data to the network by using the RSL RLL DATA REQUEST primitive.

Also, while we are tuned to any BCCH/CCCH, the L3 frames are sent from L2 to L3 in RSL RLL UNIT DATA INDICATION messages.

What is not as simple is the non-RLL part, i.e. sending the first RACH request, ...

For those, we will have to invent our own new RSLms message types, possibly reusing as many RSL information elements as possible - meaning we can share more code/structure with what we already have in libosmocore/openbsc

is there a better (simpler) way?

The RSLms approach might not be the simplest one, but I think it would be a very nice/orthogonal approach. Plus: In this model, we can eventually move the L2 implementation into the MS firmware, while talking RSLms over HLDC to L3 on the PC.

Regards, Harald