Develop Reference

Is Bonding required for BLE <-> Android, iOS device communication?

We have a BLE device and android/ios app for communication with the device.
The requirement is to have an android/ios app connect to the device without any pairing request. No security is required on the device.
Hereafter a few questions around this:
Q1. Is it possible at all to have an android/ios app connect to a BLE
device without a pairing request?
Q2. Does the bonding required for
the BLE communication?
Q3. What is the downside of having the device
with no bonding? Is it going to drop the connection while the app is
running?
Q4. Who is the initiator or pairing dialog, the app, or the
device?
Q5. When having the device with Just Works security protocol,
with no bonding and no MITM protection, should we see a pairing
dialog?

Please see answers to your questions below:-
A1. Yes it is possible to do this because pairing and connection are two separate actions. Connection is when you establish a communication channel with a remote device, while pairing is when you exchange security keys with the device and have the option of encrypting that connection.
A2. No, pairing/bonding is not required for BLE communication but it is preferred in order to have extra features and extra security on that connection.
A3. No, having no bonding does not affect the stability of your connection/communication. There are a few downsides which include less security on the connection and also potential inability to find the device if privacy is implemented. More details are provided in the link below.
A4. The app is the initiator of the pairing dialog. Specifically, the central device (the one that initiates the connection) is the one that is responsible for initiating pairing. When this central device requests to pair to the remote device, the pairing dialog is shown.
A5. I believe this is dependent on the OS type (Android/iOS) and the version number of that OS, but generally speaking the pairing dialog should not pop up because as the name suggests, it should just work. Pairing dialog pops up when MITM protection is requested.
You can find more details about pairing/bonding and general BLE communication is the answer below:-
Should one create a bond with a Bluetooth LE device
I would also recommend trying the nRF Connect app on both iOS and Android to see the difference between connection and bonding and to verify that you can have normal connections without pairing.

Based on what we got working and based on the answers to this question.
A1: Yes, it's possible if no MITM is set and the Security scheme is set to Just Works, which means no security at all.
A2: Bonding is not required for BLE communication. If the BLE device can't store the Bond information, it may lead to some issues as we got in our case. If the BLE device is able to store and use Bond information then it's recommended to Bond.
A3: The Connection will not be closed. The only downside we got is that now it takes a little more time to reconnect.
A4: The app is initiating the connection, but the dialog is shown only if the BLE device requests some type of security, it can be MITM, encryption, pin. Short answer, the pairing dialog can be avoided by changing the settings on the BLE device.
A5: When having the device with Just Works security protocol, with no bonding and no MITM protection the pairing dialog should not be presented. We got no pairing dialog experience on both iOS and Android.

To add to Youssif's answer:
A3: one difference is that connection setup will be faster since bonding allows the GATT db cache to be used. Otherwise service rediscovery must be done on every connection (if the service changed characteristic is present).
A4: on Android you can call createBond to start pairing, but on iOS there is no API for the security, so either you need to interact with a characteristic that sends an error that pairing is needed, or the remote device can send a Security Request to initiate pairing.
A5: iOS shows a dialog "accept / abort" for just works, Android doesn't.

Is there any access to send and receive radio waves using cellphone antenna?

There is so many Walkie Talkie APPs all around the web, that are using WiFi or Bluetooth to transferring the data (at least all the ones I've ever seen) and none is using inner antenna to transmit data over radio waves as a real Walkie Talkie device.
Is there any security reason for it? or is it restricted to access the antenna as a sender/receiver?

The thing you are looking for is peer-to-peer connection using mobile networks. It is impossible due to the radio services the mobile networks are licensed under vary by country as stated here.
Technically as said here You need a cellular network provider to communicate between two devices.
If you still wanna make a P2P communication Use Wi-Fi peer-to-peer (P2P) API
Follow https://developer.android.com/training/connect-devices-wirelessly/wifi-direct

Bluetooth proximity scan while paired

I have been doing a ton of research on this project I want to do, probably read every SO post (bluejacking, RFCOMM, beacons, etc...) and my head is spinning.
I want to create a system at home, using Arduino (with bluetooth le) that will scan bluetooth and detect when my phone or any of my family members phones are in proximity. Whether this is a MAC scanner or simply getting the friendly name. I would also like to get the RSSI. I dont need to do accurate distance measurement but I would like to know approximately how close they are. Does anybody have any suggestions for this approach? I know I can create an app on the phone and have it become a beacon and this may be an option but I am concerned with Android, it would have to be Marshmallow 5.0 at a minimum. I would like support to be a little lower than that, like JB or KK. So, it seems like there should be a simple scan, just looking for the MAC or name. That is all I really need.
That said, the main question I have is, can you scan for and see bluetooth devices in range if they are already paired to something? Like a Bluetooth headset? I understand bluetooth is point to point but does it allow multiple connections or at least broadcast the mac or name when paired?
This question, along with any general suggestions will help me out tremendously!
Thanks!

Bluetooth Discovery from the Listener involves Listening for Bluetooth Inquiry packets(RX) and inquiry response (TX).if done continuously this will deplete the battery. Hence most devices (including Mobile Phones and Headsets) has a policy of when to be discover-able and connectable. Headsets in particular are discoverable only when they are instructed and connectable until one device is connected to it ( There are probably exceptions where the headset has the capability to be connected to multiple phones at the same time). Most Mobile Phones are discoverable only when they are in settings screen ( Android,IOS and Windows Phones), but unlike the headset case they are connectable.
so one option with the MobilePhone is to see if it is possible to connect to them after pairing once ( for example by trying a service search on the remote device and see if it returns the service attributes, this internally makes a connection and indirectly ensures the presence of the device).
if you are specific about a mobile phone which is already paired then you might need to establish a higherlayer connection ( such as SPP) with Authentication and encryption. This would force a two way check between devices to see whether they are already paired. other less secure option is to rely on the Phone (Android and IOS phones doesn't generally Allow a higher layer connection without encryption/pairing) and try to establish an SPP connection to the phone.
Another option is to use Bluetooth-LowEnergy. But Note that security in BT was superior to BLE till BTv4.1 and same on BTv4.2.

Wi-Fi vs Bluetooth for peer to peer connection

I am developing an p2p application in android for an educational project in which I want to form groups android phones of students nearby and exchange sensor data in a university campus.
Now there are some considerations :
Devices will automatically discover each other and upon discovering connect and exchange data.
The process runs for a long time maybe 4-8 hours per day. (process of periodically sensing data and exchanging )
Now the p2p groups can be formed using either Bluetooth or WiFi (Not WiFi Direct, simple UDP packets over WiFi considering phones are connected on campus WiFi).
What are pros and cons of using Bluetooth and WiFi in this scenario in terms of reliability, power usage of phones, scalability and any other you can suggest.

Among other answers and input, I would added this answer.
First of all, before we chose WiFi or Bluetooth we need to find out the difference between those two technologies.
I have made comparison chart that covers some of the important information you might need regarding your project.
Note: There are different versions of Bluetooth's and WiFi, this chart is to represent the the general picture of Standard Bluetooth,
Bluetooth v4 and WiFi. It is always suggested to refer to manufacture
specification of each technology.
From the chart we can conclude that Bluetooth has lower power consumption vs WiFi, but at the other hand WiFi has more bandwidth than Bluetooth.
Range in general is just approximation, a lot of things affecting range like human body, obstacles, location (inside or outside), if inside; structure type and materials used inside the building, noise from other sources and devices etc.
(*) Regarding scalability, I have tested WiFi and Bluetooth v4, both system with up to 8 devices, where one of those is host (group owner, server) device and 7 others are guest (clients). see the figure below.
What regards reliability, with Bluetooth v4 I have had some time connectivity problems, but when it works than every-thing is fine.
Note: Bluetooth v4 is not back compatible with older versions of Bluetooth, so if your host is Bluetooth v4 than all other clients
should have Bluetooth v4 or vice versa.
So I will not say which one is best, but if you need longer battery life and light data communication than Bluetooth is the way. Regardless if it is Bluetooth OR WiFi you might need to start with Bluetooth to and test it, if you are happy with it than keep it, otherwise switch to WiFi.
In case you want to build your own code, the code example I followed and used previously for another university research project. It is based on 8 phones (host and client) as seen in the figure above, we collected sensor information and send it to host phone using Bluetooth 4 connection. The source code we used for that can be found here. The same project has WiFi and other type of connections.
Android official google documentation has some information and code example regarding WiFi peer to peer connection, you can follow with example of the code as well.
Regarding collecting your sensor data and sending those to one device. You could added a method that starts collecting sensor or what ever data, and after connection is established successfully than start sending it over to the other device.
As others suggest https://developers.google.com/nearby is also a way to go.

As you can rely on campus Wifi, I would definitely go with the implementation of Google Nearby APIs in my App as it was designed for such use cases...
The way it works answers your question : it makes all the heavy stuff for you, including choice between wifi or Bluetooth for better performance...

Google Nearby is definitely a good choice. You don't have to tackle all the problems when working with WiFi or Bluetooth directly. But Google Nearby only works when both devices are online and have their screens on. For a more critical review of Nearby have a look at http://blog.p2pkit.io/how-google-nearby-really-works-and-what-else-it-does
If you can not accept these limitations, you should look into other frameworks like http://www.p2pkit.io.
Disclaimer: I work for Uepaa, developing p2pkit for Android and iOS.

What are good ways (existing) to transmit data between multiple mobile phones without internet?

Background
I have an idea for an app on vacation that needs to communicate to other phones with the same app. While on vacation those phones might not all have internet as roaming can be very expensive. The data is not a lot: like 500 kB max would suffice (in json).
Every phone has a bit of info that all the other phones would like to know, but if it helps the info can be stored on 1 phone (master phone from now on) and shared later to the other phones when back home over internet.
Phones
Android, iPhone and Windows Phone
We can't assume they have NFC, IR or zigbee. Just the hardware almost every phone has like bluetooth, camera, microphone etc.
My ideas
QR codes that changes, based on new info: If the first phone is scanned the second phones QR code has data from the 1st phone and itself and the 3rd phone has data from the 1st, 2nd and 3rd (itself) until it reaches that master phone that holds all data.
Data transmission trough sound that we can't hear (or we can). Con is that I don't know if something like this exists for mobile platforms and writing it is like a 3 year master thesis project.
http://nearbytes.com
https://applidium.com
https://developer.chirp.io/
Bluetooth. Can we connect like 8 devices? Would it work consistent (connecting even my headphones can be a hassle, what about 8 phones who try to connect simultaneously)
All of these ideas have big cons. Maybe I'm overlooking a better way.
I will add a bounty to the question for the best solution
An answer that explains it with a little bit of code reference (link is ok) is always better than just: "use bluetooth man"

TL;DR
The easiest (and most supported) way of getting multiple devices to connect to each other is using WiFi. Since your goal is to achieve data transfer with no internet, the most appealing solution would be to use a Peer-to-Peer network structure.
The two major smartphone OS's (Android and iOS) have API's and documentation on creating and transferring data over a Peer-to-Peer network.
Android WiFi P2P
Apple Multipeer Connectivity
These two also have a means to encrypt the data being transferred.
Windows doesn't seem to have an API to allow multiple peers connected, but their Proximity Class will work for one device at a time.
I can give a few outlines over the different options in each major OS:
Android
Android's WiFi P2P (peer-to-peer) API was created for transferring data without internet or another network.
From their documentation:
The Wi-Fi peer-to-peer (P2P) APIs allow applications to connect to nearby devices without needing to connect to a network or hotspot (Android's Wi-Fi P2P framework complies with the Wi-Fi Direct™ certification program). Wi-Fi P2P allows your application to quickly find and interact with nearby devices, at a range beyond the capabilities of Bluetooth.
Google even has Documentation and training on this API.
iOS
Apple's Multipeer Connectivity.
Very similar to Android's P2P API, they claim:
The Multipeer Connectivity framework provides support for discovering services provided by nearby iOS devices using infrastructure Wi-Fi networks, peer-to-peer Wi-Fi, and Bluetooth personal area networks and subsequently communicating with those services by sending message-based data, streaming data, and resources (such as files).
Here is a decent looking tutorial on using Multipeer Connectivity.
--EDIT--
Another iOS way of doing this, which is a bit of a mis-utilization(?) of the tool, is by using GameKit.
However, I think that to get it to work for your purposes might result in a bit of a hack, since the "players" have to be using Game Center.
Windows
The only way (apparently) to connect phones in Windows Phone, is by using Proximity, however, that only gives you the option of connecting no more than two phones together.
They state:
Proximity is a great way to create a shared app experience between two instances of your app running on two different devices.
Those are options in each of the major mobile device OS's.
App usage could be something like:
Decide which device was going to be the "master", so that other devices can connect to it. It isn't required to know this before deploying the app, but there should be a way for the user to decide whether he is going to be a client (receiving data) or the server (pushing data).
Once it was decided between the group of devices which was going to be pushing data, that device would have to be registered as the server (in the Android P2P API, you can establish a "group owner"), and then start looking for peers by initializing the service.
Then, once the devices are connected to the master device, you can start pushing data. An additional bonus is that when using Android WiFi P2P, all communication is encrypted with WPA2, and with iOS, you can enable encryption using MCEnableEncryption (however they state that is slows down data transfer rate).
Now you would just have to pick one method to go with, and make sure that all the phones ran that OS. Because these three methods of connectivity won't work together.
All of the three methods listed are done programmatically, so there should be no strange or odd things that your user will have to do. Searching for other devices, connecting, and transferring data can all be done within your app.
More help can be provided if the question is narrowed down to specific problems, but this should be enough data to get you started.

Don't try QR or sound. I think it would be very painful to transmit 500kb of data.
Bluetooth seems like a good solution but maybe, as you already said, hard to configure.
What do you think about wifi?
At least every Android and iPhone device can create a mobile wifi hotspot. By using this, you can easily setup a environment where 8 devices are in the same LAN (without using the internet by any of your devices).
Now your "master phone" runs a simple server to synchronize data (just like an internet server would do). Every of the seven clients could receive the ip adress of you master by scanning a simple QR code or sending a short message and afterwards configure itself accordingly.

Have you checked Alljoyn?
As quoted:
"Developers can write applications for interoperability regardless of transport layer, manufacturer, and without the need for Internet access"

You can create a Wifi connection between your devices. Than after connection it creates local network between your devices. Inside this network you, of course, can interact between your devices using TCP/IP connection. It works both on Android and iOS. Simply lauch your app as server on the one device
EDIT
Note, you have to connect your devices using any network. It is possible to connect the devices by initializing your device as WiFi-router. It can be both Android and iOS. If it is possible, you can connect your devices to any wifi connection.
Than, launch your app as Server-socket, the others as clients.
for Android (java) server use this link:
https://docs.oracle.com/javase/tutorial/networking/sockets/clientServer.html
try (
ServerSocket serverSocket = new ServerSocket(portNumber);
Socket clientSocket = serverSocket.accept();
PrintWriter out =
new PrintWriter(clientSocket.getOutputStream(), true);
BufferedReader in = new BufferedReader(
new InputStreamReader(clientSocket.getInputStream()));
) {
for android device client:
try (
Socket kkSocket = new Socket(hostName, portNumber);
PrintWriter out = new PrintWriter(kkSocket.getOutputStream(), true);
BufferedReader in = new BufferedReader(
new InputStreamReader(kkSocket.getInputStream()));
)
The same idea is for iOS (Objective-C):
server
https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/NetworkingTopics/Articles/UsingSocketsandSocketStreams.html#//apple_ref/doc/uid/CH73-SW8
and client:
https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/NetworkingTopics/Articles/UsingSocketsandSocketStreams.html#//apple_ref/doc/uid/CH73-SW4

A better way could be use Ble.
It's easiers to connect the phones because you don't need user confirmation.
Seems like you can connect up to 20 devices Maximum number of peripherals on CoreBluetooth?.
To transfer 500KB should require few minutes (may be between 2 and 5).

You can track an Android device without Internet via GPS.
Connection without The Internet:
SMS
USSD
DTMF (very slow)
How to design a tracking device on USSD is mentioned at
M2M IoT Cookbook
How to develop a device based on Wireless Wide Area Network modules
You also can use the Android phone as a data logger and store under the Micro SD Card and read the card by:
Replacing the SD card to your PC
Streaming the data local by Bluetooth
Forwarding the data at home by Wi-Fi
Or Possibly:
Your app can use SMS API to transmit the DATA or other SOURCES.