the speed can not reach 2Mbit/s

[Carey]

I want use two 52DK for BLE transfer demo, one 52DK download nRF5_SDK_15.0.0_a53641a\examples\ble_peripheral\ble_app_uart",  

other 52DK download "nRF5_SDK_15.0.0_a53641a\examples\ble_central\ble_app_uart_c",

and i changed the connection interval to 7.5ms, it seems the speed for two 52832 just can reach to (251B*1000/7.5)=33.5kbyte/s;  is it right?  is it possible to transfer more fast? i heard the speed of BLE5.0 can reach to 2Mbit/s, How to achieve it? 

[Ulrich Myhre]

2Mbit is the symbol rate, and includes non-payload data like the preamble, access address, CRC and various protocol-level headers. The theoretical maximum is a bit tricky to calculate, since it depends on a lot of factors. For instance, if you can live with a slower connection (higher connection interval), you gain some speed for huge transfers. With a slightly longer connection interval, longer event length, higher DLE and ATT MTU size, you avoid much of the overhead, since you spend more time sending data in each event.

Your brief calculation also doesn't take into account that you are able to send multiple packets in each connection event. The number of packets depends on the peer, as some will have a low buffer size and start to NACK packets after a few have gotten through. If the peer is sending you data too, instead of just sending empty ACKs, this will "steal" from the event time, leading to less packets sent.

I ran a quick example locally now with PHY=2Mbit, ATT_MTU=247, DLE Size=251, connection interval = event length* = 0x140 (401.25ms) and got about 1.4Mbit/s of user-controlled (ATT) data. That is excluding the L2CAP and ATT header. This was by sending write commands in one direction, with a peer that absorbs all the data without waiting. A connection interval that long sounds like it should be quite slow, but for long transfers it will actually be beneficial, given that the other parameters allow you to spend the entire event sending data. The result is that the event ends just as the new event is starting, reaching close to 100% utilization of the radio.

* Instead of setting the event length this high, you can enable event extension instead with the `BLE_COMMON_OPT_CONN_EVT_EXT`

[Carey]

ok, now i try to send more packets in each connection event, when i use moble phone for receive, it seems the maximum can send three packets per connection( packet 251byte, interval 7.5ms, ), but if i use 52832 "ble_central\ble_app_uart_c" example to recive, it seems just can receive one packets per connection, Why?  should i modify some parameter in the code? How to change it? 

[Ulrich Myhre]

I forgot to mention that you should probably increase the TX queue for write commands or HVN (depending on your mode of transport). The peer should keep pulling events, and must also have similar parameters set for event length (or enable event extension), DLE and ATT MTU size.