How to distinguish between random and public GAP addresses?

Hello, I am writing an application in Qt that runs on the computer and interacts with a nRF51422 via BLE. Now I have noticed, that I have to tell the framework, which address type is used. When I try to connect to the nRF51 with Qt set to PublicAddress, I simply get a timeout.

I wanted to establish a connection to the nRF51 and scan for the services to determine if the device is the right one. But then I need to know what type of address I am dealing with.

Is there any algorithm to check if a BLE address is public or random? In the TI forums I read that the two MSBs differ for the three types of random addresses. But is there a similar code to distinguish a random address from a public address?

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  • Hi Andi G.,

    Luckily all that is explained in BT SIG specification. E.g. in v4.2 spec see Volume 6 (Low Energy) Part B (Link Layer) section 1.3 (Device Address) and Volume 3 (Host) Part C (GAP) section 10.8 (Random Device Address). If you want to stick with other spec version you will find similar chapter there.

    If we want to be precise we should talk about Address (MAC) field on 3 layers of BT LE:

    • Link Layer (simple 48-bit field + 2 TxAdd/RxAdd fields in PDU header - see BT SIG specification v4.2 Volume 6 (Low Energy) Part B (Link Layer) section 2.3 (Advertising Channel PDU))
    • Security Manager (providing way how to resolve Private address to Publicly known within installation base)
    • Generic Access Profile (clarifying how each type is tight to device role)

    To make it faster I leave here screenshot from official BT SIG BLE training slides, it should be evident what options are for 48-bit MAC address coding in BLE (note that BLE is using little endian so right-most bytes and bits are in fact most significant aka MSB;)

    BT LE Link Layer:

    image description


    image description

    BT LE Security Manager:

    image description

    BT LE Geneeric Access Profile:

    image description


    image description

    You obviously also could find this kind of question already answered on this forum, e.g. here;).

    Cheers Jan

    ========================

    Edit 06-July-2015

    I've added few more slides and clarification about Address field role in Low Energy Link Layer, Security Manager and Generic Access profile.

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