service element (OSI) (SE)
Communication between the service user and the service provider takes place with the aid of service primitives, which are exchanged between the directly adjacent layers via the service access point( SAP).
Here, the service primitive elements do not distinguish between communication between partner instances and communication restricted to the directly adjacent layers. They describe everything that is exchanged at the boundary of the interface between the layers in terms of information.
There are four service elements: Indicate, Confirm, Request, and Response. When a connection is established, the establishing station sends a Request to the second station, which receives the Request as Indicate and sends a Response as Confirm, which the first station receives as Confirm.
The conventions of the service primitives are defined in X.210 of the International Telecommunication Union( ITU).