Getting started on USSD and SS7

Good day all!

I've recently started working as an Erlang developer and have had some experience with writing USSD applications.

I would really like to specialize in USSD and SS7 as my two main focuses.

Can anyone point me in the right direction where I could find some material to start studying?

I've looked around, but I've mostly been able to find only implementation specific tutorials. I'm looking for some theoretical background, especailly on SS7.

Thanks in advance!!