We can identify two ways to operate SMS for PS-only network:
"SMS in MME" as elaborated in the TS 23.272 (Annex C) and the TS 29.338. It consists to a Diameter reference point (SGd) between the SMSC (IWF) and the MME
"SMS over IMS": As elaborated in the TS 23.204
I wonder why two different methods were designed.
In "SMS over IMS" the Short messages are transferred over the IMS PDN connection while in "SMS in MME", PDN connection is not mandatory.
Therefore, it seems that the "SMS in MME" replicates the existing SMS over CS that allow communication with the UE without the need of DATA channel.
The question is why the "SMS over IMS" is designed? Is it to cover the case of SMS without MSISDN?