How to increase OTA upgrade speed?

Question

Using GATT characteristic writes to transfer data is slow.Is there a cleaner and/or faster way to do faster OTA upgrade?
Also I wanted to upgrade firmware without affecting current measurements(firmware operation).
User need not to know about firmware upgrade process.
Is there any way to do this?

bjorn-spockeli · Accepted Answer

Hi Vikram, 
 the transfer speed of a firmware image will depend on how many packets are sent per connection interval (this may vary depending on the chip/chipset used by the central device) and the packet size. 
 Which BLE version the chips on both sides of the link support will also affect the datarate.
If both support BLE v4.0 the you're limited to a throughput of 124kbps. If both support v4.2 then you might be able to reach 150kbps if they support the Long MTU feature and 800kbps if they support long packets. If both sides of the link support BLE 5, then you can use the High Speed feature which may give you up to ~1.4Mbps. The unmodified bootloader will be 124-150kbps, so if you need/want higher throughput, then you will have to modify the bootloader to support the v4.2 and/or v5 features. 
 Background DFU, i.e. the firmware image is received while the application is running, is possible with the nRF5x series and is something that we are looking into adding to the SDK. However, we do not have a solution for this today. 
 Best regards 
 Bjørn