I skimmed through two sets of documents. The first is the OpenSRS programming specification (API) . Pretty straightforward and explained in Perl, but tedious since a lot of stuff go into domain registration. I'm looking at implementing a server for this protocol for my DNS-related project.

The second is the specification of SMS (GSM Short Message Service) in the GSM 03.40 document. This is more for ECE (Electronics and Communication Engineer) types, and not software-developer-me. Well it's still good to look at some background info before implementing the SMS Forum standard, SMPP, or vendor protocols like Nokia's CIMD.