How to reengineer SCADA systems for faster data transmission

Champaign, Ill. The evolution of data communication networks is creating a new environment for supervisory control and data acquisition (SCADA) systems based on frame relay, Ethernet, Internet protocols (IP), and increased data rates, according to a recent report by Data Comm for Business (DCB) Inc.

03/01/2000


Champaign, Ill. The evolution of data communication networks is creating a new environment for supervisory control and data acquisition (SCADA) systems based on frame relay, Ethernet, Internet protocols (IP), and increased data rates, according to a recent report by Data Comm for Business (DCB) Inc.

As more computer power is inserted into SCADA systems, there is increased demand for more information to be transferred, which requires faster data rates. "With these changing requirements and protocols, we are at a point where system designers should be considering challenges presented by modern networks," states Russell Straayer, DCB's president, in the report.

The report says new and old SCADA systems can usually be migrated to the latest frame relay, Ethernet, IP networks, or 9,600 bps fast-poll modems. However, successful migration requires systems designers to be aware of potential problems and incompatibilities, which can occur between legacy SCADA protocols and newer technologies. These include transmission delays, time gaps in data, control lead requirements, and what equipment is available to accommodate merging technologies.

To avoid transmission problems, DCB's report advises designers to consider several network idiosyncrasies and possible solutions. These include:

  • Frame relay , a high-speed packet switching protocol for WANs, with data packets that may not always coincide with the size of the SCADA poll/response packets. This may cause SCADA packets to be broken into several frame relay packets by the network, which includes delays between each. These time gaps can result in the SCADA polling computer seeing transmission errors when none exist. This problem can be overcome by using a hardware-based solution, such as a frame relay assembler/disassembler (FRAD) for asynchronous networks, which can accommodate byte-oriented async polling protocols. For example, DCB's Frame Relay Broadcast Polling FRAD "encapsulates" async polling protocols into frame relay format for private frame relay networks;

  • Ethernet , which also serves as a packet transmission protocol, much like frame relay. Its packets are generated without regard to incoming data protocols. Ethernet also possesses an electrical interface that differs from the RS-232 asyncronous interface common in many SCADA systems. However, by using asynchronous RS-232 entry into Ethernet via a serial server, such as DCB's Etherpath, delays can be avoided because the device uses IP protocol to encapsulate SCADA and other asynchronous data. This can eliminate delays within SCADA packets;

  • IP networks , which have the same packet characteristics as frame relay and Ethernet. There is no relationship between IP packets and incoming SCADA poll/response packets, which means timing gaps will again occur causing SCADA systems to see nonexistent error conditions. Several products can handle this problem and provide point-to-point or multiport communications over private wires via RS-232 and V.35 interface, or single-channel voice-and-data over microwave or other links. Channels can be synchronous or asynchronous up to 128 kbps; and

  • Higher-speed SCADA . Though most SCADA systems used 1,200-bps modems in the past, today's SCADA equipment is supported by more powerful computers with faster data requirements and standard lines able to pass 9,600 bps using fast poll modems. However, at higher data rates, host modems need extra time, generally known as request to send/clear to send (RTS/CTS) delays, to acquire and handle data signals. This means some polling protocols must be modified. DCB's LL9.6 Fast Poll Modem has a short RTS/CTS delay and includes multiple diagnosis features. SCADA systems can also operate over dataphone digital service (DDS) networks at speeds from 1,200-57,600 bps over multidrop networks with RTS/CTS delays of less than 1 ms, which avoids error conditions.

For more information, visit www.dcbnet.com or www.controleng.com/freeinfo .





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