How to increase the number of packets sent in a single TX transmission?

RE: How to increase the number of packets sent in a single TX transmission?

SwRa — Fri, 14 Nov 2025 13:45:58 GMT

Closing as discussion continued in #351708

RE: How to increase the number of packets sent in a single TX transmission?

HHJ — Wed, 24 Sep 2025 06:37:43 GMT

I sent the wrong file for the 15ms connection interval earlier. This one is the correct file for the 15ms connection interval.devzone.nordicsemi.com/.../15MS_5F00_2.pcapng

RE: How to increase the number of packets sent in a single TX transmission?

HHJ — Wed, 24 Sep 2025 03:45:44 GMT

I'm sorry, I think I didn't express myself clearly earlier. My timer generates and sends a set of data every 3ms. However, regardless of whether the connection interval is 7.5ms or 15ms, Bluetooth will reply in the next connection interval after receiving the second data packet. Especially with the 15ms connection interval, Bluetooth will reply after more than 10ms. I believe this situation is abnormal and does not conform to our Bluetooth logic. Currently, we are not sure where the problem lies. These are the pcapng files captured by nrf niffer for the 7.5ms and 15ms connection intervals respectively. We look forward to your reply.devzone.nordicsemi.com/.../1172.15ms.pcapng devzone.nordicsemi.com/.../0676.5ms.pcapng

RE: How to increase the number of packets sent in a single TX transmission?

SwRa — Tue, 23 Sep 2025 11:13:01 GMT

[quote user="HHJ"]ms and the transmission timing to 3ms. We found that Bluetooth can send data packets normally within the 7.5ms interval. However, after sending the second data packet, Bluetooth will force the next transmission to wait until the end of the 7.5ms connection interval, which causes blockage on the transmitting end.[/quote]

That is expected. In BLE, data can only be transmitted during a connection event, which occurs at intervals defined by the connection interval parameter. See here: https://academy.nordicsemi.com/courses/bluetooth-low-energy-fundamentals/lessons/lesson-3-bluetooth-le-connections/topic/connection-process/

Connection Event Length Extension is enabled by default in the nRF Connect SDK (SoftDevice Controller). You do not need to manually call a function like sd_ble_opt_set(BLE_COMMON_OPT_CONN_EVT_EXT, &opt) as you would in the nRF5 SDK. Refer: https://docs.nordicsemi.com/bundle/ncs-latest/page/nrfxlib/softdevice_controller/doc/scheduling.html#connection_timing_with_connection_event_length_extension

CONFIG_BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT=7500 sets the maximum connection event length (in microseconds), which is correct for controlling the event window duration.
CONFIG_BT_USER_DATA_LEN_UPDATE=y and CONFIG_BT_CTLR_DATA_LENGTH_MAX=251 are the correct settings for enabling Data Length Extension and setting the maximum data length.

Best Regards,

Swathy

RE: How to increase the number of packets sent in a single TX transmission?

HHJ — Tue, 23 Sep 2025 09:43:37 GMT

Thanks beauty

We have set the connection interval to 7.5ms and the transmission timing to 3ms. We found that Bluetooth can send data packets normally within the 7.5ms interval. However, after sending the second data packet, Bluetooth will force the next transmission to wait until the end of the 7.5ms connection interval, which causes blockage on the transmitting end.

Currently, we have configured the connection event extension with CONFIG_BT_USER_DATA_LEN_UPDATE=y, CONFIG_BT_CTLR_DATA_LENGTH_MAX=251, and the GAP event interval with CONFIG_BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT=7500.

Are we missing any key configurations such as the "Connection event length extension" in nRF52840, like setting opt.common_opt.conn_evt_evt.enable=true; err_code=sd_ble_opt_set(BLE_COMMON_OPT_CONN_EVT_EVT,&opt);? Or are there any existing configurations that need to be enabled in the main function?

RE: How to increase the number of packets sent in a single TX transmission?

SwRa — Tue, 23 Sep 2025 09:22:15 GMT

Hello,

In BLE, MTU has a default value of 23 bytes, and data length has a default value of 27 bytes. When MTU is larger than data length, the data will be segmented into chunks of the data length’s size. This means that, for your application, it appears like one message is being sent, but on the air, the data is actually split into smaller segments. To send a 242-byte data packet in a single Bluetooth LE transmission (i.e., without fragmentation into 27-byte packets), you need to increase both the Data Length and the ATT MTU in your BLE configuration. Try adding the following in your prj.conf file:

CONFIG_BT_CTLR_DATA_LENGTH_MAX=251
CONFIG_BT_BUF_ACL_RX_SIZE=251
CONFIG_BT_BUF_ACL_TX_SIZE=251
CONFIG_BT_L2CAP_TX_MTU=247

Best Regards,

Swathy