Service access point
Service access point (also service access point ) is a technical term from telecommunications . This is used to describe the interface for interaction with a communication layer at the upper limit of the same layer.
Traditionally, communication systems and protocols in telecommunications are modeled with so-called layer models, such as the OSI model . Functions that are necessary for communication are modeled and displayed in layers one on top of the other. Each layer fulfills a specific task ( e.g. data encoding / decoding ). In other words, each layer provides a certain service for the overall system, which is used by the next higher layer. An interface that is offered by a layer for its service is a service access point.
The service user (the higher layer, service user ) only accesses the service of the lower layer (the service provider ) via the service access point . So-called are for communication service elements ( service primitive ) is used. These are predefined protocols with which, for example, the higher layer sends requests to the lower layer or receives data from it.
In practice, a layer can offer several identical or different services at the same time - for example, if several connections are processed at the same time. This means that one layer can have several service access points. In many protocols it is then customary to designate these service access points by numbering, names or the like in order to distinguish them. Such an identifier is called a Service Access Point Identifier (SAPI). Higher layers then address a service via a corresponding SAPI, for example to ensure that the request can be assigned to the connection for which it is intended.
primitive
Primitives ( primitives , also called service primitives or service elements ) are abstract, implementation-independent processes for using a layer-specific service (see OSI reference model ), especially for describing communication processes. When defining the interface, primitives serve as an abstract concept and thus avoid the strict requirements of an API based on function prototypes. This only defines which data is exchanged, but not in which way the transfer takes place.
The primitives are divided into four groups:
- REQ ( request ),
- IND ( indication , message),
- RESP ( response ) and
- CONF ( Confirmation ).
With the service element REQ, a service of the underlying layer N-1 is requested by an instance of the layer N. After a request has been processed, the processing shift usually replies with CONF. A service element IND, on the other hand, is triggered by an instance of the lower layer N-1 and usually answered with RESP by the layer N above.
As a rule, an indication arises as a result of the receipt of a message initiated and sent by a communication partner via REQ. If the recipient replies to this message, e.g. B. via RESP, this then triggers the response from the original sender via CONF. This approach is based on one layer.