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  • Case ID: 282074
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Trying to receive only one packet per connection interval for every connection on a multi-link environment on SDK 1.7.0

sotillo2409

Hi,

The project is a multi-link environment of nrf52840, The 7 peripherals has the task of send data constantly of a sensors. All is working on coded_s8.

The objective is keep the higher data rate of msgs received by central, but to guarantee the uniformity between every peripheral. For that i have done a deep investigation on how the central schedule the connections, and what is the best strategy to receive the data.

The ATT_MTU is set to minimum 23 bytes because of coded_s8.

Time on air of a packet= 80+256+16+24+(23*8*8)+(24*8)+(3*8)=2'064ms

So if I disable Connection Event Length extension with:

static int EvtLen(void)
{       
        int err;
	struct net_buf *buf;
	sdc_hci_cmd_vs_conn_event_extend_t *evt_enable;

	buf = bt_hci_cmd_create(SDC_HCI_OPCODE_CMD_VS_CONN_EVENT_EXTEND,
				sizeof(*evt_enable));
	if (!buf) {
		printk("Could not allocate LLPM command buffer\n");
		return -ENOMEM;
	}
        evt_enable = net_buf_add(buf, sizeof(*evt_enable));
	evt_enable->enable = 0;

	err = bt_hci_cmd_send_sync(SDC_HCI_OPCODE_CMD_VS_CONN_EVENT_EXTEND, buf, NULL);
	if (err) {
		printk("Error enabling EVENT %d\n", err);
		return err;
	}

	printk("EVENT mode enabled\n");
	return 0;
}

And fix the maximum T event length on Kconfig with:

CONFIG_SDC_MAX_CONN_EVENT_LEN_DEFAULT=2500
Theorically making this configuration i guarantee that on a connection interval of 2500ms*7devices=17500ms/1.25=14 units i have enought time to attend the 7 devices and receive 1 packet of every one (Maybe could be packet drop but no may be systematic). Unfortunately I am not able to achieve this. Most of the times one or 2 devices keep sending half data comparing with the rest. I tried to modify a little the conn_interval without success.
If you could suggest me something more to try I will appreciate it, all code shared is done on Central
Thank you and regards.
Parents Reply Children
  • 0 in reply to sotillo2409

    Sometimes you have to restart the capture to make the device appear in the dropdown menu. Did you try that already?

    You can restart the capture by clicking the green button in the top left corner.

  • 0 in reply to Vidar Berg
    Vidar Berg said:
    restart the capture

    That solve my problem completely, I am able to follow the packets on a Connection.

    Could you resolve a final doubt of my data sniff??

    I configure Conn_interval=32.5ms

    So as I understand is, every 32.5ms aprox Master is going to send a packet to Slave(Empty PDU), if the slave have data to send then send data(L2CAP Fragment Start), the next Empty PDU of the Master the Slave send an ATT command saying something more. Could you explain-me a little bit what is the order and sense of every packet? And if you could suggest me some link to a more deep information.

    So central side isn't going to receive a message(23 bytes) in one conn_interval, otherwise is going to receive data once every 4 conn_interval. Is this normal??

    On another way as you can see on the sniff Master is sending the Empty PDU every 2*Conn_interval=65ms, is this correct??

    Sorry for make you spend more time with me, but if I achieve understand this I think that no more questions i would do on the future. 

    I am so gratefull, regards.

  • 0 in reply to sotillo2409

    Yes, the master seems to be skipping every other connection event in your case. It's not normal, but not unusual either. It can happen in multilink scenarios where the master is forced to drop connection events to maintain other active links. Do you see the same if you increase the connection interval a bit more?

    The master must sent a packet to start the connection, which will be an empty one if it has no data to send.

    It also looks like CONFIG_SDC_MAX_CONN_EVENT_LEN_DEFAULT is still too short for the master to have time to acknowledge (i.e. reply with an empty packet) your notification packets within the same connection event (you can see the connection event counter to the left of the "info" tab in Wireshark.

    You can read more about how the scheduling works in the Softdevice controller documentation here: https://developer.nordicsemi.com/nRF_Connect_SDK/doc/1.8.0/nrfxlib/softdevice_controller/doc/scheduling.html#controller-timing-activities-and-priorities

    Don't hesitate to ask if you have further questions or if any of the above is unclear.

  • 0 in reply to Vidar Berg

    Hi,

    Only for comment my final configuration.

    I need a CONFIG_SDC_MAX_CONN_EVENT_LEN_DEFAULT=8000 for achieve that on one connection event receive one msg:

    If I lower Tevent 500 less i find that only 2 events of the last 4 happen on a connection event.

    So with this Conn event and following:

    Conn_interval=(CONFIG_SDC_MAX_CONN_EVENT_LEN_DEFAULT+1.4)*Nºdevices

    I achieve the maximum throughput, near to 18kbps received by central. This value still constant independently of NºDevices.

     

    Apart from this I want to ask you for an hipotetic situation. Probably I need to combine two centrals to have more than 20 peripherals connected. There are many way to avoid noise between both centrals? As i know on One central with multiple peripherals there aren't noise problems due to Central coordinate the data transfer. But if I add another one this isn't going to happen and probably throughput go lower. There are some technique to don't disturb one with other? I hear about changing chanels of data transfer, this could solve my problem?? If I put one central working on the first half of channels availables and the second one on the final chanels for example. On advertisment  this don't have solution I guess.

    Thank you for the help,

    regards.

  • 0 in reply to sotillo2409

    Hi,

    Thanks for sharing your findings. I didn't expect the connection event length to have to be that long.

    sotillo2409 said:
    Probably I need to combine two centrals to have more than 20 peripherals connected. There are many way to avoid noise between both centrals? As i know on One central with multiple peripherals there aren't noise problems due to Central coordinate the data transfer. But if I add another one this isn't going to happen and probably throughput go lower. There are some technique to don't disturb one with other? I hear about changing chanels of data transfer, this could solve my problem?? If I put one central working on the first half of channels availables and the second one on the final chanels for example. On advertisment  this don't have solution I guess.

    I'm not sure how much degradation of throughput you can expect in this scenario, if any. It's obviously best if you are able to actually test it. Other external noise sources (wifi,etc) and range may be a bigger factor.

    BLE has 37 different data channels that are used for channel hopping to mitigate interference problems, and you normally don't have update the channel map yourself.

    Note that the channel number is displayed for each packet in the sniffer trace:

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