messages from BTLE GATT server truncated at 20 bytes

All messages from a BTLE GATT server are truncated at 20 bytes.

The server can be tested against a different client and the full payload is visible (in my case 150 bytes).

This issue is reported by other users as well (see https://forums.garmin.com/developer/connect-iq/f/q-a/196823/bluetooth-low-energy-mtu-size-for-characteristics)

As 20 bytes is the default MTU size, I guess this bug might be MTU size related.

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CYBERMAN54 over 2 years ago

crosspost, since this bug report seems to be stalled - but still has high relevance.

forums.garmin.com/.../20-bytes-truncation-bug-in-ble-stack-of-edge-devices---please-fix-this-soon
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CYBERMAN54 over 2 years ago

I can confirm this issue and add the following information:

Issue occurs on Edge1030 FW 11.10 & FW 13.40 as well as on Edge 1030plus FW 6.40

Messages are truncated, if the BLE device (operating as GATT server to the Garmin) negotiates an MTU size > 23.

For me, this looks like the Garmin BLE stack is not capable of spanning ATT packets over multiple Link Layer packets. Thus, payloads which do not fit in a single ATT packet, are truncated. Very bad.

If this shall be a feature, not a bug, for some reason, this wouldn't be BLE 4.0 specs compliant to my understanding.
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kaedenb over 2 years ago

Any update on this? I am working on an app that communicates securely with a BLE device, and I need to be able to receive the device's shared key (~200 bytes).
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flogazo over 3 years ago

Is there any more information on this issue?
Is it in some backlog? Is it a hardware-issue that's potentially resolved with newer devices?
Or is it something that might be fixed with a device FW-update or a new version of the ConnectIQ-SDK?
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flogazo over 3 years ago

I'm affected by this too. Can not get more than the first 20 bytes of any Characteristic from my continuous blood glucose monitor. That's barely past the header data. I am supposed to get 363 bytes and only get 3 messages of 20 bytes each instead of 121 each...
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