PaWR subevent data

[Katy_123]

Hi,

I am testing the PaWR examples in the NCS and I have few doubts.

1. In this page Periodic Advertising with Responses (PAwR): A practical guide , it says in section 1.3 that there are two ways for synchronization - PAST after connection OR  following the periodic advertising. Is there an example in NCS which shows the PaWR using the synchronization with the periodic advertising (without connection)? I used examples "periodic_adv_rsp" and "periodic_sync_rsp" in pairs and I can see a connection and PAST happening.

Regarding "pawr_data_request" callback, it says the callback will be called when there is buffer available?  What does this mean ? For example, How can I increment the data for subevent0 in each event? Ie, the scanner should receive 0,1,2,3,4 ... for subevent 0.

[Jakob Ruhe]

Hello Katy,

I am happy to hear that you are trying out PAwR!

1. The sample application periodic_sync_conn synchronizes to the PAwR train, advertised by the periodic_adv_conn, without using PAST. 

That said, I have only used PAST myself. I think it has some advantages because then the advertiser can share details such as which subevent and which response slot a new peripheral should have before it joins the PAwR train.

2. The pawr_data_request callback will be called periodically by the controller some time before it needs transmit data for one (or several) subevents. If you have a look in the periodic_sync_conn example and the function request_cb, it only requires i tiny modification to accomplish what you want.

This is what the loop inside the function request_cb() looks like now:

	for (size_t i = 0; i < to_send; i++) {
		buf = &bufs[i];
		buf->data[buf->len - 1] = counter++;

		subevent_data_params[i].subevent =
			(request->start + i) % per_adv_params.num_subevents;
		subevent_data_params[i].response_slot_start = 0;
		subevent_data_params[i].response_slot_count = NUM_RSP_SLOTS;
		subevent_data_params[i].data = buf;
	}

In order to only increase the counter at subevent 0 (i.e. for every new period), something like this should work:

	for (size_t i = 0; i < to_send; i++) {
		const uint8_t subevent = (request->start + i) % per_adv_params.num_subevents;
		if (subevent == 0) {
			counter++;
		}

		buf = &bufs[i];
		buf->data[buf->len - 1] = counter;

		subevent_data_params[i].subevent = subevent;
		subevent_data_params[i].response_slot_start = 0;
		subevent_data_params[i].response_slot_count = NUM_RSP_SLOTS;
		subevent_data_params[i].data = buf;
	}

 Good luck with your project!

[Katy_123]

 Jakob Ruhe Thank you for your reply.

So if the PaWR adv interval is 10 seconds, will I get this call back at least every 10 seconds ? and from the code it looks like there will be scenarios where buffer will not be available for all the subevents - What data will be sent for the subevents in this case ?

Also, I know that in normal advertising the receiver sometimes misses the ADV packets. In PAWR, will there be any cases where the subevents/responses will be missed ? Does PaWR guarantee a 100% data reception ?

[Einar Thorsrud]

Hi

Yes, you will get the pawr_data_request callback needs data for new subevent(s), so that the application can provide data.

Regarding the seocnd question, PaWR is a low-level feature that does not include retransmissions, so there is no guarantee that all data will be received. But it should be possible to build a protocol on top that does is (this is briefely described in the guilde that you linked to in your original post).