What do ISM and SRD band mean, and what is the difference between them?
ISM (Industrial, Scientific, Medical) bands are in fact bands set aside for industrial, scientific and medical equipment to radiate RF energy; 2400–2483.5 MHz is one of them. SRD (Short Range Device) is a usage class regulated for short range communication devices, defined in Europe by ERC/REC 70-03; 433.05–434.79 MHz and 863–870 MHz fall under it.
The two terms are used interchangeably in everyday speech, but their regulatory meanings differ.
ISM is a band allocation. In these bands, priority belongs to devices that generate RF energy, such as microwave ovens, industrial heaters and medical diathermy. Communication devices enter here on a secondary basis, on condition of tolerating interference. That is why Wi-Fi and Bluetooth share the 2.4 GHz band with the microwave oven.
SRD is a device class. In Europe, ERC/REC 70-03 lists one by one which sub-band, which ERP limit and which duty cycle short range devices may operate with. For 433 MHz, typically 10 mW ERP and 10% duty cycle are given, and for most sub-bands of 863–870 MHz, 25 mW ERP and 1% duty cycle.
The practical result is this: in both cases the band is licence-free, but it is unprotected. If another device operating in the same band disturbs you, you have no right of complaint; the design has to be resilient to that. Frequency hopping, CRC and retry mechanisms are therefore not a luxury but a necessity.
Full topic: 433 MHz or 2.4 GHz?