Packet format for Uncoded LE data packets

Preamble	Access Address	PDU (2–257 bytes)						CRC
1 byte (1M PHY) 2 bytes (2M PHY)	4 bytes	LL Header	Payload (0–251 bytes)				MIC (Optional)	3 bytes
		2 bytes	L2CAP Header	ATT Data (0–247 bytes)
			4 bytes	ATT Header		ATT Payload	4 bytes
				Op Code	Attribute Handle	Up to 244 bytes
				1 byte	2 bytes

source

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