Trying to achive max data rate with PCA10028

Question

Hi, i have a NRF51-DK (nRF51822 - PCA10028) board and using SDKv10 with SoftDevice S110.
I read everything i could find about this topic and i see people claiming data rate of at least 2KByte in worst case, but what i get is quite lower. 
 I started from the ble uart peripheral example, deleted everything uart-related to use only the nus and send runtime-generated data directly from the firmware. So when a button is pressed i call a data_send() function, and when i get the BLE_EVT_TX_COMPLETE event, i call data_send again. Every call to data_send changes the first character of the message sequentially. 
 These are the connection parameters i have used: 
 #define MIN_CONN_INTERVAL MSEC_TO_UNITS(7.5, UNIT_1_25_MS) 
#define MAX_CONN_INTERVAL MSEC_TO_UNITS(40, UNIT_1_25_MS) 
#define SLAVE_LATENCY 4 
#define CONN_SUP_TIMEOUT MSEC_TO_UNITS(4000, UNIT_10_MS) 
#define MAX_CONN_PARAMS_UPDATE_COUNT 3
 
 Doing so i get around 300 byte per second. Reducing MAX_CONN_INTERVAL to 20 gave me around 500byte per second, but i can't leave that because it would cause problems with IOS devices supporting a minimum of 30ms connection interval.
To test the speed i used an Android device and different apps that display data received through notification (even Nordic UART v2). 
 
 I read topics where people talked about multiple packet per connection inteval, but if i undestood correcty that's something doesn't regard firmware developing but the client device.
Even so i tried to call data_send() 4 times every BLE_EVT_TX_COMPLETE event, and doing so i got around 1Kbyte but losing some packets. 
 
 
 So, does someone know how i can speed up it?

run_ar · Accepted Answer

To speed up the transfer rate I would recommend that you fill the TX buffers between connection event. So instead of placing 4 packets per connection event, you should continue to send date a until you get the BLE_ERROR_NO_TX_PACKETS error. iOS supports up to 6 packets per connection event @ 30 ms intervals (note that I haven't tested the latest version). 
 You can also try to get a faster connection interval (Note that we have seen that throughput has actually been reduced on some iOS versions doing this as the number of packets per connection event is/was reduced.) Try asking for min 12.5 - max 22.5 ms or min 15 - max 25 ms or something similar.