The spectrum ranges from 3 Hz to 3 THz, and allocation within the spectrum is governed by the International Telecommunication Union (ITU). A band is considered a portion of the spectrum that can be allocated, licensed, sold, or freely used depending on the frequency. From an ITU perspective, the bands are categorized as follows:

Within the United States, the Federal Communications Commission (FCC) and the National Telecommunications and Information Administration (NTIA) control the frequency spectrum usage rights. The FCC administrates the non-Federal spectrum usage, while the NTIA administers the Federal usage (army, FAA, FBI, and so on).

The overall spectrum managed by the FCC ranges from the KHz spectrum through and including the GHz frequencies. The overall frequency distribution and allocation is shown below. Highlighted are the interesting frequencies that will be discussed in this book. 

The following figure shows a small portion of the allocation of frequencies within the 900 MHz to 2.7 GHz range (common for WPAN signals) and how the frequencies are currently allocated and distributed. In many areas, the usage is multi-purpose and shared:

The FCC also assigns frequencies in licensed and unlicensed spectrums. In the "unlicensed" or "licensed exempt" areas, users can operate without an FCC license but must use certified radio equipment and comply with technical requirements such as power limits and duty cycles. These are detailed in the FCC Part 15 Rules document. Users can operate within these spectrums but are subject to radio interference.

The licensed areas of the spectrum allow for exclusive usage for particular areas/locations. The allocation may be granted on a national basis or in discrete segments site by site. Since 1994, exclusivity and rights to these areas of the spectrum have been granted by auction for particular areas/segments/markets (for example, cellular market areas, economic areas, and so on). Some bands can be a hybrid of the two models, where bands may have been licensed on a site-by-site basis and later bands surrounding those licenses were auctioned for larger geographic or national areas. The FCC also allows for a secondary market and has established policies and procedures through spectrum leasing and transfer of control.  

IoT deployments will typically use the licensed regions for long-range communication, which is covered in the next chapter. The unlicensed spectrum is typically used for Industrial, Scientific and Medical (ISM) devices. For IoT, IEEE 802.11 Wi-Fi, Bluetooth, and IEEE 802.15.4 protocols all reside in the 2.4 GHz unlicensed spectrum.