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bluetoothgattbluetooth-lowenergy

How come it is always the GATT server that exposes services?

Looking at various GATT-based profiles, it seems that services are always exposed in the GATT server rather than the GATT client. For instance, the Time Profile (TIP) has the server exposing the Current Time Service (CTS). So, if a phone is to update a heart rate monitor with the current time using TIP, the phone will be the server whereas the monitor will be the client. But, being a heart rate monitor, the Heart Rate Profile expects the monitor to be a GATT server.

So, for a monitor that takes the current time from a phone, should it be a GATT client or server? Should it be set as a client whilst time syncing with the phone and set as a server otherwise? Should a custom profile be implemented such that the CTS is exposed in the client instead?

Thanks

Solution

  • Generic Attribute Profile (GATT) defines how server and client communicate with each other using Attribute Protocol for the purpose of transporting data. Client and server roles are determined when a procedure is initiated and released when the procedure is ended. Hence, a device can act in both roles at the same time.

    I would suggest you to read Bluetooth Spec. In Part G 2.2 it explains the roles and configurations.

    Client—This is the device that initiates commands and requests towards the server and can receive responses, indications and notifications sent by the server. Server—This is the device that accepts incoming commands and requests from the client and sends responses, indications and notifications to a client.

    Back to your question:

    The Time profile enables the device to get the date, time, time zone, and DST information and control the functions related the time.

    In your case, the monitor will be the GATT client when it takes the time from a phone. However, it can be a server at the same time for another procedure (operation, request etc.) with the phone.

    In short, client and server roles are not fixed to the devices. When your phone exposes the current time, it will be server. Similarly, when it gets the current time from the monitor, it will be client. no need to customize the profile. If you want your phone to get the current time from a device and expose it to another device, just implement the same profile for client and server roles to your phone.

    EDIT:
    According to TIP profile spec, to get the current time information, the GATT Read Characteristic Value sub-procedure shall be used with the handle of the Current Time Characteristic. Monitor as a client will read the Current Time Characteristic from the GATT Table of the server (in this case it is the phone). As soon as the monitor retrieves the value from phone, it can update its Current Time Characteristic Value, and expose it to its environment in three ways:

    1. Notifying it to its subscribed clients (BLE notifications). If you do it in this way, you will customize the Bluetooth TIP profile since this procedure is not defined there (I had a quick look to the document and didn't see it).
    2. Broadcasting it in the advertisement packet (Doesn't require BLE connection)
    3. Another BLE device connects to the monitor and reads the Current Time Characteristic value. This is the recommended way if you want to use Bluetooth SIG defined TIP profile as a server.