Environmental Engineering Reference
In-Depth Information
4.6.2 Remote Data Transfer
Remote transfer requires a telecommunications link between the data logger and
the central computer. The communications system may employ direct-wire cabling,
telephone lines, cellular telephone, RF telemetry, or satellite-based telemetry, or for
redundancy, a combination of these components. The main advantage of this method
is that the data can be retrieved and inspected more frequently (e.g., weekly) than
might be practical with site visits. This means that problems with the equipment
can be more quickly identified and resolved, thus likely increasing data recovery.
Many logger manufacturers now offer integral remote data collection equipment.
The main disadvantage of the remote method is the cost of the equipment. In addi-
tion, some sites have poor cellular coverage, and other, noncellular options can be
expensive.
There are two methods of remote data retrieval: those initiated by the recipient
(call out) and those initiated by the logger (phone home). The first type requires
the recipient to oversee the telecommunication operation. Steps include initiating
the call to the in-field logger, downloading the data, verifying data transfer, and
erasing the logger memory. Some call-out data logger models are compatible with
computer-based terminal emulation software packages with batch calling. Batch
calling automates the data transfer process, allowing the user to download data
from a number of monitoring sites at prescribed intervals. Batch programs can
also be written to include data verification routines. The data logger manufacturer
should be consulted to determine the compatibility of its equipment with this
feature.
A phone-home data logger automatically calls the home computer at prescribed
times to transfer data. In the past, the phone-home method could not be used to
support as many towers as the call-out method because call times had to be spaced
far apart to allow for slow or repeated transfer attempts. The newest generation of
data loggers solves this problem by using the Internet to send data out as attached
e-mail files. This allows for concurrent data transfer from multiple sites. In addition,
the data can be delivered to more than one computer, providing greater data security
and convenience.
Data loggers with remote data transfer via cellular communications are gaining
popularity because of their ease of use and reasonable cost. The cellular signal strength
and type (GSM or CDMA) at the site should be determined in advance; this can be
done with a portable phone. Where the signal strength is weak, an antenna with
higher gain can sometimes be successful. Failing that, a satellite modem linking to
the Globalstar or Iridium network is an option.
Guidelines for establishing a cellular account are usually provided by the data
logger supplier. It is important to work closely with the equipment supplier and cellular
telephone companies to resolve any questions before monitoring begins. It is best to
schedule data transfers during off-peak hours to take advantage of discounted rates
and faster network speeds.
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