A personal area network, or PAN, is a
computer network that enables communication between computer devices near a
person. PANs can be wired, such as USB or FireWire, or they can be wireless,
such as infrared, ZigBee, Bluetooth and ultrawideband, or UWB. The range of a
PAN typically is a few meters. Examples of wireless PAN, or WPAN, devices
include cell phone headsets, wireless keyboards, wireless mice, printers, bar
code scanners and game consoles.
Wireless PANs feature
battery-operated devices that draw very little current. Sleep modes commonly
are used to further extend battery life. Network protocols tend be simpler than
Wi-Fi or WiMAX (to reduce required processor power), and the transmit power is
typically less than 1 milliwatt.
In the United States, PANs for the
most part operate in two unlicensed bands: 902-928 MHz and 2.4-2.4835 GHz.
Ultrawideband devices also can operate in the 3.1-10.6 GHz band, coexisting
with other radio services by employing low overall power and ultra-low power
densities (watts/Hz).
Let’s examine three of the most
popular PAN technologies: ZigBee, Bluetooth and ultrawideband.
ZigBee is a short-range, low-power
computer networking protocol that complies with the IEEE 802.15.4 standard. In
the U.S., ZigBee devices operate in the 902-928 MHz and 2.4 GHz unlicensed
bands. The technology is intended to be less complex and less expensive than
other WPANs such as Bluetooth. Although ZigBee is a WPAN protocol, it also is
used for telemetry applications such as automatic meter reading and building
automation.
ZigBee employs direct-sequence spread
spectrum modulation with a gross data rate of 40 kb/s in the 900 MHz band and
250 kb/s in the 2.4 GHz band. Advertised transmission range is from 10 to 75
meters, but like any radio system, the actual range depends on the environment.
There are three types of ZigBee
devices: ZigBee Coordinator (ZC), ZigBee Router (ZR), and ZigBee End Device
(ZED). The ZC is the most capable device, forming the root of the network tree
and bridging to other networks. There is only one ZC per network. The ZR can run
an application function as well as act as an intermediate router, passing data
from other devices. A ZED contains just enough functionality to talk to its
parent node, which is a coordinator or a router. It can sleep most of the time,
extending its battery life.
The ZigBee Alliance is a trade
organization charged with developing and publishing the Zigbee standard and
promoting its use.
Bluetooth is a computer networking
protocol designed for short-range, low-power communications in the 2.4 GHz
unlicensed band. It was named after King Harald Bluetooth, ruler of Denmark and
Norway in the late 10th century. Sven Mattison and Jaap Haartsen, both
employees of Ericsson Mobile Platforms in Lund, Sweden, published the first
Bluetooth standard in 1994. The current version of the standard is 2.1 and
specifies gross data rates up to 3 Mb/s.
Bluetooth employs frequency-hopping
spread spectrum modulation with a rate of up to 1600 hops per second using 79
different channels, each 1 MHz wide. Because the technology uses a spread
spectrum signal and low power, it is less likely to cause harmful interference
to other 2.4 GHz devices, such as Wi-Fi radios, that often exist in the same
personal computer. There are three classes of Bluetooth devices corresponding
to different transmit power levels. Class 1, 2 and 3 devices operate at up to
100 mW, 2.5 mW and 1 mW, respectively.
Bluetooth networks normally operate
in a master-slave configuration. A master device can communicate with up to
seven active slave devices, and this network of up to eight devices is called a
piconet. Up to 255 additional devices can be inactive or parked, waiting for
wakeup instructions from the master.
The technology implements
confidentiality, authentication and key derivation using algorithms based on
the SAFER+ block cipher.
The Bluetooth Special Interest Group
is a privately held, nonprofit trade association organized to promote Bluetooth
in the marketplace and to develop Bluetooth standards.
Ultrawideband is a radio technology
useful for short-range, high-bandwidth communications that does not create
harmful interference to users sharing the same band. By FCC definition, a UWB
signal has a bandwidth that exceeds the lesser of 500 MHz or 20% of the
arithmetic center frequency. Such a bandwidth exceeds all conventional spread
spectrum radio systems, and the resulting low power density ensures the signal
does not cause harmful interference.
The FCC allows UWB devices to operate
in the 3.1-10.6 GHz band. In this band, the emission limit is -41.3 dBm per
MHz, which is the Part 15 limit for unintentional emissions in this band.
Unlike conventional radios, which continuously modulate a sinusoidal carrier,
UWB radios are short-duration pulse generators. The occupied bandwidth is
roughly equal to the inverse of the pulse duration. The duty cycle of UWB
signals is usually quite low, but the net throughput is still high because the
burst information rate during the pulse can be more than 100 Mb/s.
A pulse-based UWB method is the basis
of the IEEE 802.15.4a draft standard and working group, which has proposed UWB
as an alternative physical layer protocol to ZigBee.The WiMedia Alliance is a trade
association organized to promote UWB and develop standards.
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