Fundamentals ISM

ISM Band

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The 2.400-2.4835 GHz Industrial, Scientific, and Medical radio band used by Bluetooth, Wi-Fi, and Zigbee.

    


    
    
    

      Also known as:
      
      2.4 GHz band
      
    

    
  


  




  
  

    
ISM Band


The ISM (Industrial, Scientific, and Medical) band at 2.400--2.4835 GHz is the unlicensed radio spectrum used by Bluetooth Low Energy, Bluetooth Classic, Wi-Fi (802.11b/g/n/ax), Zigbee, Thread, and many other wireless protocols. Because it is license-free worldwide, the 2.4 GHz ISM band has become the default spectrum for short-range consumer and industrial wireless communication.


Channel Allocation in BLE


BLE divides the 2.4 GHz ISM band into 40 channels, each 2 MHz wide, spanning from 2402 MHz (channel 0) to 2480 MHz (channel 39). Three channels -- 37, 38, and 39 -- are designated as advertising channels, intentionally placed at frequencies that minimize overlap with the three most common Wi-Fi channels (1, 6, 11). The remaining 37 channels are used for data communication during active connections.


Interference and Coexistence


The crowded nature of the ISM band means BLE devices routinely share spectrum with Wi-Fi access points, microwave ovens, baby monitors, and other 2.4 GHz transmitters. BLE mitigates interference through adaptive frequency hopping, which dynamically avoids channels experiencing high error rates. The channel map maintained by the central device tracks which channels are clean and excludes degraded ones.


On SoCs that integrate both BLE and Wi-Fi radios (such as the ESP32 and ESP32-C6), coexistence arbitration logic ensures the two radios do not transmit simultaneously. This is critical because a collocated Wi-Fi transmission at +20 dBm can desensitize a BLE receiver operating just a few megahertz away.


Regulatory Constraints


While the ISM band is license-free, regulatory bodies impose limits on transmit power and duty cycle. In most regions, BLE devices are limited to +10 to +20 dBm TX power. The FCC (US), ETSI (EU), and MIC (Japan) each define slightly different maximum EIRP limits and spurious emission requirements. Products must pass certification testing to legally transmit in the ISM band, and exceeding these limits can result in harmful interference to other services sharing the spectrum.

  

  

  
  
  




  
  
Related Terms

  

  

    
    Bluetooth Low Energy
    
    Frequency Hopping
    
  





  

  
  

  
  

  
  
  

    
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Frequently Asked Questions


  

    
    

      
      

        
Our glossary covers 90+ BLE technical terms organized by category. Each term includes a definition, related terms, and links to relevant chips and guides.