After returning from my holidays, I've spent the last couple of days hacking on improving the SMS support of OpenBSC. In order to facilitate the intended store-and-forward model, we now store all incoming SMS in a SQL table. Things like validity period or even more esoteric things like SMS compression as per GSM 03.42 is obviously still missing. I try to get it working fist, and leave the gaps to be filled later.
Next will be the code for sending a SMS from an entry in the SQL table, and invoking that code every so often, based on timers and/or events such as a phone registering to the network.
The trickiest part here is how to handle the paging code. We could have a phone call and a SMS, or even more events that all want to page a phone at the same time. There needs to be some kind of arbitration and a queue, deciding what kind of event will first get access to the SDCCH that we have after paging succeeded.
There have also been suggestions to split the SMS processing into a separate process, much like in a traditional GSM network. Sounds reasonable to me, but I am not very familiar with the existing protocols (like UCP) and implementations (like Kannel). So I'll probably leave that as a second step, making the OpenBSC internal SMS handling optional at some later point. UCP would obviously also ease the integration with existing SMS operators (vSMSC and the like).