Service Receiver

Description

The Service Receiver is a term defined in TS 21.905 and TS 22.060, representing the entity that receives the service request indication primitive, containing the Service Data Unit (SDU). It is part of the service primitives model used in 3GPP to describe the interactions between layers or entities in a communication protocol stack. The service receiver is the counterpart to the service requester or service user, and together they form the endpoints of a service access point (SAP).

In the OSI and 3GPP layered architecture, service primitives are abstract representations of interactions between adjacent layers. A primitive can be a request, indication, response, or confirm. The service receiver receives an indication primitive, which signals the arrival of data or an event. The SDU is the data unit passed from one layer to another, containing the payload that the upper layer needs to process. The service receiver is thus the entity that consumes the SDU delivered by the lower layer.

The concept is generic and applies across various 3GPP specifications. For example, in the GPRS service description (TS 22.060), the service receiver could be a mobile station or a network entity that receives a service request indication. In the context of OSA, the service receiver might be an application that receives an indication from the OSA gateway. The term is used to clarify the direction of data flow and the roles of entities in service interactions.

While the term itself is abstract, it is important for specifying interfaces and protocols. It helps to define the behaviour of entities at service boundaries: the service provider (lower layer) sends an indication primitive to the service receiver (upper layer), which then processes the SDU. This model ensures that the responsibilities of each layer are clearly delineated, and that data is passed correctly.

In modern 3GPP specifications, the service receiver concept is often implicit in the definition of interfaces and protocols. For instance, in the 5G system, the Service Data Adaptation Protocol (SDAP) receives SDUs from the upper layer and maps them to QoS flows. The UE or the UPF can be considered service receivers depending on the direction of transmission. The term remains part of the foundational vocabulary, ensuring consistency across generations of specifications.

Purpose & Motivation

The Service Receiver concept exists to provide a clear, abstract model for describing how data and events are exchanged between layers or entities in a communication system. It is part of the service primitives framework, which was inherited from OSI and adapted by 3GPP to specify interfaces in a technology-neutral way.

By defining the service receiver as the entity that receives an indication primitive containing an SDU, 3GPP specifications can describe interactions without prescribing implementation details. This abstraction is crucial for interoperability: different vendors can implement the same service interface as long as they adhere to the primitive definitions. It also facilitates the specification of protocols at different layers, from the physical layer up to the application layer.

Historically, the term was introduced in Rel-4 as part of the vocabulary for GPRS and UMTS. It has been carried over unchanged through all subsequent releases, indicating its enduring relevance. While newer specifications may use different terminology (e.g., 'consumer' or 'sink'), the underlying concept remains the same: there must be a defined endpoint that receives data and events from a service provider.