How SMS works in LTE
SMS stands for short message service.The “short” part refers to the maximum size of the text messages: 160 characters (letters, numbers or symbols in the Latin alphabet). For other alphabets, such as Chinese, the maximum SMS size is 70 characters.
There are two ways to implement SMS in LTE. The ideal solution would be doing SMS using IMS.IMS over LTE is specified to transfer any form of data (e.g, voice, SMS and any other form of multi media data), IMS is still in its early stage as many network operators have not implemented IMS over their networks.
The other solution is called SG LTE. (It is like we have CS Fallback as an interim solution before they fully implement voice call over IMS).
The implementation logic of SG-SMS is very similar to WCDMA SMS. In WCDMA, we injected the SMS message into a DCCH channel and send it to the destination. It means that we carried the message over a control channel, not over a data channel. SG SMS is also using a similar concept, we send the message over a control channel. For Example,
i) UE <– NW : RRC Connection Reconfiguration
ii) UE –> NW : RR Connection Reconfiguration Comlete
iii) UE <– NW : dlInformationTransfer (embedd the SMS message – CP Data – into this message)
iv) UE –> NW : ulInformationTransfer (embedd CP-ACK into this message)
So the SMS is actually encapsulated in the NAS signaling protocol used between the UE and MME, and forwarded to and fro the MSC/VLR whereby the UE was registered in the combined EPS/IMSI attach procedure. This new interface between MME and MSC/VLR is called the Sgs interface. All entities supporting CSFB (MME, MSC and UE) are required to support SMS via Sgs although the reverse is not true (entities supporting SMS via Sgs not necessarily have to support CSFB).
SMS via Sgs is relatively easy to implement, involving only the MME, MSC and UE, out of which only the MSC belongs to the legacy CS network. SMS via Sgs also do not have known issues or deployment complexity.