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SCADA: Major Protocols & Modems

Frame Relay

In this packet protocol, the data packets do not directly correspond to the poll or response packets size of SCADA. The network thus breaks up the SCADA packet into relay packets of various frames. Delays would be caused in between frame relay data packets. When there are no actual errors, these delays or time gaps within SCADA packet would make SCADA polling system presume transmission error.


Regardless of incoming data protocols, packets of Ethernet, a packet oriented protocol is generated. Ethernet devices follow their own protocol rules depending on their needs. They do not follow the rules of the devices connected to them.

IP Networks

They possess same packet characteristics as Frame Relay and Ethernet. The incoming SCADA data packets bear no relations with the IP packets. Here again the SCADA system may presume error because of time gaps.

1200 BPS Modems

For more information, transferring data at a higher rate is very necessary. Most SCADA systems used this modem earlier. The system could find out through interface with remote devices whether the remote unit was operating allowing small amount of data collection and delivery. Now there is demand for more data volumes with faster processors and more memory.

Fast Poll Modems

These are 9600 bps faster modems that can meet this higher demand for data. SCADA systems mostly use this high-speed device. Voice grade frequencies have a bandwidth of around 3000 hertz. The phone lines, private wires and microwaves can operate at this frequency which is also adequate for fast poll 9600 bps modems. More data at higher data rates are delivered by fast poll modems. However additional time is required for these modems for proper acquisition of data signal. This “training time” is referred as the modem’s request to send/ clear to send (RTS/CTS) delay. The manner of modulating analog signal in case of this modem is different from 1200 bps modems. A complex technique called QAM is used by this modem for sending and decoding several bits of data tat causes some milliseconds delay.

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