your conclusion is interesting, i’ll answer it directly. Why we do SMS over SACCH, and not LU over SACCH ?
well, very simply because there is no need to perform a location update while on TCH. During dedicated mode, the MSC knows where is located the MS at the cell level. If a new call is incoming for this MS, then the MSC can obviously route it through the new cell without any issue. Even though it is not the same LAC as where the first call was initiated.
There is no need of paging.
The SMS is indeed send on the SACCH if the user is in dedicated mode. We need to do this because, contrarily to the LU, it is mandatory to deliver a SMS.
I believe that a second call (call waiting) is an information sent on the FACCH, but i have to check it out. (I had to check it out last time already, but i forgot)
I hope that helps 🙂 Just a thought : i wouldn’t want to be the guy who crosses a LAC border while talking on the phone, and also receiving a SMS and getting a SECOND call. what a life !