What “ISM” means
ISM stands for industrial, scientific and medical. The ITU Radio Regulations, the rules adopted and revised by World Radiocommunication Conferences, define ISM applications in No. 1.15 as the operation of equipment designed to generate and use radio frequency energy locally for industrial, scientific, medical, domestic or similar purposes, excluding applications in telecommunications.
In other words, ISM equipment uses radio energy to do work, not to send information. ITU-R Report SM.2180 lists examples, among them induction heating, magnetic resonance imaging, rubber vulcanisation, and domestic and commercial microwave ovens at 2450 MHz. Such machines can radiate radio energy, so the Regulations designate bands for them in which other services are told to expect it.
Which bands are designated
Two footnotes in the Radio Regulations name the bands. The list below is as quoted in Report SM.2180, published in 2010; check the current edition of the Regulations for any later change.
| Band | Centre frequency | Footnote |
|---|---|---|
| 6765-6795 kHz | 6780 kHz | 5.138 |
| 13553-13567 kHz | 13560 kHz | 5.150 |
| 26957-27283 kHz | 27120 kHz | 5.150 |
| 40.66-40.70 MHz | 40.68 MHz | 5.150 |
| 433.05-434.79 MHz | 433.92 MHz | 5.138, in Region 1 except the countries in No. 5.280 |
| 902-928 MHz | 915 MHz | 5.150, in Region 2 only |
| 2400-2500 MHz | 2450 MHz | 5.150 |
| 5725-5875 MHz | 5800 MHz | 5.150 |
| 24-24.25 GHz | 24.125 GHz | 5.150 |
| 61-61.5 GHz | 61.25 GHz | 5.138 |
| 122-123 GHz | 122.5 GHz | 5.138 |
| 244-246 GHz | 245 GHz | 5.138 |
The two footnotes work differently. Under 5.138, ISM use is subject to special authorisation by the administration concerned, in agreement with other administrations whose services might be affected. Under 5.150, radiocommunication services operating in those bands must accept harmful interference that ISM applications may cause. In both cases, No. 15.13 asks administrations to keep ISM radiation minimal and, outside the designated bands, below the level that causes harmful interference.
National rules then put this into practice. In the US, 47 CFR 18.301 lists ISM frequencies with tolerances, for example 915 MHz ±13.0 MHz, 2450 MHz ±50.0 MHz and 5800 MHz ±75.0 MHz.
Why communication radios use them
The ISM designation is about equipment such as ovens and heaters, but the same bands also carry familiar communication radios. The Bluetooth SIG describes both Bluetooth Classic and Bluetooth LE as operating in the 2.4 GHz unlicensed ISM band, using 2.402 to 2.480 GHz.
What permits a radio to transmit there is not the ISM footnote but each country’s licence-exempt rules. In the US, 47 CFR 15.247 sets conditions for frequency hopping and digitally modulated transmitters in 902-928 MHz, 2400-2483.5 MHz and 5725-5850 MHz, such as maximum output power and minimum numbers of hopping channels. In Europe, CEPT’s ERC Recommendation 70-03 sets out the bands and conditions for short range devices, and its appendices record each country’s implementation and restrictions.
Two consequences follow from sharing. First, communication devices in these bands are guests alongside ISM equipment and each other, and must cope with interference. Second, the rules that keep sharing workable, such as power caps, hopping requirements and duty cycle limits, shape what a device can do.
Common points of confusion
- “ISM band” and “licence-exempt band” are not the same list. The 2.4 GHz band is both. The 863 to 870 MHz range that LoRa uses in Europe does not appear in either footnote; it is a short range device band under ERC Recommendation 70-03, with its own sub-bands and duty cycles that The Things Network lists for its European frequency plan.
- Edges differ between documents. The Radio Regulations designate 2400-2500 MHz, while the FCC’s Part 15 rules cover 2400-2483.5 MHz, and the Bluetooth SIG lists 2.402 to 2.480 GHz as the part Bluetooth uses. Quote the document that governs your device.
- Some bands are regional. 902-928 MHz is an ISM band only in ITU Region 2, and 433.05-434.79 MHz is designated in Region 1 under 5.138, which requires special authorisation. A device sold in several markets may need a different band or configuration in each.
What this means for builders
Choosing a radio largely means choosing a band, and with it a set of rules. The 2.4 GHz band appears in 5.150 with no regional limit, which suits products sold worldwide, and it is shared with Bluetooth, other radios and ISM equipment such as microwave ovens. Sub-GHz choices differ by region, such as 902-928 MHz in the US and 863 to 870 MHz in Europe, where duty cycle limits apply. BLE vs Thread vs Zigbee vs LoRa compares the radios that live in these bands, and the conditions in the governing national rules, not the ISM footnote, decide what a product may transmit.