Where do I start?
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First, a few things to know: “A UUID is a universally unique identifier that is guaranteed to be unique across all space and all time” (Bluetooth 4.2 spec, Vol 3, Part B, section 2.5.1 UUID) A UUID is a 128-bit value. There are reserved UUIDs by the Bluetooth SIG that are generally represented by their 16-bit aliases. These aliases are used for convenience and represent a 128-bit value computed as follows: 128-bit value = 16-bit-value * 2^96 + BluetoothBaseUUID where the BluetoothBaseUUID is 00000000-0000-1000-8000-00805F9B34FB Examples include: 0x180F -> Battery Service UUID (128-bit UUID: 0000180F-0000-1000-8000-00805F9B34FB) 0x2A00 -> Device Name Characteristic UUID (128-bit UUID: 00002A00-0000-1000-8000-00805F9B34FB)…
The answer: 512 bytes. But to understand where that came from, let’s first look at what a characteristic really is. Attributes are the generic term describing the smallest element in the Generic Attribute Profile (GATT). So each of Services, Characteristics, and Descriptors is actually an attribute, and according to the Bluetooth 4.2 spec (Volume 3, Part F, section 3.2.9): “The maximum length of an attribute value shall be 512 octets” which leads us to the answer: 512 bytes.
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