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Entity Types
You receive a message about a particular entity. How do you choose to draw the entity that you have been informed of? We need some way to convert the information in the message, typically the Entity State PDU (ESPDU) in a way that tells us what type of entity it is. This is done with multi-value records and the semantic meaning of values in fields.
| Field Name | Field Size |
|---|---|
| Entity Kind | 8 bits |
| Domain | 8 |
| Country | 16 |
| Category | 8 |
| Subcategory | 8 |
| Specific | 8 |
| Extra | 8 |
The DIS protocol sends information about what it being described, but the size of the architectural inventory is so large that not every device in the world can be selected. No simulation can carry examples of every actual vehicle (or weapon!) that exists. The fields of the record have arbitrary semantic value, values set by SISO organization.
Here's an example of using SISO to describe a M1A1 tank:
| Field Name | Field Values | Description |
|---|---|---|
| Entity Kind | 1 | Vehicle |
| Domain | 1 | Land |
| Country | 225 | USA Vehicle |
| Category | 1 | Tank |
| Subcategory | 1 | M1A Tank |
| Specific | 3 | M1A2 Tank |
| Extra | 0 | Unused Here |
SISO maintains the value of record fields. The SISO list is fast working to aid descriptions of newly arrived vehicles, and is flexible. That of the aircraft carrier GHW Bush is available:
| Field Name | Field Values | Description |
|---|---|---|
| Entity Kind | 1 | Vehicle |
| Domain | 3 | Sea |
| Country | 225 | USA Vehicle |
| Category | 1 | Carrier |
| Subcategory | 1 | Aircraft Carrier |
| Specific | 1 | Nimitz Class Carrier |
| Extra | 10 | George HW Bush |
SISO defines hundreds of devices, including tanks, aircraft carrier ships, rifles, box cars, radios, and more.
Clients can read incoming PDUs, decode them, and make use of them. One of the things that can happen is that two simulations share similar entity types--perhaps one with M1A2 tanks with anti-mine plows, while the other simulation does not have plow-equipped tanks. Which means they can't work directly together. One way to solve that problem is to write "gateways" that change the UDP PDUs--put the simulations on two networks with a connector between their networks. The PDU is sent, and the entity type changed slightly so other simulations can communicate.
SISO's product to help DIS work is known as Enumeration and Bit Encoded Values (EBV). It is very widely used, and not only by DIS. Most other DoD protocols, including HLA RPR-FOM and TENA, also use the SISO document.