I'm writing (and copying) some Bluetooth LE scanner software to find and connect to a heart rate monitor. I used to be able to find this device by using scanForPeripheralsWithServices method to perform a generic scan by setting services to nil. But in the recent few days, I noticed that my same Bluetooth LE device (my heart rate monitor) can no longer be scanned (perhaps it is not advertising the same way it used to?) but my iPhone can still pair with it under settings and can connect to it with commercial apps such as "Polar Beat" and "Strava". So what changed? Can a Bluetooth LE device stop advertising and still be connected to (paired with) and share data as a peripheral?
The strange thing is that your device doesn't advertise.
However, it's normal that your iPhone can connect to it because the smartphone executed "bonding" procedure in the past. Bonding is another security procedure than simple pairing. With Bonding, heart rate device and smartphone exchanges keys and store them in a non volatile memory so that they can connect quickly.
With simple pairing the keys are temporary and advertising is needed.
Related
When connection to a custom BLE device from an app (android and ios) I have two options
a) first pair the device on the phone (like a headset or other devices) and then use it from the app to read and write data
b) without pairing on the phone, go inside the app, search for a device with a specific name or address, and just connect to it and read and write data.
when is one preferred over the other one ?
Its just when it needs to interact in the background or am I missing something?
The devices you see in your phones Bluetooth settings are Bluetooth Classic devices. There are some devices using both BLE and Bluetooth Classic which show up in the settings too, but the settings page is only for the Bluetooth Classic part of the device. Headphones for example are streaming music trough the Bluetooth Classic profile A2DP but might offer BLE services for additional features.
You always have to use a special app to communicate with the BLE part of a device. This could be an app provided by a manufacturer or a generic BLE scanner app such as nRF Connect.
As alexander.cpp already told you in his answer pairing (the exchange of keys) and bonding (saving of said keys) is not required for the communication with BLE devices and only needed if the device requires a secured connection.
Bonding (technically correct term for BLE, often called Pairing in non-technical speech) is mostly for security, we can send sensitive information because the receiver is verified and data is transferred in encrypted form.
Prefer to use bonding if the data is sensitive (means users feel uncomfortable if somebody they don't know receive their data). For example, I consider "current temperature = 25" as NOT sensitive. For detailed explanation, find "Bonding with a BLE device" in BLE guide by PunchThrough.
Also this is a good answer to a similar question: https://stackoverflow.com/a/42916081/10380092.
Your second question about background - no, bonding is not related to interaction in background.
I got an issue with reconnection to my BLE device.
Senario that works
- BLE Device is not paired
- Pi boots and start my App, find's the BLE device, pair and connect, receiving data
- Boot Pi and start my App, it will not pair as its already paired, it connects fine and receiving data.
Senario that does not work
- BLE Device is not pair or pair, does not matter
- If my BLE device disconnect due to power loss or out of range and gets powered up or back in range the Pi will reconnect and throw:
'System.Exception' in mscorlib.ni.dll
There is no user session key for the specified logon session. (Exception from HRESULT: 0x80070572)
If I try to reboot the Pi, the same message is thrown when I try to execute: await _readCharacteristic.WriteClientCharacteristicConfigurationDescriptorAsync(GattClientCharacteristicConfigurationDescriptorValue.Notify);
The only thing that works is if I unpair the device every time it disconnect.
This is the same issue that this guy has: https://social.msdn.microsoft.com/Forums/sqlserver/en-US/b98d77f2-bf5e-45fc-9495-1c444b54450e/uwpreconnecting-to-a-ble-csac-device-causes-exception?forum=wpdevelop
Maybe there is anyone here that has seen the same issue and solved it ?
Thanks
The reason for this particular error you are receiving is almost certainly that the BLE device itself is not storing the bonding information that is negotiated during the pairing process. For an existing BLE pairing to be re-used, the device must retain the bonding with its partner for future communication sessions.
Developers can sometimes work around this device shortcoming in their (non-Windows) PC or phone application by having the PC or Mobile app delete any previous pairings with the device already retained, and negotiate a new pairing each time it sees the Bluetooth device. This is possible because many Bluetooth devices have a static PIN, require no interaction for pairing, and remain in open pairing mode. So the app creates a new device pairing upon each communication. The Mac BLE API requires no user interaction for pairing and un-pairing as well.
But this solution is unworkable under Windows BLE API, because unlike regular Bluetooth, the Bluetooth Low Energy API requires user interaction during both the pairing, and un-pairing "ceremonies". The user must agree to every pairing, and must agree to every un-pairing. This makes this workaround solution a non-starter from a UX standpoint on Windows. My guess is the BLE device manufacturer isn't storing the bonding.
We ran into such a situation ourselves developing our own BLE driver to speak with our own BLE board; we had to re-do the firmware running our BLE device because while it would work under Mac OS and Android with the method described above, the BLE API available on the Microsoft UWP requires user interaction during the pairing ceremony. This seems the right way to do things, anyway.
I couldn't find a clear explanation what is Bluetooth le multi advertising.
For example, in Specification of the Bluetooth System, I see only
description of BLE advertising but not multi advertising.
What the difference between BLE advertising and multi advertising?
Thanks
LE Multi advertising refers to a specific offload (non bluetooth specification) feature that has been implemented in the Android releases by Google, starting from the L release.
What multi advertising means is, that your Android phone or a device with Android stack, can enable multiple advertisement trains at the same time. For instance, one specific app can set certain advertisement data (example supported services etc.), advertisement parameters and enable advertisement, and so can N number of other apps, on the same device.
At the hardware level (or physical layer), these advertisement trains are interleaved between the three advertisement channels (37, 38, 39) and thus your phone behaves as multiple peripheral devices at the same time. All these advertisement trains also have a separate Random private address, so they are seen distinctly by the remote scanner.
Bluetooth Smart has two ways of communicating. The first one is using advertisements, where a BLE peripheral device broadcasts packets to every device around it. The receiving device can then act on this information or connect to receive more information. The second way to communicate is to receive packets using a connection, where both the peripheral and central send packets. We will focus on advertisement for several reasons:
You can’t create a connection between two devices without using advertisements. Defining the data and format of advertisement packets is usually the first thing you work on when developing a BLE device.
A large number of BLE products sleep most of the time, waking up only to advertise and connect when needed. This means advertisements have a big impact on power consumption.
Users want responsive products, and the advertising interval is critical in quick connections.
Advertising is by design unidirectional. A Central device can’t send any data to the Peripheral device without a connection. But a single peripheral can advertise to multiple masters in the area.
ref:http://www.argenox.com/bluetooth-low-energy-ble-v4-0-development/library/a-ble-advertising-primer/
I got an issue with reconnection to my BLE device.
Senario that works
- BLE Device is not paired
- Pi boots and start my App, find's the BLE device, pair and connect, receiving data
- Boot Pi and start my App, it will not pair as its already paired, it connects fine and receiving data.
Senario that does not work
- BLE Device is not pair or pair, does not matter
- If my BLE device disconnect due to power loss or out of range and gets powered up or back in range the Pi will reconnect and throw:
'System.Exception' in mscorlib.ni.dll
There is no user session key for the specified logon session. (Exception from HRESULT: 0x80070572)
If I try to reboot the Pi, the same message is thrown when I try to execute: await _readCharacteristic.WriteClientCharacteristicConfigurationDescriptorAsync(GattClientCharacteristicConfigurationDescriptorValue.Notify);
The only thing that works is if I unpair the device every time it disconnect.
This is the same issue that this guy has: https://social.msdn.microsoft.com/Forums/sqlserver/en-US/b98d77f2-bf5e-45fc-9495-1c444b54450e/uwpreconnecting-to-a-ble-csac-device-causes-exception?forum=wpdevelop
Maybe there is anyone here that has seen the same issue and solved it ?
Thanks
The reason for this particular error you are receiving is almost certainly that the BLE device itself is not storing the bonding information that is negotiated during the pairing process. For an existing BLE pairing to be re-used, the device must retain the bonding with its partner for future communication sessions.
Developers can sometimes work around this device shortcoming in their (non-Windows) PC or phone application by having the PC or Mobile app delete any previous pairings with the device already retained, and negotiate a new pairing each time it sees the Bluetooth device. This is possible because many Bluetooth devices have a static PIN, require no interaction for pairing, and remain in open pairing mode. So the app creates a new device pairing upon each communication. The Mac BLE API requires no user interaction for pairing and un-pairing as well.
But this solution is unworkable under Windows BLE API, because unlike regular Bluetooth, the Bluetooth Low Energy API requires user interaction during both the pairing, and un-pairing "ceremonies". The user must agree to every pairing, and must agree to every un-pairing. This makes this workaround solution a non-starter from a UX standpoint on Windows. My guess is the BLE device manufacturer isn't storing the bonding.
We ran into such a situation ourselves developing our own BLE driver to speak with our own BLE board; we had to re-do the firmware running our BLE device because while it would work under Mac OS and Android with the method described above, the BLE API available on the Microsoft UWP requires user interaction during the pairing ceremony. This seems the right way to do things, anyway.
i know that iBeacon use only advertising channel. which means there is no need connection.
im trying to make my own beacon module which send and receive data.
im curious about what makes the packet in iBeacon format(prefix /UUID / minor/ major/ TX) in communication ?
is it firmware?
let's assume that
when i make my iPhone act as iBeacon , it will send the advertising packet. which means it sends data in iBeacon format. but after turn off the app for iBeacon, i try to use my iphone
to send some files to laptop via Bluetooth low energy mode as usual.
then it will send data in bluetooth standard format. is this right?
given that situation, my iphone can be both iBeacon and just normal phone capable of bluetooth low energy.
i think also the beacon module can be like that. how about the product recently released? like estimote, redbearlab and so on. after connection, do they receive data?
Every iBeacon product works a little differently, but it is common for a product to be connectable for configuration purposes over Bluetooth LE.
Radius Networks' RadBeacon, for example, has firmware that sends out its advertisement as needed to be a standard iBeacon. The same firmware will allow a connection over Bluetooth LE, exchanging data with an external client (the RadBeacon app for iOS). This connectability is outside standard iBeacon functionality, using proprietary techniques that are still part of the larger Bluetooth LE standard.
Your understanding is therefore correct.
Full disclosure: I am Chief Engineer at Radius Networks.