Message identifiers
Section 9.4.1.2.2 of 3GPP TS23.041 reserves the range of message identifiers 4352 - 6399 (0x1100 - 0x18FF) for public warning. This table also defines the different combinations of CAP parameters Severity, Urgency and Certainty that map to specific EU-Alert and WEA alerts.
Table 1 of ETSI TS102900 defines how to map alert levels between EU-Alert and WEA3.
The value of the message identifier to use for each alerts is selected based on:
- Alert type.
- Language: local or additional.
The following table all this information to show how the CBC shall decide what message identifier to use for each alert.
| EU-Alert | WEA3 | CAP Parameters | Message Id | |||
|---|---|---|---|---|---|---|
| Language | ||||||
| Severity | Urgency | Certainty | Local | Additional | ||
| Level 1 | Presidential | - | - | - | 4370 | 4383 |
| Level 2 | Extreme Alert | Extreme | Immediate | Observed | 4371 | 4384 |
| Likely | 4372 | 4385 | ||||
| Level 3 | Severe Alert | Expected | Observed | 4373 | 4386 | |
| Likely | 4374 | 4387 | ||||
| Severe | Inmediate | Observed | 4375 | 4388 | ||
| Likely | 4376 | 4389 | ||||
| Expected | Observed | 4377 | 4390 | |||
| Likely | 4378 | 4391 | ||||
| AMBER | AMBER | - | - | - | 4379 | 4392 |
| - | RMT | - | - | - | 4380 | 4393 |
| - | Exercise | - | - | - | 4381 | 4394 |
| - | Operator defined use | - | - | - | 4382 | 4395 |
| Level 4 | Public Safety Alert | - | - | - | 4396 | 4397 |
| - | State/Local test | - | - | - | 4398 | 4399 |
| - | Geofencing trigger | - | - | - | 4400* | - |
| Info | - | - | - | - | 6400 | - |
| * Geofencing trigger does not contain displayable text. | ||||||
The CAP profile for each country should specify how to derive alert types (i.e. message identifiers) from the value of specific parameters or groups of parameters inside a CAP Alert. These profiles may reuse what is shown in the table for CAP parameters Severity, Urgency and Certainty, or define a totally different way of deriving the alert level and, therefore, the message identifier. For example, they might use the parameter element inside the info object to explicitly state the alert level.
Serial Number
All CBS messages have a serial number that is used by various nodes (e.g. eNBs, UEs) to know whether two requests received from different sources, at different times or in different locations (case of UE) actually refer to the same or a different message.
The internal structure of this serial number is defined in section 9.4.1.2.1 of 3GPP TS23.041. In short, the serial number is two octets long and has the following fields:
- Geographic Scope (GS) [2 bits]: used to define the geographical validity of this message and how the UE shall display it. The following table is extracted from the standard.
| GS value | Display Mode | Geographical scope |
|---|---|---|
| 00 | Inmediate | Cell wide |
| 01 | Normal | PLMN/SNPN wide |
| 10 | Normal | Tracking Area wide in E-UTRAN and NG-RAN |
| 11 | Normal | Cell wide |
For PWS, the display mode should be ignored by UEs. All PWS messages for which the user of the handset has opted-in should be displayed inmediately.
Default Geographical Scope
While there is no default value defined for GS, it is presumed that most commercial CBCs allow configuring the default value that is used for all the PWS messages.
In most countries, value 02 is configured, since PWS alerts have PLMN scope. Another value can be configured in countries that require different scope for PWS alerts.
- Message code [10 bits]: differentiates between CBS messages from the same source and type (i.e. with the same Message Identifier).
- Update number [4 bits]: indicates a change of the message content of the same CBS message, i.e. the CBS message with the same Message Identifier, Geographical Scope, and Message Code. It differentiates between older and newer versions of the same CBS message within the indicated geographical area.
Message reference
Section 9.2.1 of 3GPP TS23.041 defines the concept of message reference. This is what CBS nodes (MME, eNB, UE, etc.) use to distinguish if a received CBS protocol message refers to a new broadcast message or an active one, and it is defined as the combination of serveral fields / parameters as the tern <Message Identifier, Serial Number, Cell Identifier>.
A Note on Message References
A message reference has cell granularity.
A WRITE-REPLACE-WARNING-REQUEST may translate into an update for active message references and the creation of new message references. It depends on the content of the Cell List or Warning Area List respectively.
Data coding scheme
3GPP TS23.038 defines the encoding of text messages for CBS. The data coding scheme (DCS) is a 1 octet value that contains information about:
- Type of data: plain / compressed text or binary.
- Text encoding: GSM7 or UCS2.
- Language.
TS23.038 also defines the GSM7 enconding tables for the different languages. It encodes most English and Scandinavian characters with 7 bits, and defines an extension mechanism, using shift tables, to encode additional characters with 14 bits.
GSM7 shift tables have been defined for many languages that use a reduced set of characters.
UCS2 can be used for any language and must be used for languages with a large set of characters, like Chinese or Japanese. UCS2 is a flavour of Unicode, and encodes all characters with 16 bits.
Area identifiers
Area concepts for the different technologies (LTE, 5G) are defined in different 3GPP standards, and their identifiers are defined in 3GPP TS23.003.
The CBC uses the following area concepts:
| Technologies | Identifier | Area concept | Comments |
|---|---|---|---|
| LTE | E-CGI | E-UTRAN Cell | |
| 5G | NR CGI | NR Cell | |
| LTE, 5G | TAI | Tracking Area | |
| LTE, 5G | Emergency Area |
When needed, the TC procedure clearly states which identifiers are relevant for the test.