Agriculture Reference
In-Depth Information
(IEEE, 1999) and wireless PAN, IEEE 802.15.1 (Bluetooth) (IEEE, 2002) and IEEE
802.15.4 (ZigBee) (IEEE, 2003), are used more widely for measurement and automa-
tion applications. All these standards use the instrumentation, scientific and medical
(ISM) radio bands, including the sub-GHz bands of 902-92 8MHz (United States),
868-870 MHz (Europe), 433.05-434.79 MHz (United States and Europe), and 314-
316 MHz (Japan), and the GHz bands of 2.400-2.4835 GHz (worldwide acceptable).
In general, a lower frequency allows a longer transmission range and a stronger capa-
bility to penetrate through walls and glass. However, because radio waves with lower
frequencies are more easily absorbed by various materials, such as water and trees,
and radio waves with higher frequencies are easier to scatter, effective transmission
distance for signals carried by a high frequency radio wave may not necessarily be
shorter than that by a lower frequency carrier at the same power rating. The 2.4-GHz
band has a wider bandwidth (250 Kbps) that allows more channels and frequency
hopping and permits compact antennas. The ZigBee standard is established by the
ZigBee Alliance, which is supported by more than 70 member companies. It adds
network, security, and application software to the IEEE 802.15.4 standard. Owing
to its low power consumption and simple networking configuration, ZigBee is con-
sidered the most promising for wireless sensors and has been embedded into many
commercial products worldwide.
In agricultural applications, stand-alone data loggers with standard interfaces
[analog input/output (I/O), digital I/O, serial ports, parallel ports, etc.] are widely
used. They are easy to use, easy to program, and very rugged under various environ-
mental conditions. For example, data loggers from Campbell Scientific Inc. (2011)
have been widely used in weather stations to collect environmental data. Some of
the data loggers integrate RF modules to remotely transmit data to a control cen-
ter. Recently, more and more data logger manufacturers are adding wireless com-
munication modules to their existing dataloggers. Campbell Scientific Inc. now
provides communication modules for Ethernet, Spread spectrum RF, satellite, cel-
lular devices (GPS and CDMA), etc., which can easily connect to their dataloggers.
Decagon Devices Inc. (USA) (Decagon, 2011) recently introduced Em50G wire-
less cellular data logger and Em50R wireless radio data logger, which allow long
distance networking and data communication. MicroStrain (2010) has a production
line of wireless nodes with various sensors, wireless base stations, and wireless
sensor data aggregator. Onset (2010) manufactures a series of Hobo dataloggers.
They recently launched wireless dataloggers for temperature and/or humidity for
indoor and outdoor applications. These datalogger platforms are tied closely with the
operating systems and development tools from their own manufacturers. They can
be easily networked among modules from the same manufacturer, but are difficult
to handle when connecting with those from other manufacturers. One of their major
shortcomings is that they are often expensive and less flexible, hence, not suitable for
large-scale WSN applications.
All WSN hardware platforms share features of limited onboard or on-chip
resources, severe memory constraints, and limited power access. Hence, an oper-
ating system (OS) running on them needs to be both very small in footprint and
event-driven. Originally developed by the University of California-Berkeley, Intel
Research, and Crossbow Technology, TinyOS is a free, open-source embedded OS
