 Frequently Asked Questions
Q: What is Smart SpectrumTM?
A: Data-Linc’s Smart Spectrum technology is an advanced implementation of frequency hopping that provides unparalleled reliability and security in industrial applications prone to high noise. Not all wireless spread spectrum communication technologies are created equal.(For more information on the security of Data-Linc's wireless data tranfer, read "Data-Linc Group SRM Series Wireless Security: Understanding Wireless Modem Data Transmission.")
Q: How does Smart Spectrum achieve "unparalleled reliability?"
A: Ideal for PLC, SCADA and DCS communication where system integrity is critical, Smart Spectrum uses a unique combination of radio frequency (RF) attributes to achieve this high level of performance:
- Algorithmic frequency hopping over 112 channels
- High RF data rate of 144 kbps
- Highly sensitive RF receiver
- 32-bit CRC error correction
Q: We've been using direct sequence spread spectrum for data transmission. How does algorithmic frequency hopping compare to direct sequece, and how does it work?
A: Spread spectrum radio modems typically use one of two spreading methods: direct sequence or frequency hopping. Direct sequence continuously spreads data across a wide portion of the frequency band. If a frequency is not available – because other equipment or too much noise occupies the band – then that data is lost. In a high-noise environment, the reliability of the data is dependent on the signal-to-noise ratio. The percentage of frequencies unavailable represents the percentage of data that will be lost.
Frequency hopping, on the other hand, takes incoming data and breaks it down into smaller individual packets, which are then sent on separate frequencies. Once the packets have been transmitted, the data is recompiled in its original format. If a packet cannot be successfully sent on a given frequency, it is re-sent on another. With 112 channels over which to send the data packets, Smart Spectrum ensures that your critical data gets through. See schematic of a typical frequency hopping pattern. For more information read "License-Free Spread Spectrum and FSK Wire Industrial Data Communications," an article about FHSS technology.
Q: Our PLCs operate well below data rates of 144 Kbps. Why should we get rates greater than what we need?
A: Smart Spectrum radio modems communicate with each other at 144 kbps – well above the data rate of most PLC and other industrial data acquisition and control systems. This is important because RF speed determines overall system performance. As interference increases, effective throughput decreases. If a radio modem’s RF data rate is only 22 kbps, 75% interference leaves you with an effective throughput of only 4.8 kbps. If, on the other hand, you’re using a Data-Linc Smart Spectrum radio modem, the 144 kbps RF data rate with 75% interference still leaves you with an effective throughput of 28.8 kbps – providing a reserve of performance to ensure communication integrity.
Q: What is the advantage of the highly sensitive RF receiver?
A: Smart Spectrum radio modems that have their receive sensitivity set very high allow the processor to sample several frequencies and compare the data, resulting in its ability to distinguish valid data from noise.
Q: What if an error occurs during transmission?
A: Smart Spectrum radio modems have 32-bit CRC error correction. After transmitting data over the RF link, a Smart Spectrum radio modem performs a cyclical redundancy check (CRC) on each packet. In this way, Smart Spectrum not only ensures that your data gets through, but also that your data gets through accurately.
View Data-Linc modems utilizing Spread Spectrum technology. "Introduction to Smart Spectrum" is available to download (pdf file, size 73K). The file requires Adobe Acrobat Reader to view. If you need a free copy, visit Adobe.)
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