WLAN devices need to conform to Regulatory domain or country requirements for Transmit power/spectral mask and channel allocation. A regulatory domain might be a country or a set of countries which might follow similar laws for channel utilization.

For e.g.

• The EU regulatory domain is for Europe (a set of countries)
• The US regulatory domain is for US of A (a particular country)

Each Country defines its own set of regulatory restrictions on channel, transmit power, indoor/outdoor usage as per the spectrum laws and needs of the country.

The 802.11 standard for WLAN broadcasts the regulatory requirements in the country information Element. The country information element is provided below

Fig Courtesy: 802.11 Standard

As Per the 802.11 standard

• The element ID for this element is set to a value 7
• The length of the element is variable
  • It might contain more than one triplet comprising the First Channel Number,
    Number of Channels, and Maximum Transmit Power Level fields and referred to as subband triplets
  • where dot11OperatingClassesRequired is true and the First Channel Number/Operating Extension Identifier octet has a positive integer value of 201 or greater, then that triplet comprises the Operating Extension Identifier, Operating Class, and Coverage Class fields
• The Country String field of the element is 3 octets in length. The AP and mesh STA set this field to the value contained in the dot11CountryString attribute before transmission in a Beacon or Probe Response frame. (e.g. USI – US Indoor)

The description of the sub-band triplet is provided in the following articles

Country Element Subband triplet with operating Extension identifier < 201