What controls packets per connection event?

RE: What controls packets per connection event?

umr_engnr — Mon, 02 Feb 2026 22:06:54 GMT

We have learned a lot about scheduling optimizations since this question was first posted. The largest takeaway, as it applies to this post, is that data length extension only helps on the last connection made because the scheduler will stack connections right on top of each other. For example, if you have two connections both with 100ms connection intervals and both with data event lengths of 7.5ms, your first connection will start at time 0 and due to data length extension can keep sending data for most of the 100ms; however, once connection #2 is made it is placed at 7.5ms right after the first connection. So now connection #1 is only allowed 7.5ms per connection event to send data, while connection #2 is allowed 92.5ms per connection event to send data. If yet another connection is made, then it is placed directly after connection #2, and now connection #1 and #2 only get 7.5ms, whereas connection #3 gets 85ms to send data. There is a lot of other scheduling nuance, but this was the largest reason the performance was seemingly random, and would vary widely depending on what connections were being made.

RE: What controls packets per connection event?

variant — Mon, 26 Jan 2026 20:53:17 GMT

As Amanda pointed, read the SDC/Scheduling. You will see then that if you are advertising and/or scanning while you are performing throughput measurements on a connection, you will get different throughput results.

RE: What controls packets per connection event?

Amanda Hsieh — Thu, 19 Dec 2024 20:12:23 GMT

Maybe you can analyze their sniffer logs to find out something different.

RE: What controls packets per connection event?

umr_engnr — Thu, 19 Dec 2024 05:20:53 GMT

Hi Amanda,

Do you have any thoughts on why with the same exact connections and parameters the scheduler can produce such drastically different results?

Is there any debug data we could compare between these two scenarios to try and better understand what is going on?

In the "fast" case the performance is acceptable, we just want that behavior to happen reliably.

RE: What controls packets per connection event?

Amanda Hsieh — Wed, 18 Dec 2024 23:29:58 GMT

Hi,

You are correct. See the explanation in the SDC/Scheduling, and Bluetooth: Throughput for how to increase the throughput.

-Amanda H.

RE: What controls packets per connection event?

umr_engnr — Tue, 17 Dec 2024 19:58:45 GMT

We noticed that CONFIG_BT_CTLR_SDC_CONN_EVENT_EXTEND_DEFAULT=y

Which based on the KCONFIG data seems to imply that BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT is not used for the connection event length determination.

However, it does appear to be used to set BT_CTLR_SDC_CENTRAL_ACL_EVENT_SPACING_DEFAULT.

From nrf/subsys/bluetooth/controller/KCONFIG:

config BT_CTLR_SDC_CONN_EVENT_EXTEND_DEFAULT
bool "Enable connection event extension by default"
default y if !NRF_802154_RADIO_DRIVER
help
When Extended Connection Events are disabled, the maximum connection event length is
configured with BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT.
When Extended Connection Events are enabled, the controller
will extend the connection event as much as possible, if
- Either of the peers has more data to send.
See also: Core v5.6, Vol 6, Part B, Section 4.5.6.
- There are no conflicts with other roles of equal or higher priority.

config BT_CTLR_SDC_CENTRAL_ACL_EVENT_SPACING_DEFAULT
int "Default central ACL event spacing [us]"
default 30000 if BT_ISO
default BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT
help
On the central, sets the time ACL connections are spaced apart assuming that
they are using the same connection interval.
This configuration allows the application to set a different value for event spacing
than BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT. This will result in holes in the scheduling
timeline if the connection interval is longer than the ACL event spacing.
This gap can be used for other scheduling activites like isochronous streams.

RE: What controls packets per connection event?

umr_engnr — Tue, 17 Dec 2024 19:41:34 GMT

In case it is helpful I went ahead and captured the CONFIG_BT_CTLR* settings from the net core .config file:

CONFIG_BT_CTLR=y
CONFIG_BT_CTLR_DATA_LENGTH_MAX=81
CONFIG_BT_CTLR_SDC_PERIPHERAL_COUNT=2
# CONFIG_BT_CTLR_SDC_QOS_CHANNEL_SURVEY is not set
# CONFIG_BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT_OVERRIDE is not set
CONFIG_BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT=4000
CONFIG_BT_CTLR_SDC_CONN_EVENT_EXTEND_DEFAULT=y
CONFIG_BT_CTLR_SDC_CENTRAL_ACL_EVENT_SPACING_DEFAULT=4000
# CONFIG_BT_CTLR_SDC_PERIODIC_ADV_EVENT_LEN_DEFAULT_OVERRIDE is not set
CONFIG_BT_CTLR_SDC_PERIODIC_ADV_EVENT_LEN_DEFAULT=7500
CONFIG_BT_CTLR_SDC_TX_PACKET_COUNT=3
CONFIG_BT_CTLR_SDC_RX_PACKET_COUNT=2
CONFIG_BT_CTLR_SDC_SCAN_BUFFER_COUNT=3
CONFIG_BT_CTLR_SDC_PERIODIC_SYNC_BUFFER_COUNT=3
CONFIG_BT_CTLR_SDC_RX_PRIO=6
# CONFIG_BT_CTLR_DF is not set
CONFIG_BT_CTLR_FAL_SIZE=8
CONFIG_BT_CTLR_ECDH_LIB_OBERON=y
# CONFIG_BT_CTLR_ECDH_LIB_TINYCRYPT is not set
CONFIG_BT_CTLR_ECDH_STACK_SIZE=900
# CONFIG_BT_CTLR_ECDH_IN_MPSL_WORK is not set
# CONFIG_BT_CTLR_SDC_EVENT_TRIGGER is not set
CONFIG_BT_CTLR_SDC_BIG_RESERVED_TIME_US=1600
CONFIG_BT_CTLR_SDC_CIG_RESERVED_TIME_US=1300
CONFIG_BT_CTLR_SDC_CIS_SUBEVENT_LENGTH_US=0
CONFIG_BT_CTLR_LE_ENC_SUPPORT=y
CONFIG_BT_CTLR_EXT_REJ_IND_SUPPORT=y
CONFIG_BT_CTLR_DATA_LEN_UPDATE_SUPPORT=y
CONFIG_BT_CTLR_PRIVACY_SUPPORT=y
CONFIG_BT_CTLR_EXT_SCAN_FP_SUPPORT=y
CONFIG_BT_CTLR_PHY_UPDATE_SUPPORT=y
CONFIG_BT_CTLR_PHY_2M_SUPPORT=y
CONFIG_BT_CTLR_PHY_CODED_SUPPORT=y
CONFIG_BT_CTLR_ADV_EXT_SUPPORT=y
CONFIG_BT_CTLR_ADV_PERIODIC_SUPPORT=y
CONFIG_BT_CTLR_ADV_PERIODIC_RSP_SUPPORT=y
CONFIG_BT_CTLR_SYNC_PERIODIC_SUPPORT=y
CONFIG_BT_CTLR_SYNC_PERIODIC_RSP_SUPPORT=y
CONFIG_BT_CTLR_SYNC_TRANSFER_SENDER_SUPPORT=y
CONFIG_BT_CTLR_SYNC_TRANSFER_RECEIVER_SUPPORT=y
CONFIG_BT_CTLR_ADV_ISO_SUPPORT=y
CONFIG_BT_CTLR_SYNC_ISO_SUPPORT=y
CONFIG_BT_CTLR_CENTRAL_ISO_SUPPORT=y
CONFIG_BT_CTLR_PERIPHERAL_ISO_SUPPORT=y
CONFIG_BT_CTLR_CHAN_SEL_2_SUPPORT=y
CONFIG_BT_CTLR_SCA_UPDATE_SUPPORT=y
CONFIG_BT_CTLR_CONN_RSSI_SUPPORT=y
CONFIG_BT_CTLR_ECDH_SUPPORT=y
CONFIG_BT_CTLR_LE_POWER_CONTROL_SUPPORT=y
CONFIG_BT_CTLR_CRYPTO=y
CONFIG_BT_CTLR_HCI_VS_BUILD_INFO=""
# CONFIG_BT_CTLR_TX_PWR_PLUS_3 is not set
# CONFIG_BT_CTLR_TX_PWR_PLUS_2 is not set
# CONFIG_BT_CTLR_TX_PWR_PLUS_1 is not set
CONFIG_BT_CTLR_TX_PWR_0=y
# CONFIG_BT_CTLR_TX_PWR_MINUS_1 is not set
# CONFIG_BT_CTLR_TX_PWR_MINUS_2 is not set
# CONFIG_BT_CTLR_TX_PWR_MINUS_3 is not set
# CONFIG_BT_CTLR_TX_PWR_MINUS_4 is not set
# CONFIG_BT_CTLR_TX_PWR_MINUS_5 is not set
# CONFIG_BT_CTLR_TX_PWR_MINUS_6 is not set
# CONFIG_BT_CTLR_TX_PWR_MINUS_7 is not set
# CONFIG_BT_CTLR_TX_PWR_MINUS_8 is not set
# CONFIG_BT_CTLR_TX_PWR_MINUS_12 is not set
# CONFIG_BT_CTLR_TX_PWR_MINUS_16 is not set
# CONFIG_BT_CTLR_TX_PWR_MINUS_20 is not set
# CONFIG_BT_CTLR_TX_PWR_MINUS_40 is not set
CONFIG_BT_CTLR_TX_PWR_DBM=0
CONFIG_BT_CTLR_TX_PWR_ANTENNA=0
CONFIG_BT_CTLR_TX_PWR_DYNAMIC_CONTROL=y
CONFIG_BT_CTLR_LE_ENC=y
CONFIG_BT_CTLR_ECDH=y
CONFIG_BT_CTLR_EXT_REJ_IND=y
CONFIG_BT_CTLR_LE_PING=y
CONFIG_BT_CTLR_DATA_LENGTH=y
CONFIG_BT_CTLR_PHY=y
CONFIG_BT_CTLR_CONN_RSSI=y
# CONFIG_BT_CTLR_LE_POWER_CONTROL is not set
CONFIG_BT_CTLR_FILTER_ACCEPT_LIST=y
CONFIG_BT_CTLR_PRIVACY=y
CONFIG_BT_CTLR_RL_SIZE=8
CONFIG_BT_CTLR_EXT_SCAN_FP=y
CONFIG_BT_CTLR_PHY_2M=y
CONFIG_BT_CTLR_PHY_CODED=y
CONFIG_BT_CTLR_CHAN_SEL_2=y
# CONFIG_BT_CTLR_ADV_EXT is not set
# CONFIG_BT_CTLR_SET_HOST_FEATURE is not set
# CONFIG_BT_CTLR_CENTRAL_ISO is not set
# CONFIG_BT_CTLR_PERIPHERAL_ISO is not set
# CONFIG_BT_CTLR_HCI_CODEC_AND_DELAY_INFO is not set
CONFIG_BT_CTLR_ASSERT_HANDLER=y