on bluetooth.org I saw that one BLE characteristic can have multiple field. I now searched for a while but did not get an answer about the byte order.
For example this characteristic: https://www.bluetooth.com/specifications/gatt/viewer?attributeXmlFile=org.bluetooth.characteristic.gatt.service_changed.xml
It has two fields. Is "Start of Affected Attribute Handle Range" the higher 16 bit or the lower?
Regards Maz
GATT fields are always (or at least should always be) little-endian. This is discussed in the Bluetooth Core Spec.
From v4.2 of the spec, Vol 3, Part G (which covers GATT), page 523:
2.4 Profile Fundamentals
...
• Multi-octet fields within the GATT Profile shall be sent least significant octet first (little endian).
Be very careful reading this spec because there are pieces that are in network order (big-endian), but GATT attributes are always supposed to be in little-endian.
(The only reason I a say "should always be," is that the one rule of bluetooth devices is that you will always find some device in the field that breaks the rules.... But the spec is clear.)