Session based service

Description

A session based service is an end-user service that requires application-level signalling, which is separated from service rendering. This means that the service involves a control plane that manages the session, distinct from the user plane that carries the actual media or data. This separation allows for dynamic policy and charging control based on the session state and application-level information.

In 3GPP, session based services are typically implemented using protocols such as SIP for IMS-based services. The application-level signalling (e.g., SIP INVITE, 200 OK) is used to establish, modify, and terminate the session. During this signalling, parameters such as media type, codec, and QoS requirements are negotiated. The network can then apply appropriate policies, such as QoS and charging rules, based on the session description.

The concept is closely related to Policy and Charging Control (PCC) as defined in 3GPP TS 23.203 and TS 23.503. The PCRF (Policy and Charging Rules Function) or PCF (Policy Control Function) uses application-level information from the session signalling to make policy decisions. For example, it can authorize a specific QoS for a video call or apply online charging based on the session duration.

Session based services are distinguished from non-session based services, such as best-effort Internet access, where no application-level signalling is used for service control. The separation of signalling from rendering allows the network to be aware of the service and to enforce policies accordingly. This is essential for operators to offer differentiated services and to monetize them effectively.

The definition has remained stable since its introduction in Rel-7, indicating a consistent architectural principle. It is a foundational concept for IMS and other advanced services that require dynamic control.

Purpose & Motivation

The concept of session based service was introduced to enable operators to apply dynamic policy and charging control for services that require application-level signalling. Before this, services were often treated as best-effort, and operators could not easily differentiate or charge for them based on the actual service being used. Session based services solve this by allowing the network to be aware of the service through signalling, enabling QoS enforcement and flexible charging.

Historically, as networks evolved to support multimedia services like VoIP and video calling, the need arose to distinguish between services that require signalling (e.g., IMS) and those that do not (e.g., web browsing). This distinction is crucial for resource allocation and policy enforcement. The 3GPP defined session based services in Rel-7 as part of the PCC architecture, which provides a framework for policy and charging control.

The motivation was to allow operators to offer tiered services, where users can pay for enhanced QoS or specific applications. By separating signalling from rendering, the network can identify the service and apply the appropriate policies without deep packet inspection. This also enables new business models, such as sponsored data or application-specific charging.