| Internet-Draft | SDF modeling for digital twin | February 2025 |
| Lee, et al. | Expires 29 August 2025 | [Page] |
This memo specifies SDF modeling for a digital twin, i.e. a digital twin system, and its interactions. An SDF is a format to describe Things and their associated interactions, and to represent the various kinds of information that is exchanged for these interactions. Therefore, the SDF format can be used to define the behavior of things, i.e. physical objects, and related data and interaction models in a digital twin that contain objects as components.¶
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A digital twin is defined as a digital representation of an object of interest and may require different capabilities, for example, synchronization and real-time support, according to the specific domain of application. [Y.4600]. Digital twin help organizations improve important functional objectives, including real-time control, off-line analytics, and predictive maintenance, by modeling and simulating objects in the real world. Therefore, it is important for a digital twin to represent as much real-world information about the object as possible when digitally representing the object.¶
Nowadays, digital twin technologies are applied in various domains including manufacturing, energy, medical, farm, transportation, etc. And a common format is needed to represent the objects in the domains as digital twins. SDF [I-D.ietf-asdf-sdf] can be used for modeling objects as digital twins.¶
This document specifies the modeling and guidance on how to use SDF to represent objects as digital twins.¶
This specification uses the terminology specified in [I-D.ietf-asdf-sdf] in particular "Class Name Keyword", "Object", and "Affordance".¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This section describes SDF structure with the new Class Name Keyword, sdfNonAffordance, to represent a thing or an object as a digital twin.¶
The SDF language uses six Class Name Keywords, and sdfNonAffordance is added as a new Class Name Keyword for digital twin.¶
The information attributes for digital twin are defined in [ISO23247-3]. Some of them, for example characteristics and status, can be described using existing Class Name Keywords. And the others, for example location, can be described using sdfNonAffordance.¶
{
"info": {
"title": "An example of the heater #1 in the boat #007",
"version": "2025-01-27",
"copyright": "Copyright 2025. All rights reserved.",
},
"namespace": {
"heater1": "https://example.com/heater1"
},
"defaultNamespace": "heater1",
"sdfObject": {
"boat": {
"sdfProperty": {
"value": {
"description":
"The state of the heater #1 in a the boat #007; false for off and true for on.",
"type": "boolean"
}
},
"sdfNonAffordance": {
location:{
"wgs84": {
"latitude": "35.2988233791372",
"longitude": "129.25478376484913",
"altitude": "0.0"
},
"postal": {
"city": "Ulsan",
"post-code": "44110",
"country": "South Korea"
},
"w3w": {
"what3words":"toggle.mopped.garages"
}
}
report: {
"value": {
"description":
"On February 24, 2025, the boat #007's heater #1 was on from 9 a.m. to 6 p.m.",
"type": "string"
}
}
}
}
}
}
The architecture of a digital twin based on the SDF model is illustrated in Figure 2, , following the guidelines of [ISO23247-3].¶
The Physical Layer comprises affordance and non-affordance objects. From the real-world objects, only those deemed relevant are selected for representation as digital twins. The Digital Twin Layer is structured into three sublayers: the Device Communication Sublayer, the Digital Twin Sublayer, and the Application Sublayer. The Device Communication Sublayer is responsible for monitoring and collecting data from both affordance and non-affordance objects. This sublayer provides the necessary data to synchronize the physical objects with their digital twin counterparts. The Digital Twin Sublayer ensures synchronization between the affordance and non-affordance objects and their respective digital twins using the data provided by the Device Communication Sublayer. The Application Sublayer presents the synchronized values of the digital twins to users, facilitating informed decision-making.¶
+---------------------------------------------+ - - - - - - - - - - -
| Application Sublayer |
| +----------+ +------+ +--------+ +--------+ |
| | Human | | HMI | | Apps | | Peers | |
| +----------+ +------+ +--------+ +--------+ |
+---------------------------------------------+
| Digital Twin Sublayer |
| +----------+ +-------------+ +------------+ |
| | Operation| | Application | | Resource | |
| | and | | and | | access and | |
| |management| | service | |interchange | |
| +----------+ +-------------+ +------------+ |
| +-----------------------------------------+ | Digital twin Layer
| | Digital representation of objects | |
| | +-------------+ +----------------+ | |
| | | Affordance | | Non-Affordance | | |
| | | objects | | objects | | |
| | +-------------+ +----------------+ | |
| +-----------------------------------------+ |
+---------------------------------------------+
| Device Communication Sublayer |
| +-------------+ +----------------+ |
| | Data | | Object | |
| | collection | | control | |
| +-------------+ +----------------+ |
+---------------------------------------------+ - - - - - - - - - - -
| +-------------+ +----------------+ |
| | Affordance | | Non-Affordance | |
| | objects | | objects | | Physical Layer
| +-------------+ +----------------+ |
+---------------------------------------------+ - - - - - - - - - - -
A digital twin is a partial representation of sdfThing or sdfObject that contains attributes such as sdfProperty, sdfAction and sdfEvent[ISO23247-1]. By representing sdfThing as a digital twin, crucial events that require appropriate action can be quickly detected and controlled. The requirements defined in [ISO23247-1] are applied to represent sdfThings and sdfObjects as digital twins.¶
Data related to sdfThing and sdfObject, such as sdfProperty, sdfEvent, and sdfAction, should be collected from IP and non-IP devices.¶
The collected data needs to be analyzed to understand the state of sdfThing and sdfObject.¶
The sdfThings and sdfObjects should contain data that uniquely identifies them as digital twins.¶
The sdfThings and sdfObjects should be represented as digital twins with appropriate levels of detail and accuracy, depending on the application.¶
The sdfThings and sdfObjects should be synchronized with their digital twins in real-time as appropriate for the application. Newly added or removed sdfThings and sdfObjects should be recognized and reflected in the digital twin.¶
A procedure for representing sdfThing, as a digital twin in a domain is as follows:¶
defining a purpose for expressing the observable object, as known as a physical asset or an object of interest, as a digital twin in the domain¶
organizing data based on the roles of the observable object in the domain¶
configuring the observable object into the digital twin based on the data for the purposes¶
interworking with a digital twin of each of other domains in which the observable object performs a different role¶
synchronizing the observable object and the digital twin¶
Only authorized users should have the authority to manage sdfThings and sdfObjects.¶
This document has no IANA actions.¶