Making the Right Choices for Your Wireless SCADA System
When considering a wireless data system design, everyone has a checklist of elements to consider. Some are obviously important and others tend to get overlooked. The following elements tend to fall into the latter category but have a far from insignificant impact on the long-term success and performance of your system.
Unlicensed vs. Licensed Radio, FCC Refarming
Today, the savvy wireless data customer is presented with a wide variety of communications options. In addition to licensed radio, there is now unlicensed radio with the obvious appeal of being license free. The downside is that since it utilizes uncoordinated spectrum, unlicensed radio has become somewhat unreliable as it becomes more crowded.
Unlicensed wireless SCADA networks find themselves sharing spectrum with an increasing number of industrial and consumer devices such as: cordless telephones, baby monitors, wireless LAN devices, and amateur radio operators. Also, if deployed with very high gain antennas, users may have a legal obligation to reduce the RF output power of the unlicensed radio. Additionally, radio wave propagation at higher unlicensed frequencies is relatively unfavorable as compared to the lower frequency licensed bands.
In the past, licensed frequencies were crowded and difficult to obtain. Sometimes it could take many months to obtain on-the-air operational authority. However, since 1997, FCC refarming has made it possible to obtain new communications channels and has greatly relieved communications congestion for wireless data users. Additionally, wireless data users have discovered that by utilizing professional licensing services, they can receive operational authority in a month or less.
Overall, refarming is a multi-step process that is creating additional channels where only one existed previously. It is affecting both radio manufacturers and radio users. Existing users of two-way wireless devices will eventually need to upgrade their equipment to modern refarmed equipment. This transition is necessary for two reasons: 1) Future FCC regulation will make it unattractive to continue holding a full channel of spectrum, and 2) The full channel user may receive harmful adjacent communication channel interference from newly established half channel users.
Radio Modem Hardware and PLC Protocols
Customers are also faced with a wide range of radio modem products today. In the past, it was customary to use outboard Bell 202 type 1200 bps modems and interface them to two-way voice type radios. This requires the tedious adjustment, and periodic readjustment, of audio levels between the separate modem and radio.
The Bell 202 solution ignores technology advances that have been made in the past five years. Today it is less expensive, over the life of your system, to invest in high speed integrated radio modem products which offer the advantages of easier interfacing and swapping out, higher data and polling rates, and more sensitive modem and radio technology, as well as features like wireless network diagnostics.
Additionally, many users must choose between packetized and transparent radio modem hardware. It is useful to note that PLC devices utilize communication protocols or languages that encapsulate the data stream in an envelope called a packet. This envelope surrounds the data with a message start and end marker, an origination and destination address, and a CRC or checksum. Since the PLC is already packetizing your data, it is more efficient and less costly to deploy transparent radio modem hardware, which does not add a layer of error checking and addressing.
MODBUSª (trademark of Schneider Automation, Inc.) is a popular protocol for wireless data communications. Numerous PLC manufacturers have their own unique implementation of this protocol. There are other protocols that operate similarly; some are proprietary. Generally, master slave protocols that are framed employ message addressing and error checking, and are designed for master-slave polling work well in the wireless environment. Truly transparent radio modem hardware requires RTS/CTS hardware handshaking for data flow control. Make sure that your PLC hardware supports this requirement if you utilize transparent radio modem hardware.
Wireless Network Diagnostics
Investing your hardware dollars in integrated radio modem devices also provides new features that are becoming indispensable. Wireless network diagnostics is one such feature, which can reduce communication failures, minimize the potential risk of downtime due to equipment or system malfunction, and facilitate a speedy recovery from outages. This results both in a more favorable risk management scenario and a high return on investment for your automation dollars.
In practice, diagnostics at the wireless communication level makes it possible to verify connectivity to a remote site even if your PLC or instrumentation has failed. Some diagnostic methods operate concurrently with your regular polling cycle to warn of impending communication failures. Your ability to maintain or quickly return your system to service is enhanced by the performance statistics that you can remotely obtain. Diagnostic tools can also be utilized during system deployment, and can speed along the installation process.
Redundancy and Points of Failure
SCADA system designers strive to eliminate as many single points of failure as possible. Redundancy is sometimes utilized at the control point to minimize the impact of system component failure. Redundancy also increases the system design and deployment cost. If carried to the extreme, it can make operation of the system more complicated and laborious.
In a recent study of automated SCADA systems (done by Alan W. Manning), it was found that 50 percent of automated systems are run on manual mode. Among the reasons cited was "low user confidence in the technology." If system complexity and operator workload is a concern for you, having spare components can be considered a simple and valid safeguard against failures and outages.
All system design involves trade-offs. It is essential to understand the benefits and drawbacks of the individual elements of your system design. Only then will you be able to make the right choice for your application and needs.