BLE bonding

[Roei]

Hi,

As far as I understand, every connection establishment has to start with advertising.

Event if I have already connected and bonded to a peer, after disconnection I have to start advertising in order to connect to this peer again.

Am I right?

In that case - what is the meaning of bonding here? Shouldn't the peers connect automatically again without advertising/direct advertising? Do I have another option?

Thanks!

[Einar Thorsrud]

Hi,

As far as I understand, every connection establishment has to start with advertising.

Yes, that is correct. This is a key concept of the BLE protocol. The peripheral advertises regularly, and in case of connected advertising, it will switch the radio to Rx and listen for connection requests from a central in a short window after transmitting the advertising packet. This is how BLE connections are established.

Event if I have already connected and bonded to a peer, after disconnection I have to start advertising in order to connect to this peer again.

Am I right?

Yes pairing and bonding is independent of the connection mechanism.

In that case - what is the meaning of bonding here? Shouldn't the peers connect automatically again without advertising/direct advertising?

No, that would not be possible. When the peripheral is not advertising or in a connection it is typically in sleep, with the radio completely turned off. So there would be no way to connect to it. Pairing is a completely different concept, which is how devices exchange encryption keys (used both for encrypting the link and optionally randomizing the address to that it cannot be tracked even though it advertises). Bonding is the same thing, the only difference is that when bonding, the peers agree to store the keys to be used for later.

Do I have another option?

Not really. Dependign on your use case you may not need to bond, but there is no way around advertising to initiate a connection, regardless of if bonded or not.

[Roei]

Thanks for your answer!

I will be more specific. My use case is that I want to connect and bond my NRF device with my mobile. After turning off and on the device and mobile, I want them to connect again.

Until now, I did it with direct advertising. I saved the mobile's address after bonding when they first connected, so I will use it for direct advertising when my NRF device is turned on again.

The problem is that my mobile changes it's address. I read some articles about that privacy feature. Do you know that?

What can I do about that? How can I reconnect to a mobile which changed it's address?

Thanks!

[Roei]

Hi,

ble_advertising_restart_without_whitelist() doesn't work, it cause BLE_ADV_EVT_IDLE event, instead of BLE_ADV_EVT_FAST. Only when I set whitelist_temporarily_disabled = false, it works.

Thanks!

[Einar Thorsrud]

Hi,

This works out of the box with the unmodified Current Time Application, and it is not clear to me what you have done. Why do you change whitelist_temporarily_disabled? That is only intended to be used by the module itself, it is not something you should modify.

[Roei]

Hi,

I set ble_adv_whitelist_enabled = true when initialized my module.

After that, I just operated ble_advertising_restart_without_whitelist(), and the result was BLE_ADV_EVT_IDLE.

[Einar Thorsrud]

Hi,

Please elaborate much more.

• Are you using an example?
• Which exact changes have you done from the example (preferably show a diff or similar)?
• How do you test?
• How long time does it take from the call to ble_advertising_restart_without_whitelist() until you get the BLE_ADV_EVT_IDLE?

[Roei]

I will explain again.

I want my application to start advertising with whitelist in some cases, and without whitelist in others.

How can I do that without starting advertising and restart again?

[Roei]

Yes, I used the example and I test it with my mobile, I can see when I connect or disconnect clearly, and I tested it with more mobiles to validate the whitelist. It was all good, but I need the option to start reqular advertising as well.

I get the idle event right after starting advertising (in the same ms).

[Roei]

OK, the problem is that the current advertising mode is Idle, and I need to set it to Fast before I restart advertising without whitelist. Or should I do another thing?

Thanks!

[Einar Thorsrud]

Hi,

I see. You could start advertising normally, and then restart, or slightly adjust the advertising module.

The resone calling ble_advertising_restart_without_whitelistadvertising() did not work is that the module only supports restarting advertising in the same mode, and when advertising is not active the mode is IDLE. If you want to start advertising without whitelist you could expand the library with a ble_advertising_start_without_whitelist() function like this:

uint32_t ble_advertising_start_without_whitelist(ble_advertising_t * const p_advertising,
                                                 ble_adv_mode_t            advertising_mode)
{
    ret_code_t ret;

    p_advertising->whitelist_temporarily_disabled = true;
    p_advertising->whitelist_in_use               = false;
    p_advertising->adv_params.filter_policy       = BLE_GAP_ADV_FP_ANY;
    // Set correct flags.
    ret = flags_set(p_advertising, BLE_GAP_ADV_FLAGS_LE_ONLY_GENERAL_DISC_MODE);
    VERIFY_SUCCESS(ret);

    ret = ble_advertising_start(p_advertising, advertising_mode);
    if ((ret != NRF_SUCCESS) && (p_advertising->error_handler != NULL))
    {
        p_advertising->error_handler(ret);
    }

    return NRF_SUCCESS;
}

The only difference from the restart variant is that it does not stop advertising, and it uses the specified mode instead of the current mode. Calling this function instead of ble_advertising_start() will do the trick.

[Roei]

Thanks a lot!

One more question - 

I'm still confused about the difference between pm_whitelist_set and pm_device_identities_list_set. Can you please explain what does each of them do?

[Einar Thorsrud]

Hi,

The device identities list is a list of device IRK's (Identity resolution keys). So that is needed for whitelisting peers that use privacy (which include any modern phone). Whitelisting static MAC addresses only work with devices that do not use privacy. Therefore, you will need code for both to handle both cases, like it is done in the SDK examples.