Data transmission via narrowband is the older of the two methods. Data is modulated onto the power supply network at a fixed carrier frequency of approximately 55 kHz. It is transmitted at a rate of 1,200 baud, which is why this system is often also referred to as a "low data rate system".
Sealand were already carrying out their first trials with PCT at the end of the 1970s. At the beginning of the 1980s, ThermoKing collaborated with Sealand to develop the first marketable system, known under the name of ThermoNet. Sealand and Matson were the first to use this system on a large scale, principally in the relatively closed refrigerated container trade routes in the Pacific.
Figure 23: ISO filter specification for wideband and narrowband

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