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What is the maximum length of a single packet data and the maximum length supported without packet division?

Last Updated on：2024-02-18 06:46:20

The Mesh upper transport layer supports a maximum of 380 bytes of effective data. That is, for the access layer packet sending, the opcode + data supports up to 380 bytes.
Longer data will be segmented into multiple smaller packets at the lower transport layer, with support for up to 32 packets. It is not recommended to use longer data, as segmentation at the lower level will cause significant delays.
For the rules of segmentation and the calculation of data length, please refer to the relevant documentation. Here, only access message data is introduced, which is the common communication data. Additionally, control messages are generally configuration data. Access messages are divided into unseq access messages and seq access messages.

Unseq access message, which is the non-segmented access data, has a maximum length of 15 bytes. Subtracting the MIC (4 bytes), the maximum data length is 11 bytes. This includes the opcode, which means when using Vendor Model to send data, the opcode length is 3 bytes, so the maximum data length is 8 bytes.
Seq access message, which is the segmented access data, has a maximum single packet length of 12 bytes, so the total length is 12n, where n is the number of segments. The 12n includes the MIC (4 bytes), so the actual user data length is 12n-4, minus the opcode length, which is 12n - 4 - opcode_len. If using Vendor Model, the number of segments is n = (data_len + 7) ÷ 12, and n is rounded to the nearest whole number.