According to Silicon Labs AN1137, Bluetooth Mesh employs two kinds of retransmissions:
- at the network level
- at each relay
Furthermore the document states:
The minimum delay between repetitions, called the retransmission interval = (Relay Retransmit Interval Steps + 1) * 10ms + 0-10ms random delay, is typically 15 ms per hop.
In other words it seems that the minimum resend interval is 10ms.
Bluetooth Mesh is based on undirected Bluetooth LE advertisements (Mesh Profile specification Section 3.3.1).
Now, according to the Bluetooth Specification 5.2, Section 184.108.40.206.1, the advertising interval (the time between the start of two consecutive advertising events) for undirected advertisements must be at least 20ms:
T_advEvent = advInterval + advDelay
The advertising interval (advInterval) shall be an integer multiple of 0.625 ms in the range of 20 ms to 10,485.759375 s. The advDelay is a pseudo-random value with a range of 0 ms to 10 ms generated by the Link Layer for each advertising event.
Does that mean that Bluetooth Mesh violates the usual rules of Bluetooth? And regarding the 10ms lower bound (wherever it may be specified), does that apply to network-level retransmissions as well or only to per-relay retransmissions?