I'm developing an IoT application. There is a microcontroller which controls the sensors. Smartphones connect them and perform some actions. I want smartphones to connect multiple devices and control them and in the meantime I dont want internet access of the smartphones to disconnect. What I want to ask is which wireless protocol would be good for this job. Should I use WiFi, Bluetooth, Wifi-Direct or anything else ?
Actually, according to my searches, "WiFi-Direct" seems good option to me, but I wanted ask to you whether it is a good options or not. In addition, please state which specific module can be used such as "HC-06" "ESP8266" and why? while stating which protocol is good for me. Thanks in advance :)
It's a hard question. Answer is depends on many aspects:
Let me cover these topics a little more:
This is the easiest one. Bluetooth (depending on class, usually it's II) have a range about 10m (30ft). Wi-Fi (and Wi-Fi Direct which is almost the same) have range about 30m (100ft). So if all your sensors located on a user's body (like wearables) or at the desk (like mice, keyboard) then Bluetooth is fine. If devices are spread over larger area (like temp-sensors in each room), then Wi-Fi based protocol is better.
This is also quite easy. Bluetooth is capable of connecting not more than 8 devices. One of them is master and up to 7 slave devices. Wi-Fi can handle theoretically up to 255 devices, but in practice it's less (due to interference). Usually if you have more then 20-25 devices you need another access point.
If your devices are connected to a power-line (like smart wall sockets or smart light bulbs) then they easily be connected using Wi-Fi. But if it's a button-cell battery then it better be Bluetooth Low Energy (BLE) (a.k.a Bluetooth Smart or Bluetooth 4.0). However there are examples where TV remote control running on 2 AAA batteries uses Wi-Fi Direct.
Android can do anything, but iPhone is a headache. It does not support Wi-Fi Direct, it does support Bluetooth, but only limited number of BT profiles. If you need to support other BT profiles you need to participate in Apple MFi program and got a license and certification from Apple. Another option is to use Bluetooth Smart technology (BLE). More information about Apple restrictions are here and in the FAQ.
I won't advice about concrete chip-sets, because there are so much choices on a market, also I don't know your budget. However you can try to look at well know brands like Broadcom, Texas Instruments, Marvell and etc. - they provide a chip-sets for any choice and gust.
If you need to support iPhone and all other requirements (range, amount of sensors, etc.) are OK with Bluetooth, I would recommend to use Bluetooth Smart (BLE) as a simplest choice.