For MNOs, short message service (SMS) has always been one of the major sources of income. Two-factor authentication, emergency alerts, IoT messaging, and many more services are possible due to SMS. In a data-oriented LTE network, optimal support of traditional circuit-switched services is necessary. However, LTE’s transition to an all-IP architecture initially created a dilemma for operators, and it was how to deliver SMS without traditional voice circuits. This is where innovative SMS solutions in LTE were rolled out. In this article, we will discuss the types of SMS solutions in LTE that changed the scene.
SMS Integration in LTE
SMS was introduced in the period of GSM in the early 1990s. It was first started as a simple 160-character text service. As mobile networks evolved from 2G to 3G and then to LTE (Long Term Evolution), one challenge surfaced: how to preserve this service in an all-IP environment. LTE was established by the 3GPP in release 8 and developed as a pure packet-switched system. SMS integration in LTE is the only viable solution. Whether through circuit-switched fallback (CSFB) or modern IP-based IMS (IP Multimedia Subsystem) solutions, LTE has adapted to keep SMS operation.
Classification of SMS Solutions in 4G
From smartwatches to IoT sensors, SMS still drives billions of devices in LTE networks. But how does it survive without standard voice channels? Below are the three solutions that make SMS integration in LTE possible.
1. Circuit-Switched Fallback (CSFB)
Because of LTE’s all-IP nature, SMS requires a circuit-switched mechanism in many networks. Circuit-Switched Fallback (CSFB) system helps in this situation. It is a sophisticated solution that temporarily shifts a device from LTE back to legacy 2G/3G to deliver SMS. In plain words, your phone’s network is taking a quick detour to send SMS in the old-school way before returning to the high-speed LTE network. Here’s how CSFB works:
How CSFB Works:
- Temporary Network Switch: When an SMS is triggered, the LTE device “falls back” to a 2G (GERAN) or 3G (UTRAN) network to access the MSC (Mobile Switching Center).
- RRC Connection Release with Redirection: The LTE base station (eNodeB) ends the LTE session and guides the device to the correct legacy network.
- SGs Interface: This connection between the MME (in LTE) and the MSC enables the signaling essential for fallback.
- Return to LTE: Once the SMS is delivered, the device can switch to LTE for high-speed data usage.
Use Cases:
- Legacy network areas without VoLTE or IMS support
- SMS delivery for roaming users
- Early LTE handsets without advanced capabilities
Limitations:
- Increased latency and battery drain during fallback
- Temporary disruption of the LTE data session
- Not ideal for modern, all-IP networks
2. SMS over SGs Interface (SMS-SGs)
SMS over SGs is another solution to send SMS in LTE networks. It employs the SGs interface, which connects the Mobility Management Entity (MME) in the LTE core to the MSC Server from legacy CS networks. This interface allows SMS transmission via NAS signaling, eliminating the need for CSFB.
How SGs Work:
- After the EPS/IMSI attach, the UE triggers a service request to send or receive an SMS.
- The message is encapsulated in NAS format and sent from the UE to the MME.
- The MME passes the SMS to the MSC, and then the MSC passes it to the Short Message Service Center (SMSC).
- Delivery reports follow the reverse path, confirming message status without switching to legacy networks.
- This entire process uses the SGs Application Protocol (SGsAP) over SCTP/IP.
Benefits
- SMS over LTE radio without switching to 2G/3G
- Works even in networks without IMS or VoLTE
- Useful in early LTE rollouts or mixed environments
- Reduces latency and conserves legacy network resources
Limitations
- Not scalable for large-scale or future messaging demands
- Still dependent on legacy CS infrastructure (MSC, SMSC)
- Doesn’t support rich communication features like IMS
- Limited support in 5G-forward architectures
3. SMS over IMS (IMS-SMS)
IMS is the most advanced and latest SMS solution in LTE networks. It allows short text messages to be exchanged over the IP Multimedia Subsystem (IMS), which means no more circuit fallback, just an all-IP-based architecture. It’s the preferred approach in fully VoLTE-enabled networks and sets the stage for seamless messaging, even in 5G networks.
Key Features
- IMS Architecture: Programmed on an IMS core using components like P-CSCF, S-CSCF, and the HSS, this solution sends SMS via SIP signaling to ensure safe and consistent delivery.
- Role of IP-SM-GW: The IP Short Message Gateway (IP-SM-GW) acts as a bridge between SIP-based IMS messages and legacy SMSCs to handle interworking and delivery reports.
- SIP MESSAGE: Instead of typical SMS protocols, messages are transmitted as SIP MESSAGE requests. They are compact, secure, and real-time.
- Dependent on VoLTE Support: Since this solution needs an IMS infrastructure, it’s only available on VoLTE-capable devices and networks.
Benefits
- Native messaging support in pure LTE/5G networks
- No need for CS fallback or SGs interface
- Full integration with voice, video, and data services
Drawbacks
- Requires complex IMS infrastructure
- Not universally supported in roaming services
Summing Up
In short, there are three main SMS solutions in the LTE network—CSFB, SGs, and IMS. Each has unique deployment requirements in legacy and modern networks. For operators aiming to upgrade without disrupting legacy services, choosing the right SMS solution is crucial. In light of the introduction of 5G, IMS-based SMS stands tall as the most future-proof solution right now. ComCode Technologies plays a huge role in enabling this transition by offering telecom software and consultancy for MNOs, MVNOs, private LTE, and beyond. Give us a call for a free quote on your project!