Basic HTML Version
36
Geotechnical News • December 2014
www.geotechnicalnews.com
GEOTECHNICAL INSTRUMENTATION NEWS
identify the main types. These will be
discussed in turn.
Point to point
Point to point is the simplest wireless
architecture. This comprises a remote
sensor node that communicates via
radio directly with a gateway. The
gateway provides the data backhaul to
the database, but it could be collocated
with the main data storage system.
This architecture is suitable for single
or widely dispersed monitoring points,
and might typically use GSM/GPRS
as the wireless link, or satellite for
very remote locations.
Hub & spoke
In a hub and spoke system wireless
sensor nodes communicate directly
with a ‘relay’ or ‘controller’ node.
Each sensor node needs to be within
range of such a node. The relay nodes
then in turn communicate (directly
or indirectly) with a primary control-
ler node, which acts as a gateway.
This then forwards the data to the
data storage system via a data back-
haul. This is often characterized as
a hierarchical network, as the nodes
act as ‘slaves’ to the controller or
relay nodes. Typically these systems
use low power, short range wireless
for sensor node communications and
are suitable where clusters of sensors
are required. However the relay and
controller nodes have significant addi-
tional power requirements associated
with their need to relay messages; in
practice these must be provided with
an external power source.
Mesh
In a mesh network, each node com-
municates with one or more of its
neighbours. All the nodes in the
network are equal in status, and this
is often characterized as a non-hierar-
chical network architecture. The nodes
forward data via their neighbours,
using the most efficient route in the
direction of the gateway. The gateway
then collects the data and sends it on
to the user via the data backhaul. This
architecture allows for the network to
be self-configuring, which makes it
self-healing and robust, as well as easy
to extend and amend.
Project considerations
As with any monitoring project,
a number of questions need to be
answered, explicitly or implicitly. This
will influence the choice as to whether
to use wireless or not, and the type
of wireless system to be used if this
option is selected.
What to measure and how often?
How many monitoring points are
required? Are they close together or
widely spaced? How often are read-
ings required? I mean REALLY how
often are data points required? Wire-
less is not generally suited to continu-
ous or very frequent data readings as
this places heavy demands on the bat-
tery power of the sensor node. As an
example, in many long term structural
applications, one reading an hour is
more than sufficient.
It may be required to adjust the report-
ing rate of the sensors, for example
when intense construction activity
takes place or significant movements
are observed. Some systems can sup-
port this. If so is it simple or does it
require local intervention? Can it be
done remotely via the backhaul?
Location & access
Where are the sensors to be deployed?
Are they clustered in a limited space,
or are they widely scattered? Is it in
open outdoor space or a restricted
space or even confined underground,
such as a tunnel or basement? What
facilities are available for power?
What communications facilities are
available to get the data out of the
location? Is GSM or satellite pos-
sible? If not, then is there access to the
telephone network, a data communica-
tions network, and/or the internet?
Is the location difficult to reach, and/
or hazardous to access? What time
restrictions and permissions apply
to accessing the location? Are there
maintenance liabilities with running
cables to sensors, or are they prone
to damage by engineering crews or
rodents? Is flexibility required in the
deployment of sensors? Is it required
Figure 2
.
Wireless architectures for sensing.