Handle Value Notification variable conn.interval

[samcornell]

I am using a nRF51822 to send data to an iOS device. Because latency is critical to my application, i 've set up a test program that sends a packet every second , using notifications.

Sniffing the BLE traffic with nRF Sniffer i am seeing that Slave and Master (iOS device) exchange an Empty Data PDU every conn. interval which in my case is 14 ms roughly ( i assume the real value is 13.75 because must be a multiple of 1.25 ms).

But prior to the Notification made by the Slave to the Master the conn. interval. seems to jump to 17 ms inexplicably and sometimes more even 19 ms.

I can't explain this behavior , based on my knowledge on BLE the conn. interval should remain constant. Maybe something strange is happening on iOS (Master) side ?

I've attached a screenshot.

wiresh.png

EDIT i've included the wireshark file: 1secint_nRF_ipad_poll.pcapng

[Hung Bui]

@samcornell: Could you please upload the full sniffer trace ? Could you check the delta time (from start to start) in the Nordic BLE sniffer meta ? EDIT: Start to start is not reliable. My mistake. The "end to start" is the correct one. Calculation needed if we want to find connection interval because "end to start" is dependent on the packet length. See the answer.

[samcornell]

Ok I've checked the delta time it is constant: start to start is 14.92 ms, end to start is 14.69 ms. Does this means that the conn. interval is not changing ? But then why in the image attached i have different values if i check the Time Column in Wireshark ?

[Hung Bui]

@Samcornell: The timestamps are the time the packet arrived on PC (on UART), so it may not be accurate. I would suggest to use the delta time start to start instead. The "end to start" also has an issue, so it's not usable too.

EDIT: I was wrong. "End to Start" is the correct one. It's varied because it's depends on the length of the packet.

So, if the "End to start" of 2 packets of a connection interval is "a" and "b" and the TX time of the packets are "c" and "d", the connection interval will be = a + b +c +d.

[samcornell]

Thank you very much i will use the start to start delta value then in future