How to increase MTU of NRF52840 to 247?

[laumung]

Hi there, I am developing a peripheral to central system with nrf52840 base on NUS service. Now I tried to increase the MTU of the peripheral to 247 to increase the transfer rate. I modified the NRF_SDH_BLE_GATT_MAX_MTU_SIZE to 247 and NRF_SDH_BLE_GAP_DATA_LENGTH to 251. 

#ifndef NRF_SDH_BLE_GAP_DATA_LENGTH
#define NRF_SDH_BLE_GAP_DATA_LENGTH 251
#endif

// <o> NRF_SDH_BLE_GATT_MAX_MTU_SIZE - Static maximum MTU size. 
#ifndef NRF_SDH_BLE_GATT_MAX_MTU_SIZE
#define NRF_SDH_BLE_GATT_MAX_MTU_SIZE 247
#endif

But when I used NRF Sniffer to monitor the data transfer in BLE, I noticed that the length of the packet is still 31. Did I do the right? Or any other configuration should be changed? Appreciated your kind help.

[Simonr]

Hi

The 31 bytes you're seeing is only the advertising packet transmitted over BLE (you can see this from the info column in Wireshark). The MTU size is negotiated and set once the device connects to a device and is ready to transmit data in the connection. If you want to increase the size of your advertising packet you should check out extended advertising in this blog post.

Best regards,

Simon