We are developing a device which communicates with our iOS/watchOS apps via Bluetooth LE and has to stream a lot of sensor data for an extended time period (hours). Everything works fine under iOS 15.x, but we've found out that iOS 16 betas (and the RC) changed something in the negotiation process: previously we've used 15 ms Connection Interval, but iOS 16 (and watchOS 8) most of the time negotiates 24 ms, which is too wide for our bandwidth. The long connection interval causes packet loss (9-33%), and after 3 failed retries (3x30 sec) our hardware drops the connection.
I've checked all forums and documentation available but didn't found any indication that something changed. Is there any new parameters we can look into to fix this issue?
EDIT: Changed 30 ms to 24
You've probably set both Interval Min and Interval Max at 15ms. Apple explicitly says that this may caused devices to negotiate to 30ms (see section 41.6 Connection Parameters):
If an accessory requests both an Interval Min and Interval Max of 15 ms, some devices will scale the interval to 30 ms to balance power and performance constraints.
Basically, asking for "as fast as possible, no margin" translates into "yeah, everyone asks for that, how about moderately fast?"
However, you may ask for a faster CI if you include HID:
If Bluetooth Low Energy HID is one of the connected services of an accessory, a connection interval down to 11.25 ms may be accepted by the device.
Though I believe what iOS 16 is doing exactly matches Apple's spec (they always said that 15ms could be negotiated to 30ms), you should still open a Feedback+DTS to discuss the impact on your product and use case. Sometimes these things are by accident, and sometimes they're experiments that get rolled back.