Active scanning with enabled filter causes sending SCAN_REQ to every advertiser

Hi Edvin,

thank you for your fully explainable response! :) I'm quite new in BLE and in Nordic products, so some obvious stuff is still new to me.

About sending scan response - you're right, I forgot that time between receiving first advertising data and sending SCAN_REQ / CONNECT_IND is quite tight (150us).

Right now I've implemented something similar as you suggested. I've put simple machine state and "match flags" in "no_match" callback and if data in both received packets is ok, I can determine if it's my peripherals.

Just to make sure - if my device is connectable, I have to send CONNECT_IND after receiving first packet, Trying to establish connection in callback from receiving second packet is pointless, right? Then advertiser would need to send at least 2 adv packets, right?

From the advertiser device I have small issue with whitelists.

I would like to response for SCAN_REQ only to "my" centrals. But central can send in SCAN_REQ only it's own random address. During every scanning "session" scanner will have different (random) address. Let's say I can send private static address of central during "initial phase", but it gives me nothing when scanner will send random address in SCAN_REQ.

How to properly handle it? I saw some article to make pairing and use IRK keys to resolve random address into private address, but I couldn't find any example for that.

For any help thanks in advance!

Adam

Hi Edvin,

thank you for your fully explainable response! :) I'm quite new in BLE and in Nordic products, so some obvious stuff is still new to me.

About sending scan response - you're right, I forgot that time between receiving first advertising data and sending SCAN_REQ / CONNECT_IND is quite tight (150us).

Right now I've implemented something similar as you suggested. I've put simple machine state and "match flags" in "no_match" callback and if data in both received packets is ok, I can determine if it's my peripherals.

Just to make sure - if my device is connectable, I have to send CONNECT_IND after receiving first packet, Trying to establish connection in callback from receiving second packet is pointless, right? Then advertiser would need to send at least 2 adv packets, right?

From the advertiser device I have small issue with whitelists.

I would like to response for SCAN_REQ only to "my" centrals. But central can send in SCAN_REQ only it's own random address. During every scanning "session" scanner will have different (random) address. Let's say I can send private static address of central during "initial phase", but it gives me nothing when scanner will send random address in SCAN_REQ.

How to properly handle it? I saw some article to make pairing and use IRK keys to resolve random address into private address, but I couldn't find any example for that.

For any help thanks in advance!

Adam

Kedlov said:

Just to make sure - if my device is connectable, I have to send CONNECT_IND after receiving first packet, Trying to establish connection in callback from receiving second packet is pointless, right? Then advertiser would need to send at least 2 adv packets, right?

Typically, the flow is that you receive an advertising packet, with or without a scan response (that doesn't really matter). Then you decide whether you want to connect to it or not. If you want to connect, you will call the API that will connect to the device (which depends on what SDK version you are using. Are you using the nRF5 SDK, or are you using NCS?). When this is called, the scanner will then wait for the next advertising packet from that device, and it will send the connect_ind immediately after receiving the advertisement. 

This next part is also dependent on what SDK you are using, but the principle will be the same.

In BLE you have several address types. In private addresses, you can have resolvable or unresolvable addresses. Resolvable addresses means that there is an IRK (Identity Resolving Key), which you can use to figure out whether a seemingly random address actually belongs to a known device or not. I don't know exactly how that works on the mathematical level, but the Bluetooth Low Energy stack supports this. 

What are you using to scan? Is it another nRF chip, a phone? Or something else?

And what SDK version are you using?

Best regards,

Edvin

Thanks for reply!

I'm using NCS updated to v2.0.0, based on some example projects generated from "Create a new application from sample" and now I explore most common use case scenarios which would also adjust to my projects' requirements.

Some of this requirements is:

- use radio as short as possible (connectionless beacon mode)

- get information is central node really received data or we should repeat it (data will change in some intervals but we shouldn't miss any of that data)

- from central point of view - sends scan requests only to "my" peripherals

- from peripheral point of view - sends scan responses only to "my" centrals

user26071 said:

In BLE you have several address types. In private addresses, you can have resolvable or unresolvable addresses. Resolvable addresses means that there is an IRK (Identity Resolving Key), which you can use to figure out whether a seemingly random address actually belongs to a known device or not.

Exactly this is what i needed to know, thanks! Actually after tests with some example which used bonding, when my peripheral was bonded with scanner, I was able to see its static private address, so I think that bonding and storing IRK keys are essentials which I need in my peripheral to know which scanner is "my" scanner.

user26071 said:

What are you using to scan? Is it another nRF chip, a phone? Or something else?

I will use nRF chip, for few tests I've used phone but scanning performance was too low for me. With scanner on nRF52840 I think I was able to catch almost very first advertising packet every time. Time from starting advertising to send scan response was around 4-15ms (probably depends of current scanning and adv channel)

Thanks to your reply, now I know how to figure out if scan request comes from my scanner.

Now the issue is following:

Let's say I have few hundreds peripheral nodes. Creating bond (by manually clicking some buttons for pairing mode) only to generate IRK keys would be time consuming. Let's say I could program this keys through wired connection during production or change the roles and central for few seconds would broadcast it's IRK key to all sensors in close proximity. I saw OOB pairing by using NFC and external antenna but we want to cut piece price for every peripheral node.

After this long "preamble" here comes my question ;)

Is it possible to manually create/save IRK keys which will be later used to resolve scanner private address?

For any help, thanks in advance!

Adam