SMS without MSISDN in IMS

Description

SMS without MSISDN in IMS (SMSMI) is a capability defined in 3GPP TS 29.338 that allows SMS delivery via IMS to devices which are not associated with a classic telephone number (MSISDN). In traditional SMS, the MSISDN is the primary key for routing and identifying the subscriber. However, for many IoT/M2M devices, such as sensors, meters, or vehicle modules, allocating an MSISDN per device is inefficient from a numbering resource perspective and often unnecessary for their communication patterns. SMSMI solves this by leveraging IMS identities and subscription profiles that are decoupled from the MSISDN.

Architecturally, SMSMI involves several IMS entities: the IP Short Message Gateway (IP-SM-GW), the Serving-Call Session Control Function (S-CSCF), the Home Subscriber Server (HSS), and the UE. The key innovation is the use of an alternative identifier for SMS routing. This can be an IMS Public User Identity (such as a SIP URI, e.g., sip:[email protected]) or an External Identifier (as defined for MTC). The HSS stores the subscriber profile for the device, linking this alternative identity to the IMS subscription and indicating that the subscriber is enabled for SMSMI. When an SMS is to be delivered to such a device, the originating entity (e.g., an Application Server or another UE) addresses it to this alternative identifier.

The IP-SM-GW plays a central role. It receives the SMS submission, typically encapsulated in a SIP MESSAGE request. The IP-SM-GW interrogates the HSS (via the Sh or Cx interface) to retrieve the subscriber's SMSMI service profile and to obtain the necessary routing information. Since there is no MSISDN, the HSS may provide an IMSI or an MSISDN-less subscription indicator. The IP-SM-GW then performs the interworking function to ensure proper delivery. It may need to translate the destination address and invoke the appropriate IMS service logic in the S-CSCF. The S-CSCF applies initial Filter Criteria (iFC) to route the SIP MESSAGE to the correct Application Server if needed, and ultimately to the UE if it is IMS registered.

For the UE side, the device must support IMS and be configured with its IMS Public User Identity. It registers with the IMS network using that identity. When an SMS arrives as a SIP MESSAGE, the UE's IMS client processes it as a normal SIP message. The content of the SIP MESSAGE body carries the SMS TP (Transfer Protocol) payload. This allows the device to send and receive SMS using its IMS identity, completely bypassing the need for an MSISDN in the signaling path. This model integrates seamlessly with IMS-based communication, enabling SMS services for a new class of devices within the all-IP network architecture.

Purpose & Motivation

SMSMI was created to extend SMS services to devices in an IMS environment that lack a traditional MSISDN, a common scenario in the Internet of Things (IoT) and Machine-Type Communication (MTC). The primary problem it solves is the inefficient use and exhaustion of MSISDN numbering resources. Allocating a unique, routable phone number to every sensor, smart meter, or telematics module is not scalable and is often regulated. SMSMI enables these devices to use SMS—a reliable, widely supported, and low-bandwidth messaging protocol—without consuming scarce E.164 numbers.

Historically, SMS was tightly coupled to the MSISDN as the subscriber address. As networks evolved to IMS and operators began deploying large-scale MTC services, this coupling became a limitation. Early MTC solutions sometimes used MSISDNs, but this was seen as a wasteful workaround. The motivation for SMSMI was to fully leverage IMS's identity management, which is based on URIs and not limited to E.164 numbers. This allows operators to assign more flexible identifiers (like SIP URIs or External IDs) to IoT devices, simplifying provisioning and management.

Furthermore, SMSMI addresses the operational need to support legacy SMS applications and services for new IoT devices. Many M2M applications rely on SMS for control commands, data reporting, or firmware updates due to its simplicity and universal support. By enabling SMS over IMS without an MSISDN, operators can offer these existing M2M solutions on modern, all-IP networks without redesigning the application layer. It also facilitates convergence, allowing a single IMS core to serve both traditional human subscribers (with MSISDNs) and machine subscribers (with alternative identifiers) for messaging services, thereby reducing operational complexity and cost.

Key Features

• Enables SMS delivery over IMS to subscribers without an assigned MSISDN, using alternative identifiers like SIP URI or External Identifier
• Utilizes the IP-SM-GW to perform address resolution and interworking between SMS protocols and IMS SIP messaging
• Relies on HSS profiles that indicate SMSMI subscription data and map the alternative identity to IMS subscription parameters
• Supports both mobile-originated and mobile-terminated SMS procedures for MSISDN-less devices within IMS
• Integrates with IMS service control via Initial Filter Criteria (iFC) in the S-CSCF for potential application server interaction
• Facilitates Machine-Type Communication (MTC) and IoT deployments by conserving E.164 numbering resources

Evolution Across Releases

Defining Specifications