iOS Bluetooth dual-mode; connect BLE (GATT) to an already connected BR/EDR (A2DP/HFP) stereo headset simultaneously

Question

I\'m developing a stereo headset with Bluetooth using the classic profiles (HFP, A2DP, AVRCP) as one would expect Ina stereo headset. However, I want to deploy a remote cont

Answer 1

Well, after digging down into specs and trying to understand things more clearly, I've found the answers I was looking for, even though I would have preferred to more optimistic answers... ;(

Nevertheless, let's get to it; the Bluetooth spec for 4.0 (BLE) says that;

Dual-mode gadgets cannot act as BLE peripheral and advertise its presence while still being connectable in "Classic" Bluetooth using BR/EDR.

Furthermore, CSR source-code examples for the CSR8670 dual-mode chipset I'm using all behave in the same say; BLE advertisements as peripheral are disabled when any classic BT-link is connected. Instead, the CSR source code promote that the device should act as the BLE central instead, allowing other BLE peripheral units to advertise and connect to it, all fully doable while streaming audio (acting as A2DP sink).

This does not suite my setup at all since;

1. BLE centrals consume more power than BLE peripherals, and my device needs to conserve energy
2. The dual-mode "combo"-problem of combining a BR/EDR-device with BLE peripheral functionality just moved to the phone instead, which will not work any better since we cannot expect Apple (or anyone else) to violate the BLE spec.

Instead, the recommended approach is let my stereo headset skip BLE entirely and use GATT over BR/EDR instead, also known as advertising via "vanilla", which makes sense really; I mean, I already have an ACL-link setup between two devices, why should I need to kick any sort of discovery mechanism?

Again, the Bluetooth SIG comes in handy;

https://developer.bluetooth.org/TechnologyOverview/Pages/GATT.aspx

GATT and ATT are not transport specific and can be used in both BR/EDR and LE. However, GATT and ATT are mandatory to implement in LE since it is used for discovering services

So, the SIG says "yes" to using BR/EDR as transport for GATT, but the question then instead becomes; how can I then get access to that connected BR/EDR device from within my iOS-app, where the typical scenario is to use CBCentral to scan, discover and connect to a CBPeripheral? The answer is simple; you cannot, since iOS 7.0 does not (yet?) support GATT for BR/EDR;

https://www.bluetooth.org/tpg/showDeclaration.cfm?3A000A5A005C5344535D5414403B0C0D0E2405022413010E57503F202A5A72

So, to sum things up; if you want to have a peer-to-peer setup between two dual-mode Bluetooth devices, using BOTH Bluetooth Classic profiles AND Bluetooth Smart services/characteristics, you should use GATT over BR/EDR, which is no-go for Apple-devices, but might be supported by Android (don't know, will port app to Android eventually though, but regardless it's not a big deal for Android since worst-case will imply a fallback to SPP and a simple byte-protocol to do the work that I need to get done).

That's that. Hope that I've helped someone ;) /Markus