BLE has 40 channels (wider spacing 2MHz than classic BT) Channels 37, 38, 39 are designated to be advertisement channels. Those channels are located at begining, middle at end of radio band. Location was chosen to do not interact with main 3 WiFi channels (1,6,11)

Each advertisement is transmitted on all 3 channels in 10ms intervals.

Some of the most commonly used Advertising Data Types are:

• 0x06 Incomplete List of 128-bit Services: UUIDs of the services provided by the peripheral can be advertised in 128-bit format.
• 0x02 Incomplete List of 16-bit Service Class UUID
• 0x08 Shortened Local Name and 0x09 Complete Local Name

UUIDs are 16 bytes number (128bit), like 75BEB663-74FC-4871-9737-AD184157450E. To save data, only first 32 bits are used, rest is defined by Bluetooth SIG: XXXXXXXX-0000-1000-8000-00805F9B34FB.

For 16bits UUIDs like 0x180D, rest is filled with zero: 0000180D-0000-1000-8000-00805F9B34FB