SMS over LTE: Methods and Call Flows

There are two main ways SMS can be delivered over an LTE system. This is known as SMS over LTE. It can be delivered via a Short Message Service Center (SMS-C).

• SMS Over SGs: Here, the LTE UE (User Equipment) device sends and retrieves CS (Circuit Switched) based SMS messages.
• SMS Over IP: Here, the LTE UE device sends and retrieves SIP PS (Packet Switched) based SMS messages.

SMS over SGs transfers the messages between the MSC (Mobile Switching Center) and MME (Mobility Management Entity) and transports the messages over the LTE system using the SGs interface. In this type, the LTE UE does not fall back to legacy cellular systems such as GSM or CDMA. Existing roaming conditions will be reused in this type of SMS over LTE.

In SMS over IP mode, SIP based SMSs are carried via the IMS core element. In this type of SMS over LTE, the SMS to be transmitted is encapsulated in a SIP message in the IP-SM-GW entity. This type needs an LTE UE having both IMS and SIP protocol stacks, as well as an IP Short message gateway (IP-SM-GW), an IMS core, and an HLR/HSS (Home Location Register/Home Subscriber Server) which supports SMS over IP with the help of home routing.

LTE Mobile Originating SMS Call Flow - LTE UE SMS MO Call

The figure above depicts the entire LTE mobile originating SMS call flow. SC (Service Center) is responsible for relaying and forwarding the SMS. SMS-IWMSC is the interworking MSC for the SMS system. HLR/HSS stands for Home Location Register/Home Subscriber Server.

Let us understand the LTE SMS MO Call procedure:

• After the EPS/IMSI attachment, the LTE UE triggers a service request in order to initiate mobile originating SMS.
• The SMS is encapsulated in a NAS message and transmitted to the MME entity.
• The MME forwards this SMS to the MSC/VLR.
• The MSC/VLR acknowledges the receipt of the SMS to the LTE UE.
• The SMS is forwarded to the service center (SC) which returns a delivery report back to the UE as shown in step-7 and step-8.
• The UE acknowledges receipt of the delivery report to the MSC/VLR entity.
• The MSC/VLR informs the MME that no more NAS messages are in the queue.
• A release request is sent from the MSC/VLR to the MME.

LTE Mobile Terminating SMS Call Flow - LTE UE SMS MT Call

The figure above depicts the entire LTE mobile terminating SMS call flow. The SMS retrieved by the LTE compliant UE from the service center is referred to as an LTE SMS MT Call, and the SMS originated by the UE is referred to as an SMS MO call.

Let us understand the SMS MT call procedure below:

• The service center starts the transmission of the SMS.
• Based on routing information available with the HLR, the SMS is forwarded to the right MSC/VLR in the system. The LTE UE is attached with this MSC/VLR.
• The MSC/VLR initiates a paging message to the MME.
• The MME transmits a paging message to each LTE eNodeB on the tracking areas where the UE is registered.
• After success in paging, the UE transmits a SERVICE REQUEST message to the MME.
• The MME issues a service request to the MSC/VLR.
• The MSC/VLR packetizes the SMS and forwards the same to the MME.
• The MME encapsulates the SMS in NAS format and delivers it to the UE.
• The LTE UE acknowledges receipt of the SMS to the MSC/VLR entity. It also issues a delivery report. This report is forwarded to the service center.
• The MSC/VLR acknowledges receipt of the delivery report to the UE. It also informs the MME about no NAS messages being left in the queue.
• Finally, the MSC/VLR initiates a release request to the MME.