We have a product originally based on the simple peripheral (OAD-enabled) application using SDK 5.10.00.02. The product supports one connection, and has to offload up to around 3.7 megabytes of data stored in external flash to an android client. Data is transferred using notifications, and the firmware is in a tight loop reading 16-byte records from external flash and calling GATT_Notification(), immediately retrying the GATT_Notification() call if it does not return success. (have not yet attempted larger packet sizes)
We have found that android does not respect the minimum connection interval specified in the device firmware, and connection interval is set to 15ms. MAX_NUM_PDU is currently set to 15. This is working pretty well, but depending on the location of the test, we are seeing a slowing of download performance over time. I have a few questions about the data observed in the Wireshark captures.
1) It's my understanding that, according to the BLE spec, within a connection interval, packets are to be separated by 150us. The logs show a very consistent 230us gap between the central's empty PDU and the peripheral's response packet, and then a 430us gap between the peripheral's data packet and the next empty PDU from the central. Is this to be expected?
2) The peripheral does not seem to respect the MAX_NUM_PDU value. At the beginning of the transfer, it is common for 20+ packets to be sent in one connection interval. Is this normal BLE stack behavior?
When testing at our development lab, the download rate starts out at around 800 packets/second, then by the time it reaches the end of the download, the rate can drop under 400 packets/second. If the same test is run at home, the download rate is mostly consistent throughout the download.
3) How much, and what kind of difference can it make if there are multiple wifi networks or other RF sources in the area when running BLE downloads? When running in our development lab or the customer's lab, the download rate slows over time, but always completes. When running at the board manufacturing facility, the download never completes.
The Wireshark logs do not show any errors, but I might not have it configured to show lower-level errors. (I am new to Wireshark.)
4) When the download slows over time, the logs do not show errors, but fewer packets are sent per connection interval as the download progresses. Toward the end of the download, only 5 or 6 packets might be included in a given interval. In the last packet in the connection interval, the More Data field within the BLE Link Layer Data Header is set to false, indicating there is no more data to send, but the download might be only 80% complete. Since the firmware is running the same loop reading from flash and calling GATT_Notification, I am not clear on why the application would only be able to get 5 or 6 records out during the connection interval when it was sending 20+ early in the download. I have had issues in the past where the download would slow down due to a memory leak and heap exhaustion, but that issues was addressed, and this does not seem to be the case here since the download can be repeated immediately and the performance is the same (fast start, slow finish). If there were a heap issue, I would expect subsequent downloads to start out slow and eventually fail.
Here is a snippet of the Wireshark log showing an early connection interval of an example download:
847 19.340044 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 848 19.340275 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown) 849 19.340705 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 850 19.340934 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown) 851 19.341365 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 852 19.341593 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown) 853 19.342024 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 854 19.342254 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown) 855 19.342684 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 856 19.342914 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown) 857 19.343344 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 858 19.343573 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown) 859 19.344004 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 860 19.344233 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown) 861 19.344663 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 862 19.344893 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown) 863 19.345323 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 864 19.345553 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown) 865 19.345983 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 866 19.346212 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown) 867 19.346642 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 868 19.346872 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown) 869 19.347302 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 870 19.347532 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown) 871 19.347963 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 872 19.348193 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown) 873 19.348623 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 874 19.348852 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown) 875 19.349282 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 876 19.349512 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown) 877 19.349942 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 878 19.350172 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown) 879 19.350602 Master_0xcbae658a Slave_0xcbae658a LE LL 26 Empty PDU 880 19.350832 Slave_0xcbae658a Master_0xcbae658a ATT 51 Rcvd Handle Value Notification, Handle: 0x002e (Unknown: Unknown)
Any insights would be appreciated.
