that low duty-cycle applications may average current drains of
only microamps, allowing batteries
to last years – approximating their
shelf life.
Marketing is already playing on this
benefit, with “Five years off a few AAs”
noted! Solar powering from scavenged
room light may even be feasible.
A more dramatic feature however
relates to the relaying of data to a
specified address on the 255-device
ZigBee network.
This conjures up visions of your
door keys relaying “Hi family, I’m
home” details to the beer fridge, via
perhaps a network of ZigBee devices in
light switches and ad-hoc networked
sensors throughout the house.
Unlike the simplex and error-prone
nature of 433MHz units, handshaking
for data reliability is supported too.
This makes more professional applications attractive, suiting drive-by
utility monitoring and on-demand
telemetry, etc.
Although only ratified late 2004,
many makers already offer modules,
typified by the postage-stamp-sized
“XBee” from US firm MaxStream
(www.maxstream.net).
These come as either a 1mW or a
more powerful and sensitive longrange 100mW “XBee Pro” and allow
easy RS-232 or USB connections to
suit microcontroller interfacing.
Both look very “user friendly”,
with Hayes-style “AT” command
radio modem features, although the
low Picaxe serial data rates (typically
just 2.4kbps) may rather “waste” the
250kbps Xbee horsepower.
Since high-gain 2.4GHz antennas
are compact and very easy to organise
(see www.usbwifi.orcon.net.nz), lineof-sight control ranges to a kilometre
should be feasible with even the 1mW
version
All manner of exotic applications
arise with this new technique, the
least of which could be ZigBee-fitted
and powered solar garden lamps, all
“chatting” across the neighbourhood
and winking mesmerising messages
together while exchanging wireless
data about their owners’ lifestyles.
They’ll probably be all the rage by
next Christmas…
SC
References:
Get used to the ZigBee logo . . . you’re
going to see a lot more of it shortly!
References are conveniently hosted at
www.picaxe.orcon.net.nz/zigbee.htm
February 2006 73
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