When the ISM (Industrial, Scientific, Medical) frequency bands were first designated in 1947, the goal was to ensure the reliable operation of various industrial, scientific, and medical devices. Devices in this category, such as microwave ovens, RF welders, or diathermy devices, can be used in the ISM bands without a license, as long as they emit only limited levels of radiation. Because of this license-exemption, the designated bands were later also used for short-range radio frequency communication purposes.
Embedded systems mostly fall under the Short Range Device (SRD) regulatory category in the EU. These devices generally operate in ISM bands, though there is not a complete overlap. Two significantly regulated parameters are ERP (Effective Radiated Power), and the duty cycle, which in this case refers to the proportion of transmission time. This is often very low, meaning the device can spend relatively little time transmitting data, but in the case of a wireless microphone, it can be as high as 100%.
| Permitted Frequency Band | Maximum e.r.p. | Channel access and occupation rules |
|---|---|---|
| 868.00 MHz to 868.60 MHz | 25 mW | ≤ 1% duty cycle or polite spectrum access |
| 868.70 MHz to 869.20 MHz | 25 mW | ≤ 0.1% duty cycle or polite spectrum access |
| 869.40 MHz to 869.65 MHz | 500 mW | ≤ 10% duty cycle or polite spectrum access |
# Excerpt from the ETSI EN 300 220-2 V3.3.1 standard - https://www.etsi.org/deliver/etsi_en/300200_300299/30022002/03.03.01_60/en_30022002v030301p.pdf
Everyday devices such as anti-theft systems, contactless credit card payment, gate openers and other remote controllers, wireless doorbells, and outdoor sensors all operate on permitted bands. Furthermore, well-known technologies such as Wi-Fi, Bluetooth, BLE, LoRaWAN, and others also follow strict standards.
| Frequency range | Typical application |
|---|---|
| 13.553 - 13.567 MHz | RFID, NFC (Pl. PayPass) |
| 433.05 - 434.79 MHz | Távirányítók, Szenzorok |
| 863 - 870 MHz | LoRaWAN (EU) |
| 2.4 - 2.5 GHz | Wi-Fi, BLE, Zigbee |
Wi-Fi technology is based on the IEEE 802.11 family of standards. It primarily operates both in the 2.4 GHz ISM and 5 GHz band, and with Wi-Fi 6E/7, the 6 GHz band can also be used. Wi-Fi communication is half-duplex, meaning that only one party transmits at a time. Data is transmitted in the form of packets, not continuously as in analog radio communication. Early Wi-Fi generations used DSSS modulation, while modern systems operate using OFDM or OFDMA modulation.
| Standard | Wi-Fi generation | Release year | Theoretical max. data rate |
|---|---|---|---|
| 802.11ac | Wi-Fi 5 | 2013 | 6.9 Gbit/s |
| 802.11ax | Wi-Fi 6/6E | 2019 | 9.6 Gbit/s |
| 802.11be | Wi-Fi 7 | 2024 | 46 Gbit/s |
The unit of communication is the station (STA), which refers to any device capable of Wi-Fi communication, such as a smartphone or embedded system. The central element of Wi-Fi networks is the access point (AP), whose role is to coordinate communication among the connected STA devices and to provide the connection between wireless clients and the wired network. The network created by the AP can be identified by its SSID, and in larger networks, multiple APs can work together to form a network. In such cases, clients are able to automatically switch between APs while moving, maintaining a continuous connection.
The device found in every household that is used to create a Wi-Fi signal is commonly referred to as a router, which is misleading, as these devices actually perform many more functions than simple routing. A router’s job is to establish a connection between networks and forward data packets, which it does by connecting the single network it creates to the service provider’s network. However, it also serves as an access point, a DHCP server, and generally acts as a basic firewall.