September 2008

Wireless integration adds latency

By Louis De Silvio

At the beginning, we respected legacy SCADA systems from a marketing and a practical standpoint.

Since wireless allows cost effective capabilities to add new IO points, we design our systems to both accommodate stand-alone scenarios and legacy system needs.

A further and new technology that will arrive soon is a board, which will be completely remote and not need a base station at all (using cellular).

When you add wireless points, you get a potentially serious level of granularity that was not previously available due to cost of running new wires and the rest. You also get completely new applications as another possibility. Either way, you can run the new points outside the legacy SCADA, or you can make a hybrid.

That is where the design had to be inclusive and avoid the "proprietary" stigma. Therefore, in the case of hybrid, we make the wireless "base station" radio connect to a PC that Ethernets to the SCADA, with appropriate tagging in the SCADA. Alternatively, it could connect directly to the SCADA system itself, again supporting tags. We output XML format currently.

In the case of a stand alone, we can do all the control and monitoring from the base station PC, which of course, can connect to the Internet, which in conjunction with the MESH radios, allows for the ability to remotely monitor and change PLC programming on the fly.

Therefore, we have two capabilities in one system. When you couple that with the ability to "encapsulate" any existing protocol's packets (HART, Modbus, the others) and fly them over the air with MESH radios either to an IO mapped destination or to the SCADA system, or the MESH base radio's PC, you sort of have it all covered - right down to the current ISA work being done to get a "standard" that encompasses all the legacy protocols.

Source of increased latency

When integrating wireless technology into the existing design an important consideration is to plan on the associated "latency" wire causes. Latency is the time lag between the instant data is known and the time that the system actually acts upon that information.

For example, if a pressure sensor connects to a PLC, and the pump, which created the pressure in the vessel, connects to the same PLC, latency would be the time the PLC received a pressure reading impulse, interpreted it, decided on an action, sent a command signal to the pump, and the pump received it. The entire cycle of time elapsing between receiving the initial input from the pressure sensor, and sending an output to the pump, is the latency time. There are other factors contributing latency, but in this example, it would be in either nanoseconds or milliseconds at worst.

If the pressure sensor linked to a SCADA system and through it to the pump, the time the signal traveled hundreds or thousands of feet over a wire, saw processing through the SCADA system, and traveled hundreds or thousands of feet to the target, would define that latency.

With improvements in wireless communications and the adaptation to process control and monitoring, there is an evolution and a migration toward acceptance of wireless devices where they are applicable.

While there have been strides in the security of wireless packets of information as they fly through the air that security is also a source of increased latency. That is to say, wireless adds latency, and encryption adds even more as the packets must go through encryption prior to flight and decryption upon arrival.

In addition, resolving the phenomenon of "lost packets," a virtually non-existent issue in wired applications, must also happen. These are not deal breakers though, just considerations with which one must work.

Wireless is considerably more cost effective than wiring devices.

ABOUT THE AUTHOR

Louis De Silvio (lou@industrialt.com) is an ISA member and president of Industrial Telemetry in Collinsville, Okla. His paper from the 53rd International Instrumentation Symposium is SCADAless SCADA using wireless MESHradio telemetry. Read it at http://www.isa.org/pdfs/scadaless-scada-using-wireless-mesh-telemetry.