Monitoring System Topology

I recently had a discussion with a client in Taiwan regarding the methods used to collect data from remote monitoring stations.  My client does a lot of landslide monitoring, and their technique is to install a cellular modem in every station and collect data from each.  This is a "Multi-Point Collection" network where data is collected separately from each station. Because the terrain is not only remote but also rugged and steep, they are always fighting with cell signal issues.  I have been trying to convince my client to use radios at sites with poor cellular signal, and route the data to a single station with a good signal strength.  For purposes of discussion we'll call this a "Single-Point" collection scheme - where a host station is responsible for polling and collecting data from remote stations over a local wireless network, and data is collected from only the Host station(s).  This is the technique we have incorporated into our wireless EmbankNet ™ dam monitoring system, and we have found it very efficient and cost effective for remote sites.  This blog will describe and attempt to point out the relative benefits of each of these approaches.

SCADA vs. Data Logging

Before we discuss the different forms of data collection, I should say that our discussion is centered around a methodology of monitoring called "data logging" as opposed to SCADA systems that use Programmable Logic Controllers (PLCs).  Data loggers typically are low-power devices designed to be connected to sensors and deployed in remote locations for extended periods, usually using a battery and a solar panel as a power source.  PLCs are found on the factory floor where there is abundant power and where they are dedicated to measuring and controlling things in real time.

Due to technological advances, the line between these two concepts is definitely blurring.  But for purposes of this discussion, a SCADA system is dedicated to controlling processes in real time without humans collecting and analyzing the data.  Data logging is dedicated to collecting the  sensor data for analysis, modeling and reporting.  It's important to point out that data may be collected in a SCADA system, and real-time control may be implemented in a data logging system.  But the hardware and software in each has evolved from a different core purpose.

So our discussion is about how we can collect data from data loggers using different network topologies using modems and/or radios.

Multi-Point Collection

This type of data collection using telemetry is the simplest to implement as it basically collects data from each monitoring station without reference to others.  Each station must have it's own telemetry that connects directly to the collection point at the home or office.  With this topology we periodically connect to each station directly with a server (or have each station connect to the server) and we retrieve the stored data. 

Multi-Point Collection

In the diagram above each station is connected to the Server through the Internet.  We tend to use this technique when our stations are spread over a wide area and there is no "line-of-sight" between them. The benefit of this approach is ease of deployment.  Each station stands alone and only needs some form of communication to access it.  It used to be a phone line, but now it's usually either a satellite or cellular modem.  The major limitation to this approach is that Internet connectivity is not always available at remote locations.  There is also a significant management and cost factor over the long term as service plans must be procured and maintained for each modem.  This is not a one-time occurrence or cost as technology and data plans are changing constantly and service providers may not have the same sensitivity to the importance of your data.  Service providers are creating machine-to-machine (M2M) data plans which makes management and provisioning easier.  But with over 15 years of experience using this approach I can tell you that change is the constant in this industry.

Single-Point Collection

This form of data collection involves using radios and modems in tandem to adapt to local conditions, and to consolidate data into fewer number of data collection points - called Host stations.

Single-Point Collection

This technique requires more up-front programming to configure as remote stations must send data to the Host either on a pre-determined schedule, or in response to a request from the Host. This means that radio configuration, synchronizing clocks and managing connection failures has to be added to the standard data logger programming.  Many people think that radio communications are problematic, but we have not found this to be the case.  In fact, properly programmed and configured, radio communications can be extremely robust and reliable.  And the brand of radio you use does matter.  Adding radios is a technological challenge that has to be overcome.  But it's a one-time cost in labor, and once you develop a system you can use it again in other systems you build.

The advantages to using this data collection method is more flexibility in design and layout, less long-term cost, and it's your network in that you are less dependent on a third party provider.  The disadvantages are increased complexity in programming.  You also need at least two communications ports (one for modem and one for radio) on at least the Host data logger.

Hybrid Systems

Hybrid systems use a combination of radio and cellular networks to adapt to local conditions and extend a monitoring network over a large area.  Where Internet connections are not available, or where we have a quantity of stations over a relatively small areal extent, we use a radio network to connect remote stations to a Host.

Hybrid System - Point and Multi-Point

Host stations are generally located where we have higher quality cellular service.  In this manner information can be consolidated for data collection and also shared from one Host station to another throughout the extended monitoring network.  Enhanced data visualization techniques, like web-based HMIs, can also be used at a Host station to provide real time access to data from almost anywhere. This type of hybrid data logging system with data being freely shared between separate monitoring systems starts to resemble a factory-floor SCADA system - but with a lower rate of data throughput.  Measured points are taken at a frequency appropriate for the purpose of recording the data, but the data is shared to enhance operations and improve awareness.  This will also improve the quality of the collected data as more stakeholders take an interest in data integrity.