Service request

Description

A service request is defined as one invocation of a service through a service request primitive. This term was introduced in Release 4 (Rel-4) and is specified in 21.905 and 22.060. It represents a specific instance of a service consumer asking a service provider to perform a function or provide a resource. The concept is fundamental to service-oriented interactions in 3GPP networks, where entities communicate by invoking services through well-defined primitives.

In the context of 3GPP, a service request primitive is an abstract representation of the interaction between two entities. It typically includes parameters that specify the requested service, any input data, and the expected output. The service request is the actual message or signal that carries the primitive. For example, in the GPRS (General Packet Radio Service) domain, a service request might be sent by a mobile station to initiate a packet data session. The request is processed by the network, which then provides the service if the request is valid and resources are available.

The service request concept is closely tied to the service requester, which is the entity that initiates the request. The relationship between the requester and the provider is governed by the service relationship. In modern 5G networks, service requests are often implemented as HTTP/2 requests in the Service Based Architecture (SBA), where Network Functions (NFs) expose their services via APIs. A service request in this context is an API call that invokes a specific service operation.

Service requests can be categorized based on their nature: they can be synchronous or asynchronous, and they can be one-way or request-response. The choice depends on the service and the requirements. For instance, a service request for immediate data retrieval might be synchronous, while a request for a long-running process might be asynchronous. The 3GPP specifications define the primitives and procedures for various service requests, ensuring interoperability between different implementations.

From a protocol perspective, service requests are often mapped to specific messages. For example, in the Non-Access Stratum (NAS) protocol, a Service Request message is used by the UE to request the establishment of a signaling connection or the activation of a user plane bearer. This message is part of the 5G Mobility Management (5GMM) procedures. The service request primitive is the abstract concept, while the NAS message is its concrete realization.

Understanding service requests is essential for network engineers and developers because they are the building blocks of service delivery. Efficient handling of service requests is critical for network performance, as each request consumes resources and must be processed within certain time constraints. The 3GPP specifications provide detailed requirements for service request handling, including admission control, prioritization, and error handling.

Purpose & Motivation

The service request concept was introduced to provide a standardized way to invoke services in a telecommunications network. Before its introduction, service invocation was often implicit or tied to specific protocols, making it difficult to describe and manage services in a technology-agnostic manner. The service request abstraction allows for a clear separation between the service interface and its implementation, enabling greater flexibility and interoperability.

In the early days of GPRS, service requests were used to initiate packet data sessions. As networks evolved, the concept was generalized to cover any service invocation. This generalization was necessary to support the growing variety of services and the need for dynamic service composition. The service request primitive provides a common language for describing service interactions, which is particularly important in multi-vendor and multi-operator environments.

Moreover, the service request concept supports the principles of service-oriented architecture (SOA), which emphasizes loose coupling, reusability, and discoverability. By defining service requests as discrete invocations, 3GPP enables the creation of reusable service components that can be combined in different ways. This is a key enabler for network slicing and the Service Based Architecture in 5G, where services are dynamically discovered and invoked.