What is the maximum size of data that can be encapsulated in a characteristic?

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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