Hello! We are designing an iPad app that will occasionally connect to our device over BLE. The app is kind of a maintenance/service app that will query various information from the device, for example:
- state
- battery level
- firmware version
- configuration table
- event log
- activity log
and send a few items like
- various commands such as reset to factory defaults
- a firmware upgrade
My teammate has proposed setting up a set of BLE characteristics on the device that publish most of the above individually; a characteristic for state, battery, version, config data, event data, activity data, and a pair of RX/TX channels for streaming data. Generally speaking, a lot of characteristics, each dedicated to a specific type of data.
My inclination is to just set up the RX/TX, and do everything over that one serial channel with a framed message system: "[B2234][S7][C3,27]" tells us the battery level is 2234, the state is 7, and config parameter 3 is 27. Maybe we have a second RX/TX dedicated to binary data, no framing.
So I was hoping you could help us settle this debate. Both of us have a little experience but we aren't experts at BLE.
The advantages/disadvantages of the many-characteristic approach, he says:
- PRO more transparent/explicit what is being passed
- PRO don't have to write any parsing code, data is not fettered with framing data
- CON if you want to transmit something else we need to add a whole new characteristic instead of adding a new framed command, which is more coding
- CON I'm not sure if this is true, but if we add a new characteristic to the device, do we break compatibility with the App?
The advantages/disadvantages of the single Rx/TX are:
- PRO very versatile, if we want to add a command or change the timeliness or pattern of data transmission for a certain data item, we are just modifying C code, not redoing the set of characteristics
- PRO compatibility is a non-issue, any new commands not supported yet by the other side are just ignored or NACKed
- CON as mentioned, you have to write parsing code for the framed data
- CON I think RX/TX over BLE is awkward, in that you have to "batch" up your stream to the size of the characteristic. We'd have to be careful to not lose data.
So I consider the versatility (since we will probably change our dataset a lot) of the single RX/TX to be the winning factor, and he considers the explicit transparency of the many-characteristics approach to be the winner.
I also feel like there are factors we don't know about due to inexperience. Are there costs of additional characteristics, or are they cheap? What are the compatibility issues if the two sides don't match on the characteristics.
So which approach do you guys like?
