- What it is: public warning alerts carried as cell broadcast messages over the 5G Broadcast carrier.
- Why it matters: It reuses the 5G Broadcast receiver rather than needing an alert-specific one.
- In this section: This page covers the transmit and receive roles, the cell broadcast message model and the wider warning chain.
How it works
An alert is passed to the transmitter, encoded as a Cell Broadcast Service (CBS) message carried in a System Information Block, broadcast over the FeMBMS carrier, and received and presented on a device, reusing the 5G Broadcast receiver rather than requiring an alert-specific one (ETSI TS 103 720 V1.2.1 clause 5.15.3.2; ETSI TS 103 720 V1.2.1 clause 5.15.4; ETSI TS 103 720 V1.2.1 clause 5.15.3.3; TS 36.331 V19.4.0 clause 5.2.1.3; TR 36.976 V19.0.0 clause 1; ETSI TS 103 720 V1.2.1 clause 5.15.1; ETSI TS 103 720 V1.2.1 clause 7.6).
Key specifications: ETSI TS 103 720 (5G Broadcast System for linear TV and radio services, clause 5.15.3.3 defines PWS support, the delivery of warning messages via SIB broadcast on the E-UTRAN Uu downlink), 3GPP TS 23.041 (Technical realization of Cell Broadcast Service, CBS: message structure, message identifiers, serial numbers and data coding for ETWS and CMAS), 3GPP TS 36.331 (defines SystemInformationBlockType12 and the UE actions on receiving a warning-message SIB), 3GPP TR 36.976 (overall description of LTE-based 5G broadcast) (ETSI TS 103 720 V1.2.1 clause 5.15.3.3; TS 23.041 V20.0.0 clause 1; TS 23.041 V20.0.0 clause 9.4; TS 23.041 V20.0.0 clause 9.4.1.2; TS 23.041 V20.0.0 clause 9.4.3; TS 36.331 V19.4.0 clause 6.3.1; TS 36.331 V19.4.0 clause 5.2.2.19; TR 36.976 V19.0.0 clause 1).
Public warning here follows the Public Warning System (PWS) approach: CBS is the delivery mechanism, and the alert types it can carry include the Earthquake and Tsunami Warning System (ETWS) and the Commercial Mobile Alert System (CMAS) (TS 23.041 V20.0.0 clause 2; ETSI TS 103 720 V1.2.1 clause 4.2.5; TS 22.268 V20.3.0 clause 3.1). ETSI TS 103 720 profiles the CMAS path, with the warning message in SIB12; it names ETWS only as a regional adaptation of PWS, and the ETWS system information blocks SIB10 and SIB11, although defined in TS 36.331, are not part of the TS 103 720 profile (ETSI TS 103 720 V1.2.1 clauses 4.2.5 and 5.15.3.3; TS 36.331 V19.4.0 clause 6.3.1).
Transmit and receive roles
The Emergency Alerts feature set sits on the LTE-based 5G Broadcast transmit chain, in the roles the standard itself defines: (ETSI TS 103 720 V1.2.1 clause 5.15.1)
- RAN / physical-layer transmitter. The 5G Broadcast transmitter builds the E-UTRA system information carried on the broadcast carrier. For public warning, this is where the warning-message System Information Blocks are scheduled and encoded onto the radio frame, so that a 5G Broadcast Receiver (a Receive-Only Mode device that operates in idle mode) receives the SIB12 message that carries a CMAS warning (ETSI TS 103 720 V1.2.1 clause 5.15.3.3; ETSI TS 103 720 V1.2.1 clause 5.15.1; TS 36.331 V19.4.0 clause 5.2.1.1; TS 36.331 V19.4.0 clause 6.2.2; ETSI TS 103 720 V1.2.1 clause 5.15.6; TS 36.331 V19.4.0 clause 5.2.2.19).
- SIB12 encoding. The transmitter constructs the SIB12 message body from a set of CBS fields (message identifier, serial number, data coding scheme, and the warning-message segment carrying the alert text): the fields a standards-conformant transmitter needs to populate to broadcast a given alert (for example an Extreme or Severe alert, or a presidential-level or EU-Alert Level 1 alert) (ETSI TS 103 720 V1.2.1 clause 5.15.3.3; TS 36.331 V19.4.0 clause 6.3.1; TS 23.041 V20.0.0 clause 9.4.1.2.2).
- Receiver / device. A 5G Broadcast capable device tuned to the carrier decodes the system information, detects the warning-message SIB, decodes the CBS payload, and presents the alert. ETSI TS 103 720 specifies a PWS Client in the 5G Broadcast Receiver, and TS 23.041 leaves the precise method of display outside 3GPP scope; the same procedure that decodes ordinary system information also decodes SIB10/11/12, so a warning is just another scheduled SIB (ETSI TS 103 720 V1.2.1 clause 5.15.5.1; ETSI TS 103 720 V1.2.1 clause 7.2; TS 36.331 V19.4.0 clause 5.2.2.4; ETSI TS 103 720 V1.2.1 clause 5.15.6; TS 36.331 V19.4.0 clause 5.2.2.19; TS 36.331 V19.4.0 clause 5.2.2.1; TS 36.331 V19.4.0 clause 5.2.2.17 to 5.2.2.19; TS 36.331 V19.4.0 clause 5.2.1.5; TS 36.331 V19.4.0 clause 5.2.1.3; TS 36.331 V19.4.0 clause 6.2.2).
The mapping to the standard: the transmitter plays the role of the E-UTRAN cell (the eNB in a live network); the SIB12 it emits is the same warning-message structure a live network would broadcast; and the device applies the standard UE procedure for acquiring and acting on warning-message system information (ETSI TS 103 720 V1.2.1 clause 5.15.3.3; ETSI TS 103 720 V1.2.1 clause 5.15.1; TS 36.331 V19.4.0 clause 6.3.1; ETSI TS 103 720 V1.2.1 clause 5.15.5.1; TS 36.331 V19.4.0 clause 5.2.2.19; TS 36.331 V19.4.0 clause 5.2.2.17 to 5.2.2.19).
The CBS message model on the transmit side
CBS carries a warning message as a short, self-contained data structure rather than as an IP flow, which is what lets it reach idle and non-attached devices (TS 23.041 V20.0.0 clause 2; ETSI TS 103 720 V1.2.1 clause 5.15.3.3; TS 22.268 V20.3.0 clause 4.2; ETSI TS 103 720 V1.2.1 clause 4.2.5; TS 36.331 V19.4.0 clause 6.3.1). The main fields a developer will encounter when configuring or extending the transmitter are:
| CBS field | Role | Notes |
|---|---|---|
| Message Identifier | Identifies the warning type / source (for example a CMAS or ETWS category) | Assigned in 3GPP TS 23.041 clause 9.4.1.2.2; the reference transmitter's default configuration uses the fixed identifier 0x1102 (a statement about the reference tool, not taken from the specifications), which TS 23.041 assigns to the ETWS combined earthquake and tsunami warning. The value 0x1102 is 4354, within the range 4352 to 6399 that ETSI TS 103 720 V1.2.1 clause 5.15.3.3 allows for PWS. |
| Serial Number | Distinguishes and versions a message (geographical scope, message code, update number) | Changing the update number signals an updated message for the same event (TS 23.041 V20.0.0 clause 9.4.1.2.1; TS 23.041 V20.0.0 clause 8.2). |
| Data Coding Scheme | Selects the character set / language of the message body | Follows the CBS data coding scheme rules in TS 23.041. |
| Warning Message Segment | Carries the alert text, possibly split across segments | The transmitter marks the last segment and sets the segment number; the payload is the encoded warning text (TS 36.331 V19.4.0 clause 6.3.1; TS 23.041 V20.0.0 clause 9.4.3.2.4). |
For CMAS-style alerts the message is carried in SIB12. For ETWS (defined in TS 36.331 but not profiled by ETSI TS 103 720) the primary notification is carried in SIB10 and the secondary notification in SIB11; SIB11 uses the same segment fields as SIB12, SIB10 does not (TS 36.331 V19.4.0 clause 5.2.1.5; TS 36.331 V19.4.0 clause 6.3.1; ETSI TS 103 720 V1.2.1 clause 5.15.3.3; TS 36.331 V19.4.0 clause 5.2.1.4).
What later releases added
Public warning in 3GPP dates from Release 8, when ETWS was added to the Cell Broadcast Service in TS 23.041 (TS 23.041 V8.0.0 change history, CT#41, CRs 0018, 0019 and 0020); the Cell Broadcast Service itself is older (TS 23.041 V3.1.0 clause 1, for GSM and UMTS), and the stage 1 requirements of TS 22.268 start with V9.0.0 (Release 9; TS 22.268 V9.0.0 change history). See the release history. The release descriptions record these additions: ETWS/CMAS indication in connected mode for non-Cat-M LTE-MTC UEs in Release 16 (TR 21.916 V16.2.0 clause 7.3); ePWS Message Identifiers and language-independent content in Release 16 (TR 21.916 V16.2.0 clause 12.1); PWS over SNPN in Release 17 (TR 21.917 V17.0.1 clause 6.2.3.2); and, in Release 19, PWS over satellite E-UTRAN and NG-RAN with ETWS geo-fencing (TR 21.919 V19.0.0 clause 5.3.1) and NB-IoT warning system information blocks (TR 21.919 V19.0.0 clause 5.1.3). These are described for the 3GPP system in general; this page has not checked which of them ETSI TS 103 720 V1.2.1 clause 5.15 applies to the 5G Broadcast receiver, so check that clause before relying on any of them. For Release 20 (work plan of 28 September 2026), the work plan lists MLT-REQ, Stage 1 for Multilingual Template-based alerts (TS 22.268), and the CPAS work items (TS 23.041); no Release 20 release description was available, so see the release history for the list.
The complete public warning chain
A live Public Warning System has more upstream steps than the broadcast hop this page focuses on: a Cell Broadcast Entity feeds a Cell Broadcast Centre, which runs the CBC-to-MME Write-Replace Warning procedure, and the MME distributes the warning to the relevant eNBs. This page's analysis covers the last hop of that chain, network to device over the broadcast carrier, which is where CBS/SIB12 delivery and the standard UE procedure for acting on it are defined (TS 23.041 V20.0.0 clause 3.3; TS 23.041 V20.0.0 clause 9.1.3.4.2; ETSI TS 103 720 V1.2.1 clause 5.15.1; TS 29.168 V19.0.0 clause 4.3.3.2; TS 36.331 V19.4.0 clause 6.3.1; ETSI TS 103 720 V1.2.1 clause 5.15.3.3; TS 36.331 V19.4.0 clause 5.2.2.19).
5G-MAG's own reference tools overview slide deck introduces the alert path, and the MBMS: Public Warning System board tracks the underlying developer issues. What is planned is on the project page, under Latest updates and roadmap.
Related: Media delivery over 5G Broadcast (the underlying transmit and receive platform this extends) · Standards: Media delivery over 5G Broadcast (the full specification set, including ETSI TS 103 720, that this builds on) (ETSI TS 103 720 V1.2.1 clause 5.15.1)